MODULE-21 TRENDS IN THE MODERN PERIODIC TABLE

Transcription

1 MODULE-21 TRENDS IN THE MODERN PERIODIC TABLE Valency is defined as the number of electrons an atom requires to lose, gain, or share in order to complete its valence shell to attain the stable noble gas configuration. The electrons present in the outermost shell (valence shell) of an atom are known as valence electrons. Therefore, on moving across a period, from left to right, the valency first increases somewhere till the middle and then decreases. And, on moving down in a group valency remains the same. For example:- Atomic Radius: The atomic radius is usually considered as the distance from the centre of the nucleus to the outermost shell i.e., to a point where the electron density is effectively zero. Across the period i.e., from left to right: Atomic radius decreases Down the group i.e., from top to bottom: Atomic radius increases Reason: Across the period, the effective nuclear charge increases. This is due to the fact that the number of electrons increase (in the same subshell), increasing the number of protons in the nucleus. This pulls the valence shell of electrons in an atom towards itself, thus decreasing the atomic radius. But as we move down the group, the number of orbits keeps on increasing along with the number of protons. The space required to accommodate the extra orbits takes prevalence and therefore the atomic size increases. Ionisation energy: It is the energy required to remove the most loosely bound electron from the valence shell of an isolated neutral gaseous atom.

2 Across the period i.e., from left to right: Ionisation energy increases Down the group i.e., from top to bottom: Ionisation energy decreases Reason: Across the period, the effective nuclear charge increases. This causes the atomic radius to decrease, thus getting the valence shell closer to the nucleus. This makes it difficult to remove electrons. But as we move down the group, the number of orbits keeps on increasing along with the number of electrons. The distance from the nucleus coupled with the interference of the electron between the nucleus and the valence shell renders the valence electrons weakly bound to the nucleus. Electronegativity: The tendency of an atom to attract a bonding pair (hared pair) of electron towards itself in a molecule of a compound is called electronegativity. Across the period i.e., from left to right: Electronegativity increases Down the group i.e., from top to bottom: Electronegativity decreases Reason: Across the period, the effective nuclear charge increases, thus decreasing the atomic radius. This favours the increase in electronegativity of elements across the period. But as we move down the group, the number of orbits keeps on increasing and therefore the atomic size increases and the electronegativity decreases. Metallic character: It is defined as the tendency of an atom to lose electrons. Non-metallic character: It is defined as the tendency of an atom to gain electrons. Across the period i.e., from left to right: Metallic character decreases and non-metallic character increases. Down the group i.e., from top to bottom: Metallic character increases and non-metallic character decreases. Reason: The elements to the left of the periodic table have a tendency of losing electrons easily as compared to those to the right. As we move from left to right of the period, the electrons of the outer shell experience greater pull of the nucleus. This greater force of attraction is because the effective nuclear charge increases and the size of the atom decreases from left to right. Thus, electrons of the elements to the right of the table do not lose electrons easily so are non-metallic in nature. As we move down the group the number of shells increases. This causes the effective nuclear charge to decrease due to the outer shells being further away: in effect the atomic size increases. The electrons of the outermost shell experience less nuclear attraction and so can lose electrons easily thus showing increased metallic character.

3 Effective nuclear charge (Z eff ): The nuclear charge experienced by the outermost electrons of an atom. The actual nuclear charge minus the effects of shielding due to inner-shell electrons. OR The measure of attractive force between nucleus and valence electrons. Z eff = Z- e inner shells Where, Z = atomic number and e inner shells represents number of electrons in the inner shells (except the valence electrons) How to calculate Z eff? Let us calculate the effective nuclear charge of sodium atom. The atomic number of sodium atom is 11 and the number of electrons in the inner shells [except valence electron(s)] is 10. Therefore, Z eff for sodium atom will be = +1. Now, let us also calculate the Z eff for magnesium atom. Mg (Z = 12) and number of electrons in the inner shells [except valence electron(s)] = 10. Therefore, Z eff for magnesium atom will be = +2. So, Z eff of Mg is greater than Na. Z eff trends: Z eff increases from left to right across period and it remains the same on moving from top to bottom in a group. Shielding effect: Electrons present in the inner filled shells/orbits between the nucleus and valence electrons shield the valence electrons somewhat from the effect of protons in the nucleus (also called screening effect). The electron shielding effect mainly applies to the valence electrons. Electrons found in the inner orbits will shield the attractive force from the nucleus. More the number of electrons in an atom, further the distance of valence electrons from the nucleus and greater the decrease in attraction. PREDICTING POSITION OF SOME ELEMENTS IN THE PERIODIC TABLE: GROUP NUMBER: Number of valence electrons decides the group number. If the number of valence electron is less than 3, then group number will be the number of valence electrons. If the number of valence electron is greater than or equal to 3, the group number is given by adding 10 (Group 3 to 12) to the number of valence electrons. PERIOD NUMBER: In the electronic configuration, the highest outer main shell decides the period number.

4 ELEMENT ATOMIC NUMBER ELECTRONIC CONFIGURATION VALENCE ELECTRONS HIGHEST OUTER MAIN SHELL GROUP NUMBER K(1) L(2) M(3) N(4) Sodium Third (M) 1 3 Fluorine Second 17 2 (L) Calcium Fourth 2 4 (N) Sulphur Third (M) 16 3 Carbon Second 14 2 (L) Argon Third (M) 18 3 PERIOD NUMBER

5 MODULE-21 TRENDS IN THE MODERN PERIODIC TABLE Some textual questions solved: 1) (a) What property do all elements in the same column of the Periodic Table as boron have in common? (a) All the elements in the same column as boron have the same number of valence electrons (3). Hence, they all have valency equal to 3. (b) What property do all elements in the same column of the Periodic Table as fluorine have in common? (b) All the elements in the same column as fluorine have the same number of valence electrons (7). Hence, they all have valency equal to 1. 2) Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the Periodic Table. Write the electronic configuration of these two elements. Which of these will be more electronegative? Why? Nitrogen (Z = 7) 2 5 and Phosphorus (Z = 15) Nitrogen is more electronegative than phosphorus. On moving down a group, the number of shell increases. Therefore, the valence electrons move away from the nucleus and the effective nuclear charge decreases. This causes the decrease in the tendency to attract electron and hence electronegativity decreases. 3) How does the electronic configuration of an atom relate to its position in the Modern Periodic Table? In the modern periodic table, atoms with similar electronic configurations are placed in the same column. In a group, the number of valence electrons remains the same. Elements across a period show an increase in the number of valence electrons. 4) In the Modern Periodic Table, calcium (atomic number 20) is surrounded by elements with atomic numbers 12, 19, 21, and 38. Which of these have physical and chemical properties resembling calcium? The element with atomic number 12 has same chemical properties as that of calcium. This is because both of them have same number of valence electrons (2). 5) Compare and contrast the arrangement of elements in Mendeleev s periodic Table and the Modern Periodic Table. Mendeleev s periodic table Elements are arranged in the increasing order of their atomic masses. There are a total of 7 groups (columns) and 6 periods (rows). Modern periodic table Elements are arranged in the increasing order of their atomic numbers. There are a total of 18 groups (columns) and 7 periods (rows).

6 Elements having similar properties were placed directly under one another. No distinguishing positions for metals and non-metals. Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group. Metals are present at the left hand side of the periodic table whereas non-metals are present at the right hand side.

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8 MODULE-21 (WORKSHEET-1) TRENDS IN THE MODERN PERIODIC TABLE 1. Use the periodic table on page number 86 of your NCERT text book to match each element in Column A with the element in Column B that has the most similar chemical properties. Write your answer here Column A Column B p 1. Vanadium (V) a. Boron (B) 2. Arsenic (As) b. Caesium (Cs) 3. Bromine (Br) c. Chromium (Cr) 4. Cadmium (Cd) d. Cobalt (Co) 5. Gallium (Ga) e. Hafnium (Hf) 6. Germanium (Ge) f. Iodine (I) 7. Iridium (Ir) g. Iron (Fe) 8. Magnesium (Mg) h. Nitrogen (N) 9. Neon (Ne) i. Platinum (Pt) 10.Nickel (Ni) j. Scandium (Sc) 11.Osmium (Os) k.silicon (Si) 12.Sodium (Na) l. Strontium (Sr) 13.Tellurium (Te) m.sulphur (S) 14.Tungsten (W) n. Zinc (Zn) 15.Yttrium (Y) o. Xenon (Xe) 16.Zirconium (Zr) p.tantalum (Ta) 2. Answer the following questions: (Ionization energy and electronegativity) a) What is ionization energy?. b) Explain why an atom with high ionization energy value is not likely to form a positive ion?. c) What is the period trend in the first ionization energies? Why?.

9 d) What is the group trend in the first ionization energies? Why?. e) What does the electronegativity of an element indicate?. f) What are the period and group trends in electronegativities?. 3) a) In the Periodic table how does the atomic radius vary: i) across a period from left to right? ii) Down a group from top to bottom? b) In each of the following sets place the elements in order of increasing atomic radius: i) potassium, calcium, magnesium ii) chlorine, argon, silicon iii)gallium, phosphorus, aluminium

10 MODULE-21 (WORKSHEET-2) TRENDS IN THE MODERN PERIODIC TABLE 1. The periodic table of elements has grown from the sixty-three elements Dmitri Mendeleev discovered in the 1800s to one hundred and eighteen elements. However, we will only focus on the first eighteen in this activity using the Bohr s model of an atom. Complete this chart for the first eighteen elements on the periodic table of elements. Element/Ion name Symbol Atomic number (Z) Number of electrons K-Shell L-Shell M-Shell Neon Ne S Fluorine 11 Be Oxygen Nitrogen Cl Answer the following questions: a) Try to place the first eighteen elements in the correct families. b) How many families are represented with the first eighteen elements? c) Which elements have eight electrons in their outer shell? d) Which elements have one electron in their outer energy shell? e) How many electrons does oxygen have in its outer energy shell? f) How many electrons does Lithium have in its outer energy shell? g) Identify the species from the first 18 elements given above in which number of electrons and atomic number are different.

11 MODULE-21 (WORKSHEET-3) TRENDS IN THE MODERN PERIODIC TABLE 1. Atomic Number of the elements of 3 period of Modern Periodic table are listed below. Study the data carefully and answer the questions that follow: Period 3 elements Na Mg Al Si P S C Ar Atomic Number a) What are the atomic numbers of sodium and phosphorus respectively? Write their respective electronic configurations.. b) Write down the number of valence electrons in an atom of sodium and phosphorus respectively.. c) What are the valencies of sodium and phosphorus?. d) What are the valencies of sulphur and chlorine atom?. e) Comment upon the similarity in the valency of sodium and chlorine, though they are the members of different groups.. f) If the number of valence electrons in an atom is known how would you calculate its valency?.

12 g) Going left to right in a Period, how does the valency of elements vary?. 2. A part of the modern periodic table with elements represented by the letter of English Alphabet (not the elements symbols) is given below: Observe that carefully and answer the questions that follow: Groups Periods I A II B H J L N III C F I K M O IV D G P V E a) What are the elements of group I called?. b) Select the letter which represents the most reactive metal.. c) What is the name given to group 17 elements?. d) Select the letter which represents the most reactive non-metal.. e) Which element is most likely to be a metalloid?. f) How does the metallic character of elements vary in a group and why?. g) How does the non-metallic character of elements vary in a period and why?.

13 MODULE-21 (WORKSHEET-4) TRENDS IN THE MODERN PERIODIC TABLE Answer the following questions: 1) Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice? 2) Name (a) three elements that have a single electron in their outermost shells.. (b) two elements that have two electrons in their outermost shells.. (c) three elements with filled outermost shells.. 3) (a) Lithium, sodium, potassium are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements? b) (b) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common? 4) In the Modern Periodic Table, which are the metals among the first ten elements? 5) By considering their position in the Periodic Table, which one of the following elements would you expect to have maximum metallic characteristic? 6) Which of the following statements is not a correct statement about the trends when going from left to right across the periods of Periodic Table. (a) The elements become less metallic in nature. (b) The number of valence electrons increases. (c) The atoms lose their electrons more easily. (d) The oxides become more acidic. 7) Element X forms a chloride with the formula XCl 2, which is a solid with a high melting point. X would most likely be in the same group of the Periodic Table as (a) Na (b) Mg (c) Al (d) Si

14 8) Which element has (a) two shells, both of which are completely filled with electrons?. (b) the electronic configuration 2, 8, 2?. (c) a total of three shells, with four electrons in its valence shell?. (d) a total of two shells, with three electrons in its valence shell?. (e) twice as many electrons in its second shell as in its first shell?. 9) (a) What property do all elements in the same column of the Periodic Table as boron have in common?. (b) What property do all elements in the same column of the Periodic Table as fluorine have in common?. 10) An atom has electronic configuration 2, 8, 7. (a) What is the atomic number of this element?. (b) To which of the following elements would it be chemically similar? N(Z = 7), F(Z = 9), P(Z = 15), Ar(Z = 18) 11) The position of three elements A, B and C in the Periodic Table are shown below: Group 16 Group 17 A B C (a) State whether A is a metal or non-metal.. (b) State whether C is more reactive or less reactive than A.. (c) Will C be larger or smaller in size than B?. (d) Which type of ion, cation or anion, will be formed by element A?.

15 MODULE-21 (WORKSHEET-5) TRENDS IN THE MODERN PERIODIC TABLE 1) Which of the following gives the correct increasing order of the atomic radii of O, F and N? (a) O, F, N (b) N, F, O (c) O, N, F (d) F, O, N 2) Which among the following elements has the largest atomic radii? (a) Na (b) Mg (c) K (d) Ca 3) Which of the following elements would lose an electron easily? (a) Mg (b) Na (c) K (d) Ca 4) Which of the following elements does not lose an electron easily? (a) Na (b) F (c) Mg (d) Al 5) Arrange the following elements in the order of their decreasing metallic character Na, Si, Cl, Mg, Al (a) Cl > Si >Al > Mg >Na (b) Na >Mg >Al >Si > Cl (c) Na > Al > Mg > Cl > Si (d) Al > Na> Si > Ca> Mg 6) Arrange the following elements in the order of their increasing non-metallic character Li, O, C, Be, F (a) F < O < C < Be < Li (b) Li < Be < C < O< F (c) F < O < C < Be < Li (d) F < O < Be < C < Li

16 7) Which of the following elements will form an acidic oxide? (a) An element with atomic number 7 (b) An element with atomic number 3 (c) An element with atomic number 12 (d) An element with atomic number 19 8) The element with atomic number 14 is hard and forms acidic oxide and a covalent halide. To which of the following categories does the element belong? (a) Metal (b) Metalloid (c) Non-metal (d) Left-hand side element 9) On moving from left to right in a period in the periodic table, the size of the atom. (a) increases (b) decreases (c) does not change appreciably (d) first decreases and then increases 10) Which one of the following does not increase while moving down the group of the periodic table? (a) Atomic radius (b) Metallic character (c) Valence (d) Number of shells in an element 11) Which of the following set of elements is written in order of their increasing metallic character? (a) Be Mg Ca (b) Na Li K (c) Mg Al Si (d) C O N

17 MODULE-21 (WORKSHEET-6) TRENDS IN THE MODERN PERIODIC TABLE 1) Three elements A, B and C have 3, 4 and 2 electrons respectively in their outermost shell. Give the group number to which they belong in the Modern Periodic Table. Also, give their valencies. 2) If an element X is placed in group 14, what will be the formula and the nature of bonding of its chloride? 3) Compare the radii of two species X and Y. Give reasons for your answer. (a) X has 12 protons and 12 electrons (b) Y has 12 protons and 10 electrons (a). (b). 4) Arrange the following elements in increasing order of their atomic radii. (a) Li, Be, F, N :-. (b) Cl, At, Br I :-. 5) Identify and name the metals out of the following elements whose electronic configurations are given below. (a) 2, 8, 2 :-. (b) 2, 8, 1 :-. (c) 2, 8, 7 :-. (d) 2, 1 :-. 6) Arrange the following elements in the increasing order of their metallic character. Mg, Ca, K, Ge, Ga

18 7) Write the formula of the product formed when the element A (atomic number 19) combines with the element B (atomic number 17). Draw its electronic dot structure. What is the nature of the bond formed? 8) Identify the elements with the following property and arrange them in increasing order of their reactivity (a) An element which is a soft and reactive metal. (b) The metal which is an important constituent of limestone. (c) The metal which exists in liquid state at room temperature. Increasing order of reactivity :-. 9) Properties of the elements are given below. Where would you locate the following elements in the periodic table? (a) A soft metal stored under kerosene (b) An element with variable (more than one) valency stored under water. (c) An element which is tetravalent and forms the basis of organic chemistry. (d) An element which is an inert gas with atomic number 2. (e) An element whose thin oxide layer is used to make other elements corrosion resistant by the process of anodising.

19 MODULE-21 (WORKSHEET-7) TRENDS IN THE MODERN PERIODIC TABLE 1) An element is placed in 2nd Group and 3rd Period of the Periodic Table, burns in presence of oxygen to form a basic oxide. (a) Identify the element. (b) Write the electronic configuration. (c) Write the balanced equation when it burns in the presence of air (d) Write a balanced equation when this oxide is dissolved in water (e) Draw the electron dot structure for the formation of this oxide 2) An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide. (a) Where in the periodic table are elements X and Y placed? (b) Classify X and Y as metal (s), non-metal (s) or metalloid (s) (c) What will be the nature of oxide of element Y? Identify the nature of bonding in the compound formed. (d) Draw the electron dot structure of the divalent halide

20 3) Atomic number of a few elements are given below 10, 20, 7, 14 (a) Identify the elements (b) Identify the Group number of these elements in the Periodic Table (c) Identify the Periods of these elements in the Periodic Table (d) What would be the electronic configuration for each of these elements? (e) Determine the valency of these elements 4) (i) Electropositive nature of the element(s) increases down the group and decreases across the period. (ii) Electronegativity of the element decreases down the group and increases across the period. (iii) Atomic size increases down the group and decreases across a period (left to right). (iv) Metallic character increases down the group and decreases across a period. On the basis of the above trends of the Periodic Table, answer the following about the elements with atomic numbers 3 to 9. (a) Name the most electropositive element among them. (b) Name the most electronegative element. (c) Name the element with smallest atomic size. (d) Name the element which is a metalloid. (e) Name the element which shows maximum valency..

21 5) An element X which is a yellow solid at room temperature shows catenation and allotropy. X forms two oxides which are also formed during the thermal decomposition of ferrous sulphate crystals and are the major air pollutants. (a) Identify the element X. (b) Write the electronic configuration of X.. (c) Write the balanced chemical equation for the thermal decomposition of ferrous sulphate crystals? (d) What would be the nature (acidic/ basic) of oxides formed?. (e) Locate the position of the element in the Modern Periodic Table. 6) An element X of group 15 exists as diatomic molecule and combines with hydrogen at 773 K in presence of the catalyst to form a compound, ammonia which has a characteristic pungent smell. (a) Identify the element X. How many valence electrons does it have? (b) Draw the electron dot structure of the diatomic molecule of X. What type of bond is formed in it? (c) Draw the electron dot structure for ammonia and what type of bond is formed in it?