Part A Unit-based exercise

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1 Topic 2 Microscopic World I / Microscopic World (Combined Science) Part A Unit-based exercise Unit 5 Atomic structure Fill in the blanks 1 atoms 2 solids; liquids; gases 3 metals; metalloids; non-metals 4 low 5 good 6 Boron; silicon; germanium 7 nucleus; electrons; shells 8 protons; neutrons 9 atomic 10 mass 11 isotopes 12 atomic mass 13 electronic arrangement 14 electron 15 orbital True or false 16 F Mercury is a metal that exists as a liquid at room temperature and pressure. 17 T 18 T 19 F Molten sulphur contains mobile molecules only. There are no mobile ions or electrons. It does not conduct electricity. 28

2 20 F The symbol of magnesium is Mg. 21 T The atomic number of an element is the number of protons in an atom of that element. An atom has equal numbers of protons and electrons. Therefore the atomic number of an element also equals the number of electrons in an atom of that element. 22 F The neutral atom of an element must contain equal numbers of protons and electrons (NOT neutrons). 23 F A hydrogen atom ( 1 1H) contains no neutron. 24 F A sodium atom ( 23 11Na) contains 11 protons, 11 electrons and 12 neutrons. 25 T The atomic number of fluorine is 9. A fluorine atom contains 9 protons and 9 electrons. 26 T Isotopes are different atoms of an element which have the same number of protons but a different number of neutrons. 27 F Isotopes of an element have different number of neutrons and hence they have different masses. 28 T 29 F The second electron shell can hold a maximum of 8 electrons. 30 F The electronic arrangement of a calcium atom is 2,8,8,2. Hence a calcium atom contains 4 occupied electrons shells. Multiple choice questions 31 D Option Element Symbol A Calcium Ca B Chlorine Cl C Iron Fe D Potassium K 32 C Carbon, iron and silicon are solids at room temperature and pressure. 33 D Option Symbol Element State at room temperature and pressure A Cl chlorine gas B N nitrogen gas C Ne neon gas D S sulphur solid 34 B Beryllium is a solid at room temperature and pressure. 29

3 35 A Option B X melts at C and boils at C. It is a solid at 25 C. Option C Y melts at 58 C and boils at 37 C. It is a liquid at 25 C. Option D Z melts at 52 C and boils at 114 C. It is a solid at 25 C. 36 B Option A W melts at 189 C and boils at 186 C. It is a gas at 100 C and 1 atm pressure. Option B X melts at 110 C and boils at 40 C. It is a liquid at 100 C and 1 atm pressure. Option C Y melts at 7 C and boils at 60 C. It is a solid at 100 C and 1 atm pressure. Option D Z melts at 90 C and boils at 10 C. It is a solid at 100 C and 1 atm pressure. 37 C Substance Melting point ( C) Boiling point ( C) State at room temperature and pressure W 50 5 gas X 4 81 liquid Y solid Z liquid X and Z are in liquid state at room temperature and pressure. 38 A Carbon and neon are non-metals. Germanium is a metalloid. 39 A Copper is a metal. Helium and phosphorus are non-metals. 40 D Option Symbol Element Metal / non-metal A Ba barium metal B Be beryllium metal C Cs caesium metal D Kr krypton non-metal 41 C Metals are good conductors of electricity and insoluble in water (i.e. Y). 42 D Sillicon is insoluble in water. 43 D Option A An atom must have equal numbers of protons and electrons (NOT neutrons). Option B The mass of one proton is approximately equal to that of electrons. Option C A neutron carries no charge. 44 A Atomic number of X = 10 = number of protons in an atom = number of electrons in an atom Number of neutrons = mass number atomic number = = 12 30

4 45 B An atom has equal numbers of protons and electrons. Therefore the atom has 28 protons and thus its atomic number is 28. Mass number = number of protons + number of neutrons = = C Atomic number = 23 = number of protons = number of electrons Number of neutrons = mass number atomic number = = D Particle Atomic number Mass number Number of neutrons = mass number atomic number S Na Mg Si P P contains the same number of neutrons as 32 16S. 48 C Isotopes are different atoms of an element which have the same number of protons and electrons, but a different number of neutrons. 49 D Atom Atomic number Mass number Number of neutrons I II III IV atoms II and IV are isotopes of potassium. 6 x x B Relative atomic mass of lithium = 100 = x x B Relative atomic mass of X = 100 = x x x C Relative atomic mass of X = 100 =

5 53 D Let the relative abundance of 79 X and 81 X be (100 y)% and y% respectively. 79 x (100 y) + 81 x y Relative atomic mass of X = y + 81y = = 79.9 y = D Most elements have more than one isotope and the different isotopes of each element have different masses. The relative isotopic mass and relative abundance of each isotope of an element in nature must be considered when calculating the relative atomic mass of the element. 55 B The relative abundance of 69 X and a X are 65.0% and 35.0% respectively. Relative atomic mass of X = a = x 65 + a x = 69.7 a = B An atom of X has 15 electrons. The first 10 electrons fill up the first and second electron shells while the last 5 electrons go into the third shell. 57 C Option Electronic arrangement of atom Atomic number of element Name of element Metal / metalloid / non-metal A 2,1 3 lithium metal B 2,2 4 beryllium metal C 2,3 5 boron metalloid D 2,4 6 carbon non-metal 2,3 represents the electronic arrangement of an atom of a metalloid. 58 C Option Symbol Element Electronic arrangement of atom A Cl chlorine 2,8,7 B P phosphorus 2,8,5 C S sulphur 2,8,6 D Si silicon 2,8,4 the question shows the electron diagram of an atom of sulphur (symbol S). 59 B (1) Mercury is a metal which is a liquid at room temperature and pressure. (3) Not all metals are stored in paraffin oil, e.g. magnesium and copper are not stored in paraffin oil. 60 D 61 A (2) Gallium is a metal, not a metalloid. 32 (3) The crystalline form of silicon (a metalloid) can conduct electricity at room temperature.

6 62 A (2) Isotopes of an element have the same number of protons. Hence they have the same atomic number. (3) Isotopes of an element have different number of neutrons and hence different masses. 63 A (1) P is an atom of phosphorus (a non-metal). (2) Q is an atom of sulphur (a non-metal). (3) P and Q are atoms of different elements. They are NOT isotopes. 64 B (1) Particles X and Z have different number of protons but the same number of neutrons. Hence they have different masses. (2) Particles X and Y have equal numbers of protons and electrons, but a different number of neutrons. Hence they are isotopes. (3) Particles Y and Z have different number of electrons. Hence they have different electronic arrangements. 65 B (1) X is phosphorus. It is a solid at room temperature and pressure. (2) The electronic arrangement of an atom of X is 2,8,5. Hence there are 5 electrons in the outermost shell of an atom of X. (3) Number of neutrons in an atom of X = mass number atomic number = = A (1) X is sodium. Hence it is a metal. (2) The electronic arrangement of an atom of X is 2,8,1. Hence there are 11 electrons and 11 protons in an atom of X. (3) X is sodium. Its symbol is Na. 67 A (1) Number of neutrons in a 60 27Co atom = mass number atomic number = = 33 (2) The atomic number of 60 27Co is 27. Hence a 60 27Co atom contains 27 protons. (3) Isotopes of an element have the same number of electrons. 68 B Both statements are true. However, the state of an element cannot be explained by the fact whether it is a metal or non-metal. 69 A 70 C An atom is electrically neutral because it has equal numbers of protons and electrons (NOT neutrons). 71 B The atomic number of an element is the number of protons in an atom of the element. A 32 16S atom contains 16 protons. 72 C X and 54 26Y represent different elements. They are NOT isotopes. 33

7 73 A 74 A Most elements have more than one isotope and the different isotopes of each element have different masses. The relative isotopic mass and relative abundance of each isotope of an element in nature must be considered when calculating the relative atomic mass of the element. 75 C Isotopes of an element have the same number of protons but a different number of neutrons. Hence they have different masses. Unit 6 The periodic table Fill in the blanks 1 atomic number 2 groups; periods 3 group 4 period 5 metalloids; non-metals 6 alkali 7 paraffin oil 8 increases 9 hydrogen; sodium hydroxide 10 alkaline earth 11 halogens 12 greenish yellow; reddish brown; black 13 decreases 14 noble gases 15 octet 16 Argon 17 positive 18 negative 19 2; 12; ; 7; 10 34

8 True or false 21 F In the periodic table, all the elements are arranged in order of increasing atomic number. 22 T 23 T Across the second period, the elements change from metals through metalloids to non-metals. 24 F Atoms of elements in the same period have the same number of occupied electron shells. The atom of each element in Period 3 has 3 occupied electron shells. 25 F The electronic arrangement of a sulphur atom is 2,8,6. A sulphur atom has 3 occupied electron shells. Hence sulphur belongs to Period 3 of the periodic table. 26 T The electronic arrangement of an aluminium atom is 2,8,3. An aluminium atom has 3 outermost shell electrons. Hence aluminium is a Group III element. 27 T Sodium is a Group I element, an alkali metal. 28 F Argon belongs to Group 0. It is a noble gas. 29 T Magnesium is a Group II element, an alkaline earth metal. 30 T Neon is a noble gas and belongs to Group 0 of the periodic table. 31 T Potassium is a reactive metal and must be stored in paraffin oil to prevent it from reacting with the air. 32 F The melting point of the Group I elements decreases down the group. Hence the melting point of sodium is lower than that of lithium. 33 F Beryllium and calcium belong to the same group (Group II). They have similar chemical properties, NOT the same chemical properties. 34 T 35 F Iodine vapour is purple in colour. 35

9 36 T 37 F 35 Cl and 37 Cl have the same electronic arrangement and hence the same chemical properties. 38 F A helium atom has a duplet structure in its outermost shell. 39 T 40 T An oxygen atom has an electronic arrangement 2,6. It tends to gain 2 electrons in order to obtain the stable electronic arrangement of a neon atom (2,8). An oxide ion forms. Multiple choice questions 41 D Option A Elements in the periodic table are arranged in order of increasing atomic number. Option B The vertical columns are called groups. Option C The horizontal rows are called periods. 42 C Option A The number of occupied electron shells in atoms of elements in the same group increases down the group. e.g. Group I element Electronic arrangement of atom Number of occupied electron shells Lithium 2,1 2 Sodium 2,8,1 3 Potassium 2,8,8,1 4 Option B The atomic number of elements in the same group increases down the group. Option D Elements in the same group have similar, not the same, chemical properties. 43 C Option A The reactivity of Period 2 elements changes across the period. Apart from the noble gases, the most reactive elements are near the edges of the periodic table and the least reactive ones are in the centre. H Li Be B C N O F Na Mg Al Si P S Cl K Ca 36

10 Option B The atom of each element in Period 2 has two occupied electron shells, NOT 2 outermost shell electrons. Option D Across the period, the elements change from metals through metalloids to non-metals. 44 B The electronic arrangement of a carbon atom is 2,4. A carbon atom has 4 outermost shell electrons. Hence carbon belongs to Group IV of the periodic table. 45 C The group number of an element equals the number of outermost shell electrons in its atom. An atom of element X has 6 outermost shell electrons. Hence X belongs to Group VI of the periodic table. 46 A An atom of X has an electronic arrangement 2,8. X is a noble gas and belongs to Group 0 of the periodic table. 47 C An atom of element X has an electronic arrangement 2,8,5. It has 5 outermost shell electrons. Hence X belongs to Group V of the periodic table. 48 B Option Element Group number A Boron III B Bromine VII C Chlorine VII D Silicon IV 49 A Option Element Group A argon 0 neon 0 B C D carbon chlorine calcium potassium magnesium sodium IV VII II I II I Argon and neon belong to the same group. 37

11 50 D Lithium and sodium belong to Group I of the periodic table. Element Atomic number Electronic arrangement of atom Lithium 3 2,1 Sodium 11 2,8,1 The chemical properties of an element depend on the number of outermost shell electrons in its atom. Lithium and sodium have the same number of outermost shell electrons in their atoms. Hence they have similar chemical properties. 51 B Option Atomic number of element Electronic arrangement of atom A Group to which the element belongs 7 2,5 V 13 2,8,3 III B C D 9 2,7 VII 17 2,8,7 VII 11 2,8,1 I 18 2,8, ,8,4 IV 20 2,8,8,2 II Elements with atomic numbers 9 and 17 belong to the same group. Hence they have similar chemical properties. 52 D Aluminium belongs to Group III of the periodic table. Element Atomic number Difference in atomic number Electronic arrangement of atom B 5 8 2,3 Al 13 2,8,3 18 Ga 31 2,8,18,3 The difference in atomic number between the first three successive Group III elements is either 8 or 18. This is because the second electron shell can hold 8 electrons while the third electron shell can hold 18 electrons. Hence the atomic number of an element belonging to the same group as aluminium could be , i.e B The atomic number of sulphur is 16. The electronic arrangement of an atom of sulphur is 2,8,6. An atom of sulphur has 3 occupied electron shells. Hence sulphur belongs to Period 3 of the periodic table. 38

12 54 A Option A B C D Element Atomic number Electronic arrangement of atom Period to which the element belongs argon 18 2,8,8 3 aluminium 13 2,8,3 3 beryllium 4 2,2 2 silicon 14 2,8,4 3 chlorine 17 2,8,7 3 nitrogen 7 2,5 2 phosphorus 15 2,8,5 3 oxygen 8 2,6 2 Argon and aluminium belong to the same period. 55 B Option C Elements in the same group have the same number of outermost shell electrons in their atoms. Option D The reactivity of elements changes across a period. Apart from the noble gases, the most reactive elements are near the edges of the periodic table and the least reactive ones are in the centre. H Li Be B C N O F Na Mg Al Si P S Cl K Ca 56 D Option A Element X is fluorine, a non-metal. Option B The electronic arrangement of an atom of X is 2,7. An atom of X has 2 occupied electron shells. Hence X belongs to Period 2 of the periodic table. Option C Fluorine is a gas at room temperature and pressure. Option D X belongs to Group VII of the periodic table. 39

13 57 C Option A The reactivity of Group I elements increases down the group. Hence potassium is more reactive than sodium. Option B Sodium gives a golden yellow flame in flame test. Option C The density of sodium is lower than that of water. Hence it floats on water. Option D Sodium reacts with water to form hydrogen gas. 58 A The melting point of Group I elements decreases down the group, i.e. decreases with increasing atomic number. 59 B Option A Chlorine is a greenish yellow gas. Option B Chlorine belongs to Group VII of the periodic table. Its atom has 7 outermost shell electrons. Option C Sodium chloride is used to manufacture chlorine. Electrolysis of concentrated sodium chloride solution gives chlorine, hydrogen and sodium hydroxide. Option D The reactivity of Group VII elements decreases down the group. 40 Hence fluorine is more reactive than chlorine.

14 60 D Option Element Atomic number Name of element Group to which the element belongs A W 4 beryllium II B X 11 sodium I C Y 12 magnesium II D Z 19 potassium I Group II elements are less reactive than Group I elements. The reactivity of Group I elements increases down the group. Na K Be Mg Hence Z (potassium) is the most reactive metal. 61 A X conducts electricity and reacts readily with dilute hydrochloric acid to give hydrogen. X is probably a metal. 62 A Option A The atomic number of calcium is 20. The electronic arrangement of an atom of calcium is 2,8,8,2. The atom has 4 occupied electron shells. Hence calcium is in Period 4 of the periodic table. Option B Calcium is a Group II element, i.e. an alkaline earth metal. Option C Calcium reacts with non-metals to form salts. Option D Calcium reacts steadily with cold water, but does NOT catch fire. 63 C The melting point of Group VII elements increases down the group, i.e. increases with atomic number. 41

15 64 C Option A Element Chlorine Bromine Iodine Colour greenish yellow reddish brown black Chlorine, bromine and iodine are all coloured substances. Option C The reactivity of Group VII elements decreases down the group, i.e. decreases with increasing relative atomic mass. Option D Chlorine, bromine and iodine can react with sodium sulphite solution. e.g. aqueous bromine + sodium sulphite sodium sulphate + hydrogen bromide 65 B Argon is used to fill electric light bulbs because it does not react with the metal filament in the light bulb. 66 C Option C A helium atom has 2 outermost shell electrons. Option D The boiling point of noble gases increases down the group. 67 A Noble gas Atomic number Difference in atomic number Electronic arrangement of atom 42 He Ne 10 2,8 Ar ,8,8 Kr ,8,18,8

16 The difference in atomic number between the first four successive noble gases is either 8 or 18. This is because the second electron shell can hold 8 electrons while the third electron shell can hold 18 electrons. Therefore if the atomic number of A is x, then the atomic number of B could be x C The reactivity of elements changes across a period. Apart from the noble gases, the most reactive elements are near the edges of the periodic table and the least reactive ones are in the centre. Group I elements are the most reactive metals. H Li Be B C N O F Na Mg Al Si P S Cl K Ca The reactivity of Group I elements increases down the group. Hence element g is the most reactive metal. 69 D Element j is bromine, which exists as a liquid at room temperature and pressure. 70 A Group I elements are metals. They are good conductors of electricity. Their melting points and densities are much lower than the average values for metals. 71 D Option A B C D Description Group I elements have relatively low melting points. Atoms of Group I elements have 1 outermost shell electron. Group I elements are relatively soft. The reactivity of Group I elements increases down the group. Hence rubidium is more reactive than potassium. 72 C Option A B C D Description Strontium and calcium are Group II elements. They are reactive metals. They are extracted from their ores by electrolysis. Strontium is a Group II element. Hence its atom has 2 outermost shell electrons. Strontium is a reactive metal. It will react with oxygen in the air to form an oxide layer on the surface. Hence strontium tarnishes when exposed to the air. The densities of Group II elements are higher than that of water. Hence strontium sinks in water. 43

17 73 B Option A Krypton belongs to Group 0 of the periodic table. The atomic number of krypton is 36. The electronic arrangement of an atom of krypton is 2,8,18,8. Hence an atom of krypton has an octet structure in its outermost shell. Option B The electronic arrangement of an atom of krypton is 2,8,18,8. A krypton atom has 4 occupied electron shells. Hence krypton belongs to Period 4 of the periodic table. Option C All Group 0 elements are colourless gases at room temperature and pressure. Option D The density of Group 0 elements increases down the group. The density of krypton is higher than that of air. Hence a balloon full of krypton falls in the air. 74 B Option B A magnesium atom has an electronic arrangement 2,8,2. It tends to lose two electrons in order to obtain the stable electronic arrangement of a neon atom (2,8). A magnesium ion forms. Options C and D Losing electrons does not affect the number of protons and neutrons the magnesium atom has. Hence its atomic number and mass number remain the same. 75 D Option Species Electronic arrangement Number of electrons A B C D Li H 1 1 O 2 2,8 10 Cl 2,8,8 18 Na + 2,8 10 S 2 2,8,8 18 Ne 2,8 10 F 2,8 10 Ne and F have the same number of electrons. 76 C Species Number of protons neutrons electrons Fe Fe

18 77 D An atom of X loses two electrons to form the X 2+ ion with an electronic arrangement 2,8,8. Hence the electronic arrangement of an atom of X is 2,8,8,2. The atomic number of X is 20. It is calcium. Option Description A, B and C X is a Group II element, an alkaline earth metal. D An atom of X has 4 occupied electron shells. Hence X is a Period 4 element. 78 B An anion carrying 1 negative charge has one more negative charge than positive charge. The anion has 36 electrons and hence it should have 35 protons. 79 D As M 2+ ion has two more positive charges than negative charges, it should have 30 protons. Hence an atom of M should have 30 protons as well. The atomic number of M is 30. M is zinc (Zn). 80 C Particle X is an oxide ion O 2. Particle Y is an isotope of oxygen. 81 B (1) Across the second period of the periodic table, the elements show a gradual decrease in atomic size. Group I II III IV IV V VI VII Element Lithium Beryllium Boron Atomic radius (pm) (1 pm = m) Carbon (graphite) Carbon (diamond) Nitrogen Oxygen Fluorine (2) Across the second period, the elements change from metals through metalloids to non-metals. (3) Across the second period, the melting point of elements rises to Group IV and then falls to low values. Group I II III IV IV V VI VII Element Lithium Beryllium Boron Carbon (graphite) Carbon (diamond) Nitrogen Oxygen Fluorine Melting point ( C) A (1) Elements in the same group show a gradual increase in relative atomic mass. Group IV element Symbol Relative atomic mass Carbon C 12.0 Silicon Si 28.1 Germanium Ge 72.6 Tin Sn

19 (2) The number of occupied electron shells in atoms of Group IV elements increases down the group. Hence the atomic size of the elements increases down the group. Group IV element Symbol Atomic number Electronic arrangement of atom Carbon C 6 2,4 Silicon Si 14 2,8,4 Germanium Ge 32 2,8,18,4 Tin Sn 50 2,8,18,18,4 (3) All Group IV elements have 4 outermost shell electrons in their atoms. 83 A Element X is argon. An atom of X has an electronic arrangement 2,8,8. (1) An atom of X has three occupied electron shells. Hence X is in Period 3 of the periodic table. (2) An atom of X has 8 outermost shell electrons. Hence X is in Group 0 of the period table. (3) 40 18X has 22 neutrons. 84 A (1) A lithium atom has 2 occupied electron shells, a sodium atom has 3 while a potassium atom has 4. Hence the atomic size is in the order lithium < sodium < potassium. (2) The chemical reactivity of Group I elements increases down the group. Hence the chemical reactivity is in the order lithium < sodium < potassium. (3) The melting point of Group I elements decreases down the group. Hence the melting point is in the order lithium > sodium > potassium. 46

20 85 A (2) The densities of Group II elements are higher than that of water. Hence they sink in water. (3) Group II elements can react with water. Therefore they are NOT stored in water. 86 D (1) The reactivity of halogens decreases down the group, i.e. decreases with increasing relative atomic mass. 87 B Element Atomic number Name of element Metal / non-metal X 11 sodium metal Y 16 sulphur non-metal Z 17 chlorine non-metal (1) All the three elements belong to the third period of the periodic table. (2) X is an alkali metal. 88 A (2) Helium is unreactive because it has a duplet structure in the outermost shell of its atom. 89 C (1) A helium atom has 2 outermost shell electrons while other noble gas atoms have 8 electrons in their outermost shells. (2) Noble gas Symbol Relative atomic mass Helium He 4.0 Neon Ne 20.2 Argon Ar 39.9 Krypton Kr 83.8 Xenon Xe Radon Rn 222 The relative atomic mass of noble gases increases down the group. 90 D Element X is magnesium. (1) Magnesium is a reactive metal. It will react with oxygen in the air to form an oxide layer on the surface. Hence magnesium tarnishes when exposed to the air. (2) The density of magnesium is higher than that of water. Hence it sinks in water. 47

21 91 D (1) Caesium is an alkali metal. It reacts with water to give an alkaline solution. (2) Caesium gives a characteristic flame colour in flame test, just as other alkali metals, such as sodium and potassium, do. (3) The melting point of Group I elements decreases down the group. The melting point of sodium is less than 100 C. Hence that of caesium should be less than 100 C as well. 92 C (1) Strontium is a Group II element. There are 2 outermost shell electrons in a strontium atom. Hence a strontium atom tends to lose 2 electrons in order to obtain a stable electronic arrangement. An ion carrying two positive charges is formed. (2) The reactivity of Group II elements increases down the group. Hence strontium is more reactive than calcium. 93 C (1) The reactivity of halogens decreases down the group. Hence the reactivity of halogens is in the order chlorine > bromine > iodine. 48

22 (2) Halogen State at room temperature Density at 25 C Chlorine gas Bromine liquid 3.12 Iodine solid 4.93 Hence the density of the halogens is in the order chlorine < bromine < iodine. (3) Halogen Electronic arrangement of atom Atomic radius (pm) (1 pm = m) Chlorine 2,8,7 99 Bromine 2,8,18,7 114 Iodine 2,8,18,18,7 133 The number of occupied electron shells in atoms of halogens increases down the group. Hence the atomic size of halogens is in the order chlorine < bromine < iodine. 94 D (1) There is a gradual increase in the colour intensity of elements down Group VII. Astatine is probably coloured. (3) There is a gradual change in state of elements down Group VII, from gas to liquid then to solid. Astatine is probably a solid at room temperature and pressure. 95 D X is a Group II element as there are 2 outermost shell electrons in its atom. X is barium, which is below calcium and strontium in Group II of the periodic table. (1) The densities of Group II elements are higher than that of water. Hence X is denser than water. (2) X (barium) gives a characteristic flame colour in flame test as other Group II elements, such as calcium, do. (3) The reactivity of Group II elements increases down the group. As calcium reacts with dilute hydrochloric acid, X (barium) can probably react with dilute hydrochloric acid as well. 96 A A hydrogen atom has 1 proton and 1 electron. It forms a H + ion upon losing 1 electron. [H] + + e Hence a H + ion has 1 proton only. 49

23 97 A (1) An oxygen atom has an electronic arrangement 2,6. It gains 2 electrons to form an oxide ion (with an electronic arrangement 2,8). Hence an oxygen atom and an oxide ion have the same number of occupied electron shells. (2) and (3) Gaining electrons does not affect the number of protons and neutrons the oxygen atom has. Hence its atomic number and mass number remain the same. 98 A Species Electronic arrangement of species O 2 2,8 Li + 2 K + 2,8,8 Hence the species O 2 has the same electronic arrangement as a neon atom. 99 B Atom X can form a stable ion X. It can be deduced that atom X has 7 outermost shell electrons. Hence X is a halogen. e.g. (1) and (2) Gaining an electron does not affect the number of protons and neutrons X has. Hence ion X and atom X have the same number of neutrons and nuclear charge. (3) Atom X gains 1 electron to obtain an octet structure in its outermost shell. Ion X and atom X have the same number of occupied electron shells. 100 A X is a Group VII element, Y is a noble gas and Z is a Group I element. (1) Atom of X gains 1 electron to form a stable anion X. Anion X has the same electronic arrangement as atom Y. Atom of Z loses 1 electron to form a stable cation Z +. Cation Z + has the same electronic arrangement as atom Y. Hence anion X and cation Z + have the same electronic arrangement. (2) X and Z are different elements. Atoms of X and Z have different number of protons. Hence anion X and cation Z + have different number of protons. (3) X, Y and Z belong to different periods of the periodic table. 50

24 101 B 102 A Element Electronic arrangement of atom Lithium 2,1 Neon 2,8 Atoms of lithium and neon have 2 occupied electron shells. Hence they belong to Period 2 of the periodic table. 103 B Element Electronic arrangement of atom Nitrogen 2,5 Oxygen 2,6 Atoms of nitrogen and oxygen have 2 occupied electron shells. Hence nitrogen and oxygen belong to the same period of the periodic table. 104 D Across the second period of the periodic table, the melting point of elements rises to Group IV and then falls to low values. Elements in the second period of the periodic table have 2 occupied electron shells in their atoms. 51

25 105 B Across the third period of the periodic table, the atomic size of the elements decreases gradually. Across the third period of the periodic table, the elements change from metals through metalloids to non-metals, i.e. the metallic character of the elements decreases. Group I II III IV V VI VII Period 3 element Sodium Magnesium Aluminium Silicon Phosphorus Sulphur Chlorine metals metalloid non-metals Type of element metallic character decreasing 106 A The chemical properties of an element depend on the number of outermost shell electrons in its atom. Atoms with the same number of outermost shell electrons react in a similar way. Sodium and potassium belong to the same group. Both of them have 1 outermost shell electron in their atoms. Therefore sodium and potassium have similar chemical properties. 107 D The density of Group II elements show a gradual increase down the group (except magnesium and calcium). Element Density (g cm 3 ) Beryllium 1.85 Magnesium 1.74 Calcium 1.55 Strontium 2.60 Barium 3.51 The reactivity of Group II elements increases down the group. 52

26 108 C The reactivity of halogens decreases down the group. The number of occupied electron shells in atoms of halogens increases down the group. Hence the atomic size also increases down the group. Halogen Electronic arrangement of atom Atomic radius (pm) (1 pm = m) Fluorine 2,7 72 Chlorine 2,8,7 99 Bromine 2,8,18,7 114 Iodine 2,8,18,18, A 110 C An argon atom has 8 electrons in its outermost shell. It is unreactive. Its chemical properties are different from that of a chloride ion. 53

27 Unit 7 Ionic and metallic bonds Fill in the blanks 1 a) conductors b) aqueous solution; electrolytes c) non-conductors 2 ionic 3 negative; positive 4 polyatomic 5 four; two 6 three; two 7 purple; permanganate 8 chromium(iii) 9 chemical formula 10 metallic True or false 11 F An ionic bond is the strong forces of attraction between oppositely charged ions. 12 F Ionic bond usually occurs when metal atoms combine with non-metal atoms. 13 T A calcium atom has an electronic arrangement 2,8,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable argon atom. A fluorine atom has an electronic arrangement 2,7. It tends to gain 1 electron to obtain the electronic arrangement of a stable neon atom. When calcium and fluorine react, the 2 electrons released by the calcium atom are accepted by two fluorine atoms. 54

28 14 F Element X (atomic number = 11) is sodium. A sodium atom has an electronic arrangement 2,8,1. It tends to lose 1 electron to obtain the electronic arrangement of a stable neon atom. Element Y (atomic number = 16) is sulphur. A sulphur atom has an electronic arrangement 2,8,6. It tends to gain 2 electrons to obtain the electronic arrangement of a stable argon atom. When sodium and sulphur combine, two sodium atoms are required to release the 2 electrons needed by the sulphur atom. Hence the chemical formula of the compound formed is Na 2 S, i.e. X 2 Y. 15 T A magnesium atom has an electronic arrangement 2,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable neon atom. An oxygen atom has an electronic arrangement 2,6. It tends to gain 2 electrons to obtain the electronic arrangement of a stable neon atom. When magnesium and oxygen react, the 2 electrons released by the magnesium atom are accepted by one oxygen atom. Hence the chemical formula of the compound formed is MgO. 16 T Element X (atomic number = 20) is calcium. A calcium atom has an electronic arrangement 2,8,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable argon atom. Element Y (atomic number = 7) is nitrogen. A nitrogen atom has an electronic arrangement 2,5. It tends to gain 3 electrons to obtain the electronic arrangement of a stable neon atom. When calcium and nitrogen react, three calcium atoms are required to release the 6 electrons needed by the two nitrogen atoms. 55

29 Electron diagram of the compound formed: Ca (Only electrons in the outermost shells are shown.) The chemical formula of the compound formed is Ca 3 N 2, i.e. X 3 Y F Potassium is a Group I element. Its atom tends to lose 1 electron to obtain the electronic arrangement of a stable noble gas atom. Astatine is a Group VII element. Its atom tends to gain 1 electron to obtain the electronic arrangement of a stable noble gas atom. When potassium and astatine combine, the electron released by the potassium atom is accepted by the astatine atom. (Only electrons in the outermost shells are shown.) The chemical formula of the compound formed is KAt. 18 F Iron(II) ion in aqueous solution is pale green in colour. 19 T The orange colour of a potassium dichromate solution comes from the dichromate ions. 20 F Metallic bond is a type of bond in which positive metal ions are held together by a sea of mobile electrons. 56

30 Multiple choice questions 21 A Alcohol is made up of carbon, hydrogen and oxygen. Compounds made up of non-metals are nonconductors. 22 C Electrode X is the positive electrode while electrode Y is the negative electrode. Bromide ions carrying negative charges move towards the positive electrode. bromide ions electrons bromine atoms bromine molecules Lead(II) ions carrying positive charges move towards the negative electrode. lead(ii) ions + electrons lead atoms Hence a reddish brown gas (bromine) is formed at the positive electrode, i.e. electrode X. A white shiny solid (lead) is formed at the negative electrode, i.e. electrode Y. 23 A Option A Molten lead(ii) bromide is decomposed into lead and bromine by electricity. Options B, C and D In solid state, ions in the compound are held together by strong attraction. They are not free to move. Hence solid lead(ii) bromide does not conduct electricity. When lead(ii) bromide becomes molten, the lead(ii) ions and bromide ions become mobile. Hence molten lead(ii) bromide can conduct electricity. 24 C Potassium (a metal) and oxygen (a non-metal) combine to form an ionic compound. 25 C Element Atomic number Name of element a 6 carbon b 9 fluorine c 10 neon d 11 sodium Element b (fluorine, a non-metal) and element d (sodium, a metal) combine to form an ionic compound. 26 B Element d is a non-metal and element b is a metal. They combine to form an ionic compound. 27 A Metallic bonds are found in metals only, e.g. copper. 57

31 28 C Electron transfer during the reaction between potassium and oxygen: 29 B Element X forms a stable X 2+ ion. Its atom probably has 2 outermost shell electrons. Thus X is probably a Group II element. Element Y forms a stable Y 2 ion. Its atom probably has 6 outermost shell electrons. Thus Y is probably a Group VI element. Option B Calcium is a Group II element while sulphur is a Group VI element. Electron transfer during the reaction between calcium and sulphur: (Only electrons in the outermost shells are shown.) 30 D Element X is magnesium. It belongs to Group II of the periodic table. An atom of X loses 2 electrons to obtain a stable electronic arrangement. Elements X and Y form an ionic compound with the chemical formula XY 2. That means the 2 electrons released by an atom of X are accepted by two atoms of Y. Hence it can be deduced that an atom of Y needs 1 more electron to obtain a stable electronic arrangement. For atoms of non-metals in Group V, VI and VII, they gain 8 group number electrons in order to obtain stable electronic arrangements. An atom of Y gains 1 electron in order to obtain a stable electronic arrangement. Hence Y probably belongs to Group VII of the periodic table. 58

32 Electron transfer during the reaction between X and Y: (Only electrons in the outermost shells are shown.) 31 C Element X is sodium. It belongs to Group I of the periodic table. An atom of X loses 1 electron to obtain a stable electronic arrangement. Elements X and Y form an ionic compound with the chemical formula X 2 Y. That means the 2 electrons released by two atoms of X are accepted by one atom of Y. Hence it can be deduced that an atom of Y needs 2 more electrons to obtain a stable electronic arrangement. For atoms of non-metals in Group V, VI and VII, they gain 8 group number electrons in order to obtain stable electronic arrangements. Hence Y probably belongs to Group VI of the periodic table. Electron transfer during the reaction between X and Y: (Only electrons in the outermost shells are shown.) 32 C An atom of X loses 2 electrons to form a stable X 2+ ion. Hence the atom probably has 2 electrons in its outermost shell. An atom of Y gains 3 electrons to form a stable Y 3 its outermost shell. ion. Hence the atom probably has 5 electrons in 59

33 33 B Option Compound Chemical formula Electron diagram A aluminium oxide Al 2 O 3 The chemical formula is not in the form of XY. B magnesium oxide MgO C lithium fluoride LiF D sodium chloride NaCl the positive ions and the negative ions in MgO have the same electronic arrangement. 34 D A magnesium atom has an electronic arrangement 2,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable neon atom. Element X (atomic number = 7) is nitrogen. A nitrogen atom has an electronic arrangement 2,5. It tends to gain 3 electrons to obtain the electronic arrangement of a stable neon atom. When magnesium and nitrogen react, three magnesium atoms are required to release the 6 electrons needed by the two nitrogen atoms. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) The chemical formula of the compound formed is Mg 3 N 2, i.e. Mg 3 X 2. 60

34 35 C Element X is aluminium. An atom of X has an electronic arrangement 2,8,3. It tends to lose 3 electrons to obtain the electronic arrangement of a stable neon atom. Element Y is oxygen. An atom of Y has an electronic arrangement 2,6. It tends to gain 2 electrons to obtain the electronic arrangement of a stable neon atom. When X and Y react, two atoms of X are required to release the 6 electrons needed by the three atoms of Y. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) The chemical formula of the compound formed is Al 2 O 3, i.e. X 2 Y B An atom of X loses 1 electron to form a stable X + ion. It can be deduced that the atom of X has 1 electron in its outermost shell. Hence element X is probably a Group I element. An atom of Y gains 3 electrons to form a stable Y 3 ion. It can be deduced that the atom of Y has 5 electrons in its outermost shell. Hence element Y is probably a Group V element. X could be Li (lithium) and Y could be N (nitrogen), i.e. Option B is the correct answer. 37 D The reactivity of Group I elements increases down the group. Hence potassium is more reactive than lithium. 61

35 The reactivity of Group VII elements decreases down the group. Hence fluorine is more reactive than chlorine. Therefore fluorine and potassium (Option D) would react with each other most vigorously. 38 B Element Atomic number Name of element w 9 fluorine x 14 silicon y 18 argon z 20 calcium Element w (fluorine) is a reactive non-metal while element z (calcium) is a reactive metal. They react with each other readily to form an ionic compound. 39 A The chemical formula of ammonium ion is NH C Nickel(II) ion in aqueous solution is green in colour. 41 A 42 C 43 D Ion Cr 3+ Cu 2+ Fe 3+ Zn 2+ Colour in aqueous solution green blue or green yellow-brown colourless 44 B The aqueous solution of compound XZ is colourless. Hence X 2+ (aq) and Z 2 (aq) ions are colourless. The blue colour of aqueous solution of compound WZ is due to the W 2+ (aq) ions. Hence W 2+ (aq) ion is blue in colour. The orange colour of aqueous solution of compound XY is due to the Y 2 (aq) ions. Hence Y 2 (aq) ion is orange in colour. 62

36 45 D A coloured patch develops near the positive electrode because negative permanganate ions which are purple in colour move towards the positive electrode. Positive potassium ions move towards the negative electrode. However, we cannot see the potassium ions because they are colourless. 46 A The chemical formula of potassium dichromate is K 2 Cr 2 O B The chemical formula of ammonium sulphate is (NH 4 ) 2 SO 4. The compound consists of 4 elements. 48 C A rubidium ion (Rb + ) carries 1 positive charge. A carbonate ion (CO 2 3 ) carries 2 negative charges. The simplest ratio of Rb + and CO 2 3 in rubidium carbonate is 2 : 1. Hence the chemical formula of the compound is Rb 2 CO D A calcium ion (Ca 2+ ) carries 2 positive charges while a phosphate ion (PO 4 3 ) carries 3 negative charges. The simplest ratio of Ca 2+ to PO 4 3 in calcium phosphate is 3 : 2. Hence the chemical formula of the compound is Ca 3 (PO 4 ) B M forms a sulphate with the chemical formula M 2 (SO 4 ) 3. The sulphate ion (SO 4 2 ) carries 2 negative charges and the net charge of the compound must be zero. The simplest ratio of ion of M to sulphate ion in the sulphate is 2 : 3. It can be deduced that the ion of M carries 3 positive charges. Use the following steps to work out the chemical formula of the chloride of M: Step Chloride of M 1 Write down the symbols of ions in the compound. M Cl 2 Write down the number of charges of each ion on the top of each symbol. 3 Cross multiply the numbers and the symbols. 3 1 M Cl 3 1 M Cl = M 1 = Cl 3 4 Combine the symbols. MCl 3 63

37 51 D Thorium forms a hydroxide with the chemical formula Th(OH) 4. The hydroxide ion (OH ) carries 1 negative charge and the net charge of the compound must be zero. It can be deduced that the thorium ion carries 4 positive charges. The permanganate ion (MnO 4 ) also carries 1 negative charge. Hence the chemical formula of thorium permanganate is Th(MnO 4 ) D Option A Strontium is a Group II element, an alkaline earth metal. Option B The reactivity of Group II elements increases down the group. Hence strontium is more reactive than calcium. Option C A strontium ion (Sr 2+ ) carries 2 positive charges while a chloride ion (Cl ) carries 1 negative charge. The simplest ratio of Sr 2+ to Cl in strontium chloride should be 1 : 2. Hence the chemical formula of the compound is SrCl 2. Option D The densities of Group II elements are higher than 1 g cm 3 while that of sodium is lower. Hence the density of strontium is higher than that of sodium. 53 A Option A There is a gradual change in state of elements down Group VII. Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids at room temperature and pressure. Option B The reactivity of Group VII elements decreases down the group. 64 Hence chlorine is more reactive than astatine.

38 Option C Astatine is in Period 6 of the periodic table. Option D A calcium ion (Ca 2+ ) carries 2 positive charges while the ion of astatine (At ) carries 1 negative charge. The simplest ratio of Ca 2+ to At in the compound formed between calcium and astatine should be 1 : 2. Hence the chemical formula of the compound is CaAt C Cation X + has an electronic arrangement 2,8. Hence an atom of X should have an electronic arrangement 2,8,1. Element X is sodium. Option A X (sodium) is a solid at room temperature and pressure. Option B An atom of X has 3 occupied electron shells. Hence X is in Period 3 of the periodic table. Option C X (sodium) reacts vigorously with water to give sodium hydroxide and hydrogen. Option D The cation of X (X + ) carries 1 positive charge while the oxide ion (O 2 ) carries 2 negative charges. The simplest ratio of X + to O 2 in the oxide of X should be 2 : 1. Hence the chemical formula of the oxide is X 2 O. 55 A (1) Calcium (a metal) and fluorine (a non-metal) combine to form an ionic compound. 56 B Cation X 2+ has an electronic arrangement 2,8,8. Hence an atom of X has an electronic arrangement 2,8,8,2. Element X is calcium. (1) A calcium ion (Ca 2+ ) carries 2 positive charges while a hydride ion (H ) carries 1 negative charge. The simplest ratio of Ca 2+ to H in the compound formed between calcium and hydrogen should be 1 : 2. Hence the chemical formula of the compound is CaH 2, i.e. XH 2. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) (2) An atom of X has 4 occupied electron shells. Hence X is in Period 4 of the periodic table. 65

39 57 A (1) The reactivity of Group I elements increases down the group. As potassium reacts vigorously with water, caesium should also react with water vigorously. (2) The densities of all Group I elements are quite low. (3) A caesium ion (Cs + ) carries 1 positive charge while a hydroxide ion (OH ) carries 1 negative charge. The simplest ratio of Cs + to OH in caesium hydroxide should be 1 : 1. Hence the chemical formula of the compound is CsOH. 58 D (1) Oxides of Group I elements are crystalline solids. (2) Metallic bonding exists in metals. (3) A rubidium ion (Rb + ) carries 1 positive charge while a sulphate ion (SO 4 2 ) carries 2 negative charges. The simplest ratio of Rb + to SO 4 2 in rubidium sulphate should be 2 : 1. Hence the chemical formula of the compound is Rb 2 SO B (1) A barium atom has 2 outermost shell electrons. It forms a stable ion by losing 2 electrons. Hence the ion carries 2 positive charges. (2) The reactivity of Group II elements increases down the group. Hence barium is more reactive than calcium. 66

40 (3) A barium ion (Ba 2+ ) carries 2 positive charges while a sulphide ion (S 2 ) carries 2 negative charges. The simplest ratio of Ba 2+ to S 2 in barium sulphide should be 1 : 1. Hence the chemical formula of the sulphide is BaS. 60 B (1) A strontium ion (Sr 2+ ) carries 2 positive charges while a carbonate ion (CO 3 2 ) carries 2 negative charges. The simplest ratio of Sr 2+ to CO 3 2 in strontium carbonate should be 1 : 1. Hence the chemical formula of the compound is SrCO 3. (2) In solid state, ions in strontium carbonate are held together by strong attraction. They are not free to move. Hence solid strontium carbonate does not conduct electricity. (3) Carbonates of Group II elements are insoluble in water, e.g. calcium carbonate. Hence strontium carbonate is insoluble in water. 61 B Element X (atomic number = 7) is nitrogen. (1) X (nitrogen) is a gas at room temperature and pressure. (2) An atom of X has an electronic arrangement 2,5. It has 2 occupied electron shells. Hence X is in Period 2 of the periodic table. (3) A magnesium atom has an electronic arrangement 2,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable neon atom. Element X (atomic number = 7) is nitrogen. A nitrogen atom has an electronic arrangement 2,5. It tends to gain 3 electrons to obtain the electronic arrangement of a stable neon atom. When magnesium and nitrogen react, three magnesium atoms are required to release the 6 electrons needed by the two nitrogen atoms. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) The chemical formula of the compound is Mg 3 N 2, i.e. Mg 3 X 2. 67

41 62 D Compound Chemical formula Electron diagram Lithium oxide Li 2 O Potassium sulphide K 2 S Sodium oxide Na 2 O the positive ions and negative ions in potassium sulphide and sodium oxide have the same electronic arrangement. 63 A Ion Chemical formula Colour in aqueous solution Chromium(III) Cr 3+ green Nickel(II) Ni 2+ green Permanganate MnO 4 purple 64 C (1) Electrode X is the positive electrode. An orange colour develops near electrode X. This is because negative dichromate ions move towards the positive electrode. (2) Electrode Y is the negative electrode. Hydrogen ions (H + ) around electrode Y will gain electrons to form hydrogen gas. 65 A (3) Metallic bonding exists in metals only. 66 A An oxygen atom has an electronic arrangement 2,6. It tends to gain 2 electrons in order to obtain the stable electronic arrangement of a neon atom (2,8). 67 C A helium atom has 2 outermost shell electrons. 68

42 68 C A magnesium atom has an electronic arrangement 2,8,2. It has 3 occupied electron shells. Hence magnesium is in Period 3 of the periodic table. 69 A Element X (atomic number = 20) is calcium. A calcium atom has an electronic arrangement 2,8,8,2. It tends to lose 2 electrons to obtain the electronic arrangement of a stable argon atom. Element Y (atomic number = 7) is nitrogen. A nitrogen atom has an electronic arrangement 2,5. It tends to gain 3 electrons to obtain the electronic arrangement of a stable neon atom. When calcium and nitrogen react, three calcium atoms are required to release the 6 electrons needed by the two nitrogen atoms Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) The chemical formula of the compound is Ca 3 N 2, i.e. X 3 Y D Some transition metals (e.g. Fe) can form M 3+ ions. Unit 8 Covalent bonds Fill in the blanks 1 covalent 2 bond pair 3 lone pair 4 diatomic 5 monoatomic 6 one 7 three; one 8 three 9 dative covalent 10 nitrogen; lone pair 69

43 True or false 11 T 12 F Nitrogen exists as diatomic molecules. (Only electrons in the outermost shells are shown.) 13 T Carbon and silicon are non-metals. They combine to form a covalent compound (silicon carbide, SiC). Structure of silicon carbide: 14 F Neon is a noble gas. A neon atom has 8 outermost shell electrons. A special stability is obtained when this happens. Hence neon will NOT form a compound with nitrogen. 15 F Electron diagram of water: Total number of electrons in a water molecule = number of electrons in one oxygen atom + 2 x number of electron in one hydrogen atom = x 1 = 10 70

44 16 T In a SiCl 4 molecule, one Si atom forms a single bond with each of four Cl atoms. Electron diagram of SiCl 4 : (Only electrons in the outermost shells are shown.) 17 T Covalent bonding exists in both iodine and oxygen. (Only electrons in the outermost shells are shown.) 18 F Covalent bonding exists in hydrogen chloride while ionic bonding exists in silver chloride. 19 F A phosphorus atom has an electronic arrangement 2,8,5. It needs 3 more electrons to obtain the electronic arrangement of a stable argon atom (2,8,8). A hydrogen atom has an electronic arrangement 1. It needs 1 more electron to obtain the electronic arrangement of a stable helium atom (2). In order to obtain stable electronic arrangements, one phosphorus atom forms a single bond with each of three hydrogen atoms. The chemical formula of the compound is PH 3. Electron diagram of PH 3 : (Only electrons in the outermost shells are shown.) 71

45 20 T A dative covalent bond is formed when an ammonia molecule and a hydrogen ion combine to form an ammonium ion (NH 4 + ). The nitrogen atom in the ammonia molecule supplies its lone pair electrons to the hydrogen ion. Hence covalent bonds exist in ammonium chloride. Multiple choice questions 21 D Element X (atomic number = 7) is nitrogen. A nitrogen atom has an electronic arrangement 2,5. It needs 3 more electrons to obtain the electronic arrangement of a stable neon atom (2,8). Each nitrogen atom can obtain the electronic arrangement of a neon atom by sharing three of its electrons with another nitrogen atom. 22 A An atom of an element with atomic number 9 has an electronic arrangement 2,7. It needs 1 more electron to obtain the electronic arrangement of a stable neon atom (2,8). Each atom can obtain the electronic arrangement of a neon atom by sharing one of its outermost shell electrons with another atom. (Only electrons in the outermost shells are shown.) 23 D Sulphur and oxygen are non-metals. They combine to form a covalent compound. 24 D Option A Lithium (a metal) and nitrogen (a non-metal) combine to form an ionic compound. Option B Mercury (a metal) and fluorine (a non-metal) combine to form an ionic compound. Option C Neon is a noble gas. A neon atom has 8 outermost shell electrons. A special stability is obtained when this happens. Hence neon will NOT form a compound with nitrogen. Option D Fluorine and chlorine are non-metals. They combine to form a covalent compound. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) 72

46 25 C Element Atomic number Name of element a 3 lithium b 14 silicon c 17 chlorine d 18 argon Option A Element a (lithium) and element c (chlorine) combine to form an ionic compound. Option B Argon (element d) is a noble gas. An argon atom has 8 outermost shell electrons. A special stability is obtained when this happens. Hence argon will NOT form a compound with lithium (element a). Option C Element b (silicon) and element c (chlorine) combine to form a covalent compound. Option D Element b (silicon) will NOT form a compound with element d (argon). 26 D The element exists as diatomic molecules. It is a non-metal. Nitrogen in Group V exists as diatomic molecules. 27 C A nitrogen atom has an electronic arrangement 2,5. It needs 3 more electrons to obtain the electronic arrangement of a stable neon atom (2,8). A fluorine atom has an electronic arrangement 2,7. It needs 1 more electron to obtain the electronic arrangement of a stable neon atom (2,8). In order to get stable electronic arrangements, one nitrogen atom combines with three fluorine atoms to form a molecule. 28 D Option A Helium exists as monoatomic molecules. Option B In a fluorine molecule, each fluorine atom shares one of its outermost shell electrons with another fluorine atom. (Only electrons in the outermost shells are shown.) Option C In a hydrogen chloride molecule, one hydrogen atom forms a single bond with one chlorine atom. (Only electrons in the outermost shells are shown.) 73

47 Option D A silicon atom has an electronic arrangement 2,8,4. It needs 4 more electrons to obtain the electronic arrangement of a stable argon atom (2,8,8). A hydrogen atom has an electronic arrangement of 1. It needs 1 more electron to obtain the electronic arrangement of a stable helium atom (2). In order to get stable electronic arrangements, one silicon atom combines with four hydrogen atoms. 29 C A neon atom has 8 electrons in its outermost shell. A special stability is obtained when this happens. The atom has very little tendency to share electrons with other neon atoms. Hence neon exists as atoms. 30 B Element X (atomic number = 16) is sulphur. A sulphur atom has an electronic arrangement 2,8,6. It needs 2 more electrons to obtain the electronic arrangement of a stable argon atom (2,8,8). A chlorine atom has an electronic arrangement 2,8,7. It needs 1 more electron to obtain the electronic arrangement of a stable argon atom (2,8,8). In order to obtain stable electronic arrangements, one sulphur atom forms a single bond with each of two chlorine atoms. The chemical formula of the compound is SCl 2, i.e. XCl 2. Electron diagram of SCl 2 : (Only electrons in the outermost shells are shown.) 31 C In the compound, each atom of X contributes 1 electron for sharing with another atom of X and 1 electron for sharing with an atom of Y. Each atom of X contributes 2 electrons for sharing as it needs 2 electrons to obtain a stable electronic arrangement. It can be deduced that an atom of X has 6 outermost shell electrons. 32 D In the compound, the atom of X contributes 2 electrons for sharing with two atoms of Y. The atom of X contributes 2 electrons for sharing as it needs 2 electrons to obtain a stable electronic arrangement. It can be deduced that an atom of X has 6 outermost shell electrons. Element X belongs to Group VI of the periodic table (i.e. oxygen in this question). 74

48 Each atom of Y contributes 1 electron for sharing as it needs 1 electron to obtain a stable electronic arrangement. It can be deduced that an atom of Y has 7 outermost shell electrons. Element Y belongs to Group VII of the periodic table (i.e. fluorine in this question). 33 C In the compound, each atom of X contributes 1 electron for sharing with another atom of X and 2 electrons for sharing with two hydrogen atoms. Each atom of X contributes 3 electrons for sharing as it needs 3 electrons to obtain a stable electronic arrangement. It can be deduced that an atom of X has 5 outermost shell electrons. 34 D Molecule Electron diagram (only electrons in the outermost shells are shown) Number of pairs of bond pair electrons C 2 H 4 6 CO 2 4 H 2 S 2 PCl

49 35 D Molecule Electron diagram (only electrons in the outermost shells are shown) Number of pairs of lone pair electrons on the underlined atom CH 4 0 HCN 0 NH 3 1 SCl A An atom of X has 5 outermost shell electrons. Hence X probably belongs to Group V of the periodic table. An atom of Y has 7 outermost shell electrons. Hence Y probably belongs to Group VII of the periodic table. An atom of X needs 3 more electrons to obtain a stable electronic arrangement. An atom of Y needs 1 more electron to obtain a stable electronic arrangement. In order to obtain stable electronic arrangements, one atom of X forms a single covalent bond with each of three atoms of Y. A covalent compound forms. The chemical formula of the compound is XY 3. Electron diagram of XY 3 : (Only electrons in the outermost shells are shown.) 76

50 37 C An atom of X gains 3 electrons to form an anion X 3 with an electronic arrangement 2,8. Hence the electronic arrangement of an atom of X is 2,5. Element X is nitrogen. Option A X (nitrogen) belongs to Group V of the periodic table. Option B X (nitrogen) is a gas at room temperature and pressure. Option C X (nitrogen) exists as diatomic molecules. (Only electrons in the outermost shells are shown.) Option D X (nitrogen) and fluorine are non-metals. They combine to form a covalent compound. 38 D Compound formed between Electron diagram (only electrons in the outermost shells are shown) magnesium and fluorine lithium and oxygen chlorine and fluorine chlorine and oxygen 77

51 39 B Option Substance Bonding type A B C D aluminium mercury sodium calcium chloride hydrogen chloride silver chloride carbon dioxide nitrogen oxygen iodine methane sulphur dioxide metallic bonding ionic bonding covalent bonding ionic bonding covalent bonding covalent bonding 40 C In Al 2 X 3, the aluminium ion carries 3 positive charges. The simplest ratio of Al 3+ to the ion of X in the compound is 2 : 3 and the net charge of the compound is zero. Hence the ion of X carries 2 negative charges. In Period 3 of the periodic table, sulphur forms an anion carrying 2 negative charges. Hence X is sulphur. To obtain stable electronic arrangements, one sulphur atom forms a single bond with each of two hydrogen atoms. The chemical formula of the compound formed is H 2 S, i.e. H 2 X. Electron diagram of H 2 S: (Only electrons in the outermost shells are shown.) 41 D In the compound between element X and chlorine, the atom of X contributes 3 electrons for sharing with 3 atoms of chlorine. The atom of X contributes 3 electrons for sharing as it needs 3 electrons to obtain a stable electronic arrangement. Therefore it has 5 outermost shell electrons. An atom of X forms a stable anion X 3 by gaining 3 electrons. Use the following steps to work out the chemical formula of the compound formed between X and calcium: Step Compound formed between X and calcium 1 Write down the symbols of ions in the compound. Ca X 2 Write down the number of charges of each ion on the top of each symbol. 2 3 Ca X 3 Cross multiply the numbers and the symbols. 2 3 Ca X = Ca 3 = X 2 4 Combine the symbols. Ca 3 X 2 78

52 42 A Option B Atoms of elements in the same period have the same number of occupied electron shells. Option C The melting point of Period 2 elements rises to Group IV and than falls to low values. Option D Lithium and oxygen combine to form an ionic compound. Fluorine and oxygen combine to form a covalent compound. Across the second period, from lithium to fluorine, the elements change from metals to non-metals. The oxides of the elements change from ionic to covalent. Group I II III IV V VI VII Element lithium beryllium boron carbon nitrogen oxygen fluorine forms an ionic oxide forms a covalent oxide 43 D In a piece of metal, metal ions are packed tightly together in a regular pattern to form a giant structure. 79

53 44 B Element X (atomic number = 9) is fluorine. Element Y (atomic number = 13) is aluminium. The chemical formula of the compound formed between X and Y is YX 3 (or AlF 3 ). Formula mass of the compound = relative atomic mass of Y + 3 x relative atomic mass of X = x 19.0 = D Element X (atomic number = 6) is carbon. Element Y (atomic number = 16) is sulphur. The chemical formula of the compound formed between X and Y is XY 2 (or CS 2 ). Relative molecular mass of the compound = relative atomic mass of X + 2 x relative atomic mass of Y = x 32.1 = C Phosphorus and oxygen are non-metals. They combine to form a covalent compound. 47 A Elements a, b, e and f are non-metals while elements c and d are metals. (1) Element a (a non-metal) and element f (a non-metal) combine to form a covalent compound. (2) Element b (a non-metal) and element d (a metal) combine to form an ionic compound. (3) Element c (a metal) and element e (a non-metal) combine to form an ionic compound. 48 B Option Molecule Electron diagram (only electrons in the outermost shells are shown) With single bonds only? (1) CH 4 yes (2) CO 2 no (3) H 2 O yes 80

54 49 D Option Molecule Electron diagram (only electrons in the outermost shells are shown) With multiple bond(s)? (1) CO 2 yes (2) N 2 yes (3) HCN yes 50 D Chlorine and bromine are in Group VII of the periodic table. (1) Each chlorine / bromine atom has 7 outermost shell electrons. It gains 1 electron to obtain a stable electronic arrangement. An ion carrying 1 negative charge is formed. (Only electrons in the outermost shells are shown.) (2) Each chlorine / bromine atom can obtain a stable electronic arrangement by sharing one of its outermost shell electrons with another chlorine / bromine atom. Hence chlorine / bromine exists as diatomic molecules. Electron diagram of a chlorine / bromine molecule: (Only electrons in the outermost shells are shown.) (3) Both chlorine and bromine can react with sodium sulphite solution. e.g. aqueous bromine + sodium sulphite sodium sulphate + hydrogen bromide 81

55 51 B In a methane (CH 4 ) molecule, the carbon atom forms a single bond with each of the four hydrogen atoms. Electron diagram of methane: (1) The number of bonding electrons contributed by each hydrogen atom in the molecule is 1. (2) The number of bonding electrons contributed by the carbon atom in the molecule is 4. (3) Total number of electrons in the molecule = number of electrons in one carbon atom + 4 x number of electron in one hydrogen atom = x 1 = B (2) Electron diagram of magnesium bromide: (Only electrons in the outermost shells are shown.) 53 A (1) In the compound, each atom of X contributes 1 electron for sharing with another atom of X and 1 electron for sharing with an atom of Y. Each atom of X contributes 2 electrons for sharing as it needs 2 electrons to obtain a stable electronic arrangement. It can be deduced that there are 6 electrons in the outermost shell of an atom of X. 82

56 (2) An atom of Y contributes 1 electron for sharing as it needs 1 electron to obtain a stable electronic arrangement. It can be deduced that there are 7 electrons in the outermost shell of an atom of Y. (3) Each atom of X in the compound has 2 lone pairs of electrons. 54 B The electron diagram of ammonium chloride is shown in the question. (Only electrons in the outermost shells are shown.) (2) There are 5 electrons in the outermost shell of an atom of Y (nitrogen). 55 A The electron diagram of a metal carbonate is shown in the question. (Only electrons in the outermost shells are shown.) (1) An atom of X loses 2 electrons to form a stable X 2+ ion. Hence the atom probably has 2 electrons in its outermost shell. (2) There are 4 electrons in the outermost shell of an atom of Y (carbon). (3) There are 6 electrons in the outermost shell of an atom of Z (oxygen). 83

57 56 A (1) Carbon combines with oxygen to form a compound with the chemical formula CO 2. (2) Lead combines with chlorine to form a compound with the chemical formula PbCl 2. (3) Lithium combines with oxygen to form a compound with the chemical formula Li 2 O. 57 A In ammonium nitrate (NH 4 NO 3 ), the cations and anions are held together by ionic bonding, but each polyatomic ion is a group of atoms held together by covalent bonding. Electron diagram of an ammonium ion: Electron diagram of a nitrate ion: (Only electrons in the outermost shells are shown.) 58 D Element Atomic number Name of element Metal / non-metal X 9 fluorine non-metal Y 12 magnesium metal Z 16 sulphur non-metal (1) X (a non-metal) and Y (a metal) react to give an ionic compound. (2) X and Z are non-metals. They form a compound by electron sharing. (3) Y (magnesium) and Z (sulphur) react to form a compound with the chemical formula YZ. Electron diagram of the compound formed between Y and Z: (Only electrons in the outermost shells are shown.) 84

58 59 D (1) Group II elements are less reactive than Group I elements. Na K Be Mg Hence sodium is more reactive than magnesium. (2) Phosphorus and chlorine are non-metals. They combine to form a covalent compound. (3) Electron diagram of the compound formed between silicon and chlorine: (Only electrons in the outermost shells are shown.) 60 C (1) A Group IV element seldom forms an ion. (3) X and Y are non-metals. They combine to form a covalent compound. 61 B Bromine and chlorine belong to the same group as their atoms have the same number of outermost shell electrons. 62 C Neon is a noble gas. A neon atom has 8 outermost shell electrons. A special stability is obtained when this happens. Hence neon will NOT form a compound with nitrogen. 63 B Phosphorus and chlorine form a covalent compound as they are both non-metals. 85

59 64 D Hydrogen and chlorine are non-metals. They combine to form a covalent compound. Electron diagram of the compound formed between hydrogen and chlorine: (Only electrons in the outermost shells are shown.) 65 C Calcium carbonate is an ionic compound. Each carbonate ion (CO 2 3 ) is a group of four atoms held together by covalent bonding. Unit 9 Relating the properties of substances to structures and bonding Fill in the blanks 1 giant ionic 2 giant covalent 3 simple molecular 4 giant metallic 5 water; non-aqueous 6 mobile 7 Allotropes 8 oxygen; covalent; oxygen 9 three; covalent; Van der Waals 10 water; non-aqueous 11 simple molecular; covalent bond; van der Waals forces 12 mobile electrons 86

60 True or false 13 F Allotropes are two (or more) forms of the same element in which the atoms or molecules are arranged in different ways. Quartz is a form of silicon dioxide while graphite is a form of carbon. Hence quartz and graphite are NOT allotropes. 14 T Both silicon and diamond have a giant covalent structure. Structure of silicon: 15 F Silicon carbide has a giant covalent structure. Structure of silicon carbide: 16 T Electron diagram of silane (SiH 4 ): (Only electrons in the outermost shells are shown.) 87

61 17 F Carbon disulphide (CS 2 ) has a simple molecular structure. Electron diagram of carbon disulphide: (Only electrons in the outermost shells are shown.) 18 T The melting point of sugar is low and it is a non-conductor of electricity. It can be deduced that sugar has a simple molecular structure. 19 T 20 F Ionic compounds do not conduct electricity in solid state. In solid state, ions in the compound are held together by strong ionic bonds. They are not free to move. 21 T Sodium chloride is hard due to the strong ionic bonds between oppositely charged ions. Relative motion of the ions is restricted. 22 F The melting point of graphite (3 730 C) is higher than that of diamond (3 500 C). 23 F In quartz, each silicon atom is joined to four oxygen atoms by covalent bonds, while each oxygen atom is joined to two silicon atoms by covalent bonds. Structure of quartz: 24 T In graphite, the carbon atoms are arranged in flat parallel layers. There are weak van der Waals forces between the adjacent layers in graphite. The layers can easily slide over each other. Hence graphite has a slippery feel. Structure of graphite: 88

62 25 T Diamond has a giant covalent structure consisting of a network of covalent bonds. Relative motion of the atoms is restricted. This makes diamond very hard. Diamond is harder than graphite. In graphite, the layers of carbon atoms are held by weak van der Waals forces. The layers can slide over each other easily. 26 T Graphite is a good conductor of electricity. Zinc-carbon dry cells use graphite as electrodes. 27 F Dry ice consists of separate carbon dioxide (CO 2 ) molecules. In each molecule, strong covalent bonds hold the carbon and oxygen atoms together. The carbon dioxide molecules are packed close to one another in a regular pattern. Weak van der Waals forces hold the molecules together. Structure of dry ice: 28 F In iodine, strong covalent bonds hold the atoms in each molecule together. Much weaker van der Waals forces hold the separate molecules together. Structure of iodine: 29 F Iodine is slightly soluble in water. The weak attractive forces between iodine and water molecules are not strong enough to overcome the attractive forces between the water molecules. 30 F Carbon dioxide has a simple molecular structure while silicon dioxide has a giant covalent structure. Hence carbon dioxide and silicon dioxide have different physical properties. 31 T The melting point of hydrogen chloride is lower than that of potassium chloride. Hydrogen chloride has a simple molecular structure. Little heat is needed to separate the molecules. Potassium chloride has a giant ionic structure. A lot of heat is needed to overcome the strong ionic bonds between the ions during melting. 89

63 32 T Silver conducts electricity due to the movement of mobile electrons. When silver is connected to a battery, mobile electrons in the metal flow towards the positive terminal of the battery. At the same time, electrons flow into the other end of the metal from the negative terminal of the battery. Multiple choice questions 33 B 34 A Structure of sodium chloride: 35 C Option Substance Electrical conductivity A argon a non-conductor B potassium a conductor; not chemically changed during conduction C potassium fluoride D tetrachloromethane a non-conductor an electrolyte; conducts electricity in molten state or aqueous solution, and decomposed by electricity during conduction 36 D Zinc chloride is an ionic compound. It does not conduct electricity in solid state. In solid state, ions in the compound are held together by strong ionic bonds. They are not free to move. 90

64 37 C To melt an ionic compound, a lot of heat is needed to overcome the strong attractive forces (ionic bonds) between the ions. Therefore ionic compounds have high melting points. 38 C Magnesium oxide is an ionic compound. The ions in it are held together by ionic bonds. 39 B Option Chloride Structure of chloride A HCl simple molecular structure B KCl giant ionic structure C SCl 2 simple molecular structure D PCl 3 simple molecular structure To melt KCl, a lot of heat is needed to overcome the strong ionic bonds between the ions. The attractive forces between the molecules in HCl, SCl 2 and PCl 3 are weak. Little heat is needed to separate the molecules. Hence KCl has the highest melting point. 40 A Diamond has a giant covalent structure consisting of a network of covalent bonds. Relative motion of the atoms is restricted. Diamond is the hardest substance known. Structure of diamond: 41 C In silicon dioxide, each silicon atom is joined to four oxygen atoms by covalent bonds, while each oxygen atom is joined to two silicon atoms by covalent bonds. Hence silicon dioxide has a giant covalent structure. Structure of silicon dioxide: 42 B Quartz has a giant covalent structure. To melt quartz, a lot of heat is needed to overcome the strong covalent bonds between the atoms. Therefore quartz has a high melting point. 91

65 43 C Option A Calcium (Ca) tarnishes in moist air. This is because calcium reacts with oxygen in the air to form an oxide layer on the surface. Option B Sodium (Na) is stored in paraffin oil as it can react with oxygen in the air. Option C Silicon dioxide (SiO 2 ) has a giant covalent structure. It is the most stable in moist air. Option D Sulphur dioxide (SO 2 ) can react with moisture in air to form an acid. 44 D Graphite has a layered structure. Weak van der Waals forces exist between the layers. The layers can easily slide over each other. Hence graphite has a slippery feel and can be used as a lubricant. Structure of graphite: 45 D Structure of dry ice: 46 C Option Substance Structure A diamond giant covalent structure B mercury giant metallic structure C nitrogen simple molecular structure D quartz giant covalent structure 92

66 47 C Option Substance Structure A calcium oxide giant ionic structure B graphite giant covalent structure C iodine simple molecular structure D sodium giant metallic structure iodine consists of separate molecules. 48 D Option Oxide Structure A MgO giant ionic structure B Al 2 O 3 giant ionic structure C SiO 2 giant covalent structure D Cl 2 O simple molecular structure Electron diagram of Cl 2 O: (Only electrons in the outermost shells are shown.) 49 C In solid carbon dioxide, van der Waals forces hold the molecules together. 50 C The attractive forces between bromine molecules are weak. Little heat is needed to separate the molecules. Hence bromine has a low melting point. It exists as a liquid at room temperature and pressure. 51 A Option A For a substance with a simple molecular structure, the attractive forces between the molecules are weak. Little heat is needed to separate the molecules. Hence the substance has low melting and boiling points. Option B A substance with a simple molecular structure is generally quite soluble in non-aqueous solvents. Option C The substance melts at 10 C and boils at 58 C. Hence it is a liquid at room temperature and pressure. Option D Substances with simple molecular structures do not conduct electricity. 52 C Options A, B and D Common salt, sugar and sodium nitrate are soluble in water. Option C Sulphur has a simple molecular structure. It is insoluble in water but soluble in nonaqueous solvents. 93

67 53 C Option Atomic number of element Name of element Compound formed between the element and chlorine A 10 neon no compound formed B 11 sodium an ionic compound formed C 16 sulphur a covalent compound formed D 20 calcium an ionic compound formed Element X with an atomic number 16 (sulphur) reacts with chlorine to form a covalent compound with a simple molecular structure. Electron diagram of the compound formed between sulphur and chlorine: (Only electrons in the outermost shells are shown.) 54 B Element X is phosphorus. Option A X (phosphorus) is a solid at room temperature and pressure. Option B To obtain stable electronic arrangements, one atom of X (phosphorus) bonds with three hydrogen atoms. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) Option C There are 15 electrons in an atom of X. Option D White phosphorus is composed of P 4 molecules. The molecules are held together by van der Waals forces. 55 A Element Atomic number Name of element X 8 oxygen Y 12 magnesium Z 17 chlorine 94

68 Option A X (oxygen) and Z (chlorine) are non-metals. They combine to form a covalent compound. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) Option B Y (magnesium) has a giant metallic structure. Option C Z (chlorine) exists as a gas at room conditions. Option D X (oxygen) supports combustion, but it is NOT flammable. 56 B Element Atomic number Name of element X 7 nitrogen Y 14 silicon Z 20 calcium Option A X (nitrogen) is a gas at room temperature and pressure. Its melting point is quite low. Option B Y (silicon) has a giant covalent structure similar to that of diamond. Option C Z (calcium) gives a brick-red flame in flame test. Option D X (nitrogen) and Z (calcium) combine to form an ionic compound with a giant ionic structure. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) 57 A Substance Structure Melting point ( C) CO 2 simple molecular structure 56 (5.2 atm); sublimes (1 atm) SiO 2 giant covalent structure Na 2 O giant ionic structure

69 58 A Option A W is probably a metal and hence it should NOT be a brittle solid. Metals are malleable and ductile. Option B X has high melting and boiling points, and it is a non-conductor of electricity. It probably has a giant covalent structure. Option C Y is a liquid at room temperature and pressure, and it is a good conductor of electricity. It is probably mercury. Hence it is probably a good conductor of heat as well. Option D Z has low melting and boiling points, and it is a non-conductor of electricity. It is probably a non-metal with a simple molecular structure. 59 B Y is a metal that melts at 39 C and boils at 357 C. It is a liquid at room temperature and pressure. Therefore Y is likely to be mercury. 60 B Both W and X have low melting points, do not conduct electricity and are slightly soluble / insoluble in water. Hence they probably have simple molecular structures. W melts at 7 C while X melts at 46 C. Hence only X exists as a simple molecular solid at room temperature. 61 B X (a metal) and Y (a non-metal) combine to form an ionic compound. Option A The ionic compound formed between X and Y has a giant ionic structure. Option B To melt or boil an ionic compound, a lot of heat is needed to overcome the strong attractive forces (ionic bonds) between the ions. Therefore ionic compounds have high melting and boiling points. The compound formed between X and Y should be a solid at room temperature and pressure. Option C Electron diagram of the compound formed between X and Y: (Only electrons in the outermost shells are shown.) The chemical formula of the compound is XY. Option D The molten compound contains mobile ions and hence it can conduct electricity. 62 C Element Electronic arrangement of atom Name of element X 2,8,5 phosphorus Y 2,7 fluorine X and Y are non-metals. They combine to form a covalent compound Z. 96

70 Electron diagram of the compound Z: (Only electrons in the outermost shells are shown.) Option A The bonds in Z are formed by electron sharing. Option B Z has a simple molecular structure. Option C Z has a low melting point. Option D The chemical formula of Z is XY C Chloride of Melting point ( C) Structure of the chloride Bonding in the chloride Metal / non-metal X 82 simple molecular structure covalent bonding X is a non-metal Y 808 giant ionic structure ionic bonding Y is a metal Option B The chloride of X is a covalent compound with a simple molecular structure. Option C The chloride of Y melts at 808 C. Hence it is a solid at room temperature and pressure. Option D The chloride of Y conducts electricity in molten state. 64 C Magnesium is a good conductor of electricity due to the presence of mobile electrons. 65 D The ions in copper are packed closely and the metallic bonds holding them together are very strong. To melt a piece of copper, a lot of heat is needed to overcome the strong attractive forces. 97

71 66 A Silver conducts electricity due to the movement of mobile electrons. When silver is connected to a battery, mobile electrons in the metal flow towards the positive terminal of the battery. At the same time, electrons flow into the other end of the metal from the negative terminal of the battery. 67 D A metal conducts electricity in both solid and molten states. It is also insoluble in water. 68 B Option Substance Melting point ( C) Boiling point ( C) State at 50 C and 1 atm pressure A bromine 7 59 solid B chlorine liquid C oxygen gas D hydrogen bromide gas Only chlorine exists as a liquid at 50 C and 1 atm pressure. 69 C All the four substances have simple molecular structures. In these substances, strong covalent bonds hold the atoms in each molecule together. Much weaker van der Waals forces hold the separate molecules together. e.g. van der Waals forces exist between oxygen molecules: 98

72 70 D Option Substance Structure (1) oxygen simple molecular structure (2) potassium oxide giant ionic structure (3) silicon dioxide giant covalent structure 71 D Option Substance Can conduct electricity? Particles responsible for the conduction (1) graphite yes mobile delocalized electrons (2) molten zinc chloride yes mobile ions (3) magnesium sulphate solution yes mobile ions 72 C (1) The chemical formula of silicon carbide is SiC. (2) Silicon carbide has a giant covalent structure. To melt it, a lot of heat is needed to overcome the strong covalent bonds between the atoms. Hence it has a high melting point. (3) Silicon carbide has a giant covalent structure. It is insoluble in water because the atoms are held together by strong covalent bonds and it is very difficult to separate the atoms. 73 A (2) The structure of germanium is similar to that of silicon. It has a giant covalent structure. To melt it, a lot of heat is needed to overcome the strong covalent bonds between the atoms. Hence germanium has a high melting point (937 C). (3) Electron diagram of the fluoride of germanium: (Only electrons in the outermost shells are shown.) The fluoride has a simple molecular structure. 99

73 74 A This form of solid carbon is composed of C 60 molecules. It probably has a simple molecular structure. (1) Substances with simple molecular structures are usually insoluble in water. (2) Diamond is the hardest substance known. The attractive forces between C 60 molecules are weak. Hence this form of solid carbon is softer than diamond. (3) The melting point of graphite is higher than that of this form of solid carbon. To melt graphite, a lot of heat is needed to overcome the strong covalent bonds between the atoms. The attractive forces between the C 60 molecules are weak. Little heat is needed to separate the molecules. 75 B (1) Selenium and hydrogen are non-metals. They combine to form a covalent compound. (2) Electron diagram of chloride of selenium: (Only electrons in the outermost shells are shown.) The chloride of selenium has a simple molecular structure. (3) Selenium (a non-metal) and a Group I element (a metal) combine to form an ionic compound. A selenium atom has 6 outermost shell electrons. In the above reaction, a selenium atom gains 2 electrons to obtain a stable electronic arrangement. An ion carrying two negative charges (Se 2 ) forms. 76 B Structure of graphite: (1) Graphite is slippery because weak van der Waals forces exist between the carbon layers in it. Hence the layers can easily slide over each other. (2) Graphite has a layered structure. Within each layer, each carbon atom is covalently bonded to three other atoms. To melt graphite, the covalent bonds between the atoms must be overcome. Graphite has a high melting point; that means a lot of heat is needed to overcome the covalent bonds. The fact that graphite has a high melting point is an evidence to support that covalent bonds are strong. 100 (3) Graphite can conduct electricity due to the presence of delocalized electrons.

74 77 A (1) X exists as diatomic molecules. It has a simple molecular structure. (2) Weak van der Waals forces hold the molecules together. Hence X has a very low melting point. (3) Within each molecule, a strong covalent bond holds the two atoms together. 78 A (3) Sodium chloride melts at 808 C. It is still a solid at 750 C and thus cannot conduct electricity at 750 C. 79 A (2) SiH 4 has a simple molecular structure as deduced from its low melting point. (3) H 2 S is a gas at room temperature and pressure. 80 A Property of substance Structure of substance Giant covalent Giant ionic Giant metallic Melting point high high high Electrical conductivity at room temperature non-conductor (except graphite) non-conductor conductor X may have a giant covalent or giant ionic stucture. 81 B Element Atomic number Name of element Electronic arrangement of atom X 19 potassium 2,8,8,1 Y 16 sulphur 2,8,6 (1) When X and Y combine, two atoms of X lose 2 electrons and the electrons are accepted by one atom of Y. An ionic compound with a chemical formula X 2 Y forms. Electron diagram of the compound: (Only electrons in the outermost shells are shown.) (2) The ionic compound formed between X and Y is insoluble in non-aqueous solvents. (3) The aqueous solution of the compound formed between X and Y contains mobile ions. Hence it can conduct electricity. 82 B Element Atomic number Name of element Electronic arrangement of atom X 8 oxygen 2,6 Y 9 fluorine 2,7 101

75 To obtain stable electronic arrangements, one atom of X forms a single bond with each of two atoms of Y. Electron diagram of the compound formed: (Only electrons in the outermost shells are shown.) (1) A covalent compound with a simple molecular structure is formed. (2) The chemical formula of the compound is XY 2, i.e. OF 2. (3) The compound is slightly soluble in water. 83 D (1) An oxide ion carries 2 negative charges. The simplest ratio of ion of X to O 2 in X 2 O 3 is 2 : 3 and the net charge of the compound is zero. Hence the ion of X carries 3 positive charges. Use the following steps to work out the chemical formula of the chloride of X: Step Chloride of X 1 Write down the symbols of ions in the compound. X Cl 2 Write down the number of charges of each ion on the top of each symbol. 3 Cross multiply the numbers and the symbols. 3 1 X Cl 3 1 X Cl = X 1 = Cl 3 4 Combine the symbols. XCl 3 (2) X (a metal) and oxygen (a non-metal) combine to form an ionic oxide. Hence X 2 O 3 has a giant ionic structure. (3) X 2 O 3 (an ionic oxide) conducts electricity in molten state due to the presence of mobile ions. 84 A Element Atomic number Name of element Electronic arrangement of atom Relative atomic mass P 9 fluorine 2, Q 17 chlorine 2,8, (1) P and Q are non-metals. They combine to form a covalent compound by electron sharing. Electron diagram of compound X: (Only electrons in the outermost shells are shown.) 102

76 (2) Relative molecular mass of X = relative atomic mass of P + relative atomic mass of Q = = 54.5 (3) X has a simple molecular structure. It is a gas at room temperature and pressure. 85 A Element Atomic number Name of element Electronic arrangement of atom Relative atomic mass P 7 nitrogen 2, Q 12 magnesium 2,8, (1) P (a non-metal) and Q (a metal) combine to form an ionic compound by electron transfer. Electron diagram of the compound X: (Only electrons in the outermost shells are shown.) (2) X has a giant ionic structure. It should have a high melting point and hence it should be a solid at room temperature and pressure. (3) Formula mass of X = 2 x relative atomic mass of P + 3 x relative atomic mass of Q = 2 x x 24.3 = D Not all ionic compounds are soluble in water. For example, calcium carbonate is insoluble in water. Attractive forces exist between ions in an ionic compound and water molecules. However, the attractive forces between water molecules are weaker than ionic bonds. 87 A To melt potassium chloride, a lot of heat is needed to overcome the strong ionic bonds between the ions. Hence potassium chloride has a high melting point. 103

77 88 D Iodine is very soluble in non-aqueous solvents. The attractive forces between molecules of non-aqueous solvents are similar to those between iodine molecules. Hence molecules of iodine and non-aqueous solvents can mix together easily. 89 C Many covalent compounds consist of separate molecules held together by weak van der Waals forces. The molecules separate easily when heated. Therefore these covalent compounds exist as gases, liquids or solids with low melting points. 90 C Graphite can conduct electricity but diamond cannot. Diamond and graphite are different forms of carbon. 91 C Calcium carbonate is an ionic compound but it is insoluble in water. Ammonia is a covalent compound but it dissolves well in water. 92 D Sugar is a non-conductor of electricity. It does not form ions when dissolved in water. 93 B When iodine sublimes, it absorbs heat so as to overcome the van der Waals forces between iodine molecules. 94 C Carbon dioxide has a simple molecular structure while silicon dioxide has a giant covalent structure. Therefore carbon dioxide and silicon dioxide have very different physical properties. 95 A Metals are good conductors of electricity due to the movement of mobile electrons in a metal. When heating one end of a piece of metal, the mobile electrons get more energy. They move faster and collide with neighbouring electrons. This helps to transfer the heat. 104

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