MASTERS TUITUTION CENTRE Periodic Classification of Elements

Transcription

1 Periodic Classification of Elements Before the nineteenth century only a few elements were known. These elements could be easily studied individually. With the passage of time, many more new elements were discovered. More and more of their compounds were prepared. Study of these elements and compounds individually became more difficult. So, it was felt that these elements should be classified into a few groups to make their study systematic and easier. One of the earliest attempts towards the classification of elements was to divide these into metals and nonmetals. This method of classification failed because most of the elements fell into the category of metals, whereas only a few elements were nonmetals. There were a few elements which showed the properties of both metals and nonmetals. Since then, many attempts were made by various scientists to classify elements in a more systematic way. A few important attempts for the classification of elements are described below. EARLIER ATTEMPTS FOR PERIODIC CLASSIFICATION OF ELEMENTS What are Dobereiner's Triads? Dobereiner, in 1829, grouped certain elements in the groups of three, called triads. The three elements in a triad had similar chemical properties. When the elements in a triad were arranged in the order of increasing atomic masses, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic mass of the other two elements. It was found that the middle member of a triad had physical properties that were nearly the average of the other two. Three Dobereiner's triads are given in Table In these triads, the atomic mass of the middle element is nearly equal to the mean of the first and third element in each triad. Dobereiner could arrange only a few elements out of 63 elements known at that time, in the form of triads. Although, he got some success but the idea of classification into triads could not be applied to all the elements. So, the idea of classifying the elements into triads was abandoned. What is the Newlands' Law of Octaves? In 1864, John Newlands, an English chemist and a musician, arranged the elements known at that time, in the order of their increasing atomic masses. He observed that, "the properties of each element resembled those of the eighth element before it, and of the eighth element following it". In other words, he found that "the properties of the elements were repeated at every eighth element like the eighth note of an octave in music." From the arrangement given above, it is observed that every eighth element has similar properties, i.e., Li, Na and K have similar properties. Newlands called this as the law of octaves. The Newlands' classification did not work well for heavier elements. FEW QUESTIONS TO UNDERSTND Q.l. Why was the necessity of classifying the elements into certain groups felt? Ans. With the discovery of more and more elements and their compounds, it became very difficult to study and remember the properties of all these individually. So, a need to classify them into certain groups was felt. Q.2. X, Y and Z are three members of a Dobereiner's triad. The atomic mass of X is 7 u and that of Y is 23 u. What is the atomic mass of Z? Ans. The triad is X, Y, Z. Let the atomic mass of Z be x. Then, according to Dobereiner, 1 P a g e Periodic Classification of Elements by deepak sir

2 TRY YOUSELF 1. Name the elements which form the Dobereiner's triads. 2. What was the basis of Dobereiner's classification? 3. What is Newlands' law of octaves? 4. What are the limitations of the Newlands' law of octaves? D'mitri Ivanovich Mendeleef (I839-I907) D'mitri I. Mendeleef, the youngest of a family of seventeen, was born in Siberia (Russia). Mendeleef was a versatile genius. He Invented an accurate barometer. His most Imaginative and brilliant contribution to chemistry was the periodic arrangement of the elements in the form of a table. Mendeleef used Arabic numerals to Identify the periods, and Roman numerals to Identify the groups. MENDELEEFS PERIODIC LAW The Russian chemist D'mitri Mendeleef studied the chemical properties of all the 63 elements and their compounds known at that time. From the observed similarities, he postulated the periodic law commonly called as the Mendeleefs periodic law. What is Mendeleefs periodic law? When Mendeleef arranged the elements in the order of increasing atomic masses, he observed that the elements with similar properties appeared at regular intervals. This led him to state his famous periodic law in The Mendeleefs Periodic Law states that; "Properties of the elements are a periodic function of their atomic masses." While working on this classification of elements, he wrote, " When I arrange the elements according to the magnitude of their atomic masses beginning with the smallest, it becomes evident that there exists a kind of periodicity in their properties. I designate by the name "Periodic Law", the mutual relation between the properties of the elements and their atomic masses. These relations are applicable to all the elements and have the nature of a periodic function." MENDELEEF'S PERIODIC TABLE When Mendeleef arranged elements in the order of increasing atomic masses, he obtained a chart consisting of vertical columns and horizontal rows. He called this chart Periodic Table. This periodic table is commonly known as the Mendeleefs periodic table. Mendeleef laid more stress on similarity in the properties of the elements. The order of increasing atomic masses was not strictly followed. Thus, when an element did not seem to fit into the expected position, he left a gap for it. He predicted that new elements would fill these gaps as and when discovered. What are the essential features of Mendeleef's periodic table Mendeleef's periodic table had the following essential features. The vertical columns were called groups, and the horizontal rows were called periods. There were in all eight groups (Group I to Group VIII), and seven periods. To accommodate more elements, the periods 4, 5, 6 and 7 were divided into two halves. The first half of the elements were placed in the upper left corner, and the second half in the lower right corner of each box. The elements in each group resembled with each other in many properties. All elements of a group, show the same group valency. The group valency is equal to the group number, or 8 - group number. Mendeleef's periodic table did not include the noble gases because these gases (or elements) were not known at that time. Mendeleef named the yet undiscovered elements by prefixing a Sanskrit numeral -eka (for one), -dwi (for two), -tri (for three) etc., before the name of the element just above it in the periodic table. For example, the undiscovered element below silicon in group IV was named as eka-silicon. In the formula for oxides and hydrides at the top of the columns, the letter "R" is used to represent any of the elements in the group. Note the way formulae are written. For example, the hydride of carbon CH4 is written as RH4 and the oxide C02, as R02. 2 P a g e Periodic Classification of Elements by deepak sir

3 The properties of elements changed gradually in any period while going from left to right. Mendeleef's periodic table had certain gaps. Mendeleef predicted the existence and the properties of six yet-to-be discovered elements. He named these elements as eka-boron,eka-aluminium,eka-silicon, eka-manganese, dwi-manganese, and eka-tantalum. All these elements were later discovered and found to have properties similar to those predicted by Mendeleef. For example, scandium (Sc), gallium (Ga) and germanium (Ge) discovered later had properties similar to those predicted for eka -boron, eka-alumiriium and eka -silicon respectively. The predicted and the observed properties of eka-silicon (germanium) are presented in Table What are the drawbacks in Mendeleefs periodic table Mendeleefs periodic table was a brilliant attempt for the classification of elements, but it had certain drawbacks in it. Inversion in the periodic table. In Mendeleefs table, certain pairs of elements were placed in the reverse order of atomic masses. For example, cobalt (Co) having higher atomic mass had been placed before nickel (Ni) having lower atomic mass. Also, argon (40) was placed before potassium (39). This contradicted his periodic law. Position of isotopes. Isotopes have similar chemical properties, but different atomic masses. According to Mendeleefs periodic law, therefore, isotopes of an element must be given separate places in the periodic table (because the mass number is different). But, isotopes were not given separate places in the periodic table. Grouping of chemically dissimilar elements. Certain chemically dissimilar elements have been grouped together in Mendeleefs table. For example, Cu and Ag have no resemblance with alkali metals (Li, Na, K, etc.), but these have been grouped together in Group I. Separation of chemically similar elements. Certain elements which appear to be chemically similar (like Cu and Hg, Au and Pt etc.), have been placed in separate groups. Position of hydrogen. Hydrogen forms both positive ion like alkali metals, and negative ion like halogens. Thus, hydrogen resembles both. Therefore, it can be placed both in Group I and Group VII. In Mendeleefs table, the position of hydrogen was not made clear. FEW QUESTIONS TO UNDERSTAND Q.l. Which property of the atoms formed the basis of Mendeleefs classification? Ans. Atomic mass. Q.2. Many scientists before Mendeleef also used atomic mass as the basis of classification, but why did only Mendeleef succeed? Ans. The secret of Mendeleefs success was that although the classification was based on atomic mass, but at many places he did not follow this rule rigidly. He laid more stress on the similarity in the properties. Q.3. Why did Mendeleef leave many gaps in his periodic table? Ans. Mendeleef predicted that there were many elements yet to be discovered. So, when none of the elements known at that time fit into a particular position, he left a gap there. Later, when more elements were discovered these were found to fit into these gaps. Q.4. Give one example of the element discovered after Mendeleefs Periodic Table. Ans. One of the elements discovered after Mendeleef designed the Periodic Table, was gallium. Mendeleef predicted the existence and properties of this element, named by him as eka-aluminium. The properties of gallium agreed with those predicted by Mendeleef for ekaaluminium. Q.5. An element 'X' is in the third group of the periodic table. What is the formula of its oxide. Ans. The element is in the third group. Therefore, its valency is 3. Oxygen in oxides has a valency of -2. So, the formula of its oxide is X203. Q.6. What valency will be shown by an element having atomic number 17? Ans. The atomic number of element is 17. So, there are 17 protons in its nucleus. Its atom, therefore, would contain 17 electrons. These electrons are distributed in various shells as 2, 8, 7. Thus, there are seven valence electrons. This atom would require only one electron to complete its shell. Thus, its valency is -1. Q.7. Magnesium forms the following compounds. Magnesium oxide - MgO, Magnesium hydroxide - Mg(OH)2, Magnesium sulphate -MgS04 Radium belongs to the same group. Write the formulae of radium oxide, radium hydroxide, and radium sulphate. Ans. Radium belongs to the same group as magnesium. So, it should have the same valency. Therefore, it should form the same type of compounds. So, the formulae of Radium oxide is RaO, Radium hydroxide is Ra(OH)2, and Radium sulphate is RaS04. Q.8. Use Mendeleef periodic table to predict the formulae for the oxides of following elements. K, C, Al, Si, Ba Ans. Element: K C AI Si Ba Oxide: K20 C02 Al2O3 SiO2 BaO 3 P a g e Periodic Classification of Elements by deepak sir

4 Try your self 1. State the periodic law on which Mendeleef's periodic table was based. 2. How many elements were known at the time of Mendeleef? Name one element which was not known at the time of Mendeleef, but when discovered later found to have the same properties as predicted by Mendeleef. 3. Draw a layout sketch of the Mendeleef s periodic table, and label the groups and periods in this table. 4. Hydrogen could be placed both in Group I as well in Group VII. Give three points to support this statement. MODERN PERIODIC LAW What is the modern periodic law In 1913, H.G.J. Moseley in England, proved that the more fundamental property of an element is its atomic number'. Therefore, he suggested that the basis of classification of elements should be atomic number. This led to the modern periodic law, which states that, "The properties of elements are periodic function of their atomic number." How did atomic number prove a better basis of classification? By choosing atomic number as the basis of classification, the anomalies in the original Mendeleef's periodic table got removed, as described below. Inversion in Mendeleef's table. The anomaly of having the element of higher atomic mass element before the element with lower atomic mass, is removed because the element having higher atomic mass had the lower atomic number in such cases. For example, the atomic number of Ar is 18 and that of K is 19, although the atomic mass of Ar is higher than that of K. Position of isotopes. Although isotopes of an element have different atomic masses, but they have same atomic number. Hence, according to the atomic number as the basis of classification, these should be allotted the same place in the table. MODERN LONG FORM PERIODIC TABLE Taking atomic number as the basis of classification of elements, Bohr constructed a periodic table known as the Long Form Periodic Table, in In 1984, IUPAC (International Union of Pure and Applied Chemistry) recommended that the groups in the long form periodic table be numbered from the left, as 1 to 18. The long form periodic table relates the position of an element to its electronic configuration. What are the features of the long form periodic table (IUPAC) The general features of the Long Form Periodic Table (IUPAC 1984) are: The long form periodic table (IUPAC 1984) consists of 18 vertical columns called groups. Thus, there are 18 groups in long form periodic table. These groups are numbered 1 to 18 from the left, using Arabic numerals. The periodic table is divided into four blocks based on electronic configuration. s-block. This block consists of groups 1 and 2. p-block. This block consists of groups 13 to 18. The elements belonging to s- and p- blocks are called main group elements. d-block. This block consists of groups 3 to 12. Elements lying in this block are called transition elements. f- block. This block is placed below the main table. The elements belonging to group are called inner-transition elements. The elements of certain groups are given specific family names. The elements belonging to the group 1 are collectively called alkali metals. The elements belonging to the group 2 are collectively called alkaline earth metals. The elements belonging to the group 17 are collectively called halogens. The elements belonging to the group 18 are collectively called noble gas elements. Metals are located in the left and the central parts of the periodic table. Nonmetals are located in the upper right portion of the periodic table. There are seven (7) horizontal rows called periods in this periodic table. Thus, there are seven periods in long form periodic table. First period contains 2 elements, and is called Shortest period. Second and Third periods contain 8 elements each, and are called Short periods. Fourth and Fifth periods contain 18 elements each, and are called Long periods. Sixth period contains 32 elements, and is called Longest period. Seventh period is incomplete, and is called Incomplete period. Each period begins with the element that has only one electron in a new main shell. Extra points to note Hydrogen has been assigned a separate position at the top of group I. It is not possible to have an element with atomic number 1.5 placed between hydrogen and helium because, atomic number cannot have a fractional value. A gradual change in the properties of elements in a group is also known as vertical relationship in the periodic table. All elements in a group contain the same number of valence electrons. Each period starts with an element in which a new shell starts getting filled with an electron, and ends up with a noble gas which has all the Inner and outer shells completely filled. All elements in a period contain the same number of shells. 4 P a g e Periodic Classification of Elements by deepak sir

5 What are the characteristics of a group A vertical column in the periodic table is called a group. There are 18 groups in the long form periodic table. These are numbered from 1 to 18 starting from the left. On the extreme left of the periodic table is Group 1. On the extreme right end, is the Group 18. All elements in any group of the periodic table show similar properties due to similar outer electronic configuration and a gradual change in the properties due to decreasing attraction between the nucleus and the valence electrons as we go down the group. What are the characteristics of a period A horizontal row of elements in the periodic table is called a period. There are seven periods in the periodic table. Each period starts with Group I (alkali metal group), and ends with Group 18 (noble elements group). The first period elements (H and He) have only one shell - K-shell. K-shell starts getting filled at H (K : 1), and is completely filled at He (K : 2). The second period elements (Li, Be, B, C, N, O, F and Ne) have two shells - K and L shells. The L-shell starts getting filled with Li (K, L : 2, 1), and is completely filled with Ne (K, L : 2, 8). Third period elements have three shells - K, L and M shells. It starts with Na (K, L, M : 2, 8, 1), and ends with Ar (K, L, M : 2, 8, 8). Similarly, the fourth period elements have four shells - K, L, M and N, and so on. What are the merits of long form periodic table The long form periodic table has the following merits. Long form of the periodic table is based upon atomic numbers which is a more fundamental property of the atom. It relates the position of an element in the periodic table with its electronic configuration. It removes all the anomalies and drawbacks of the Mendeleef's table based on increasing atomic mass, by changing the basis of classification to atomic number. For example, all the isotopes of an element are now placed in the same position because they all have the same atomic number. The anomaly of higher atomic mass element being placed before the lower atomic mass element is removed when these elements are arranged in the order of increasing atomic number. It shows changes in the properties of various elements while moving across a period or down a group more clearly. EXAMPLE. Nitrogen (atomic no. 7) and phosphorus (atomic no. 15) belong to group 15 of the periodic table. Write the electronic configuration of these two elements in terms of K, L, M, N shells. Predict whether these are metallic or nonmetallic. Both nitrogen and phosphorus belong to Group 15 of the periodic table and contain five electrons in their outermost shell. Therefore, both nitrogen and phosphorus are nonmetals. EXAMPLE. The positions of three elements, A, B and C in the periodic table are shown below: (i)state whether C is a metal or nonmetal. (ii) State whether C is more reactive or less reactive than A. (iii) Will C be larger or smaller than B in size? (iv) Which type of ion, cation or anion, will be formed by element C? SOLUTION, (a) Group 17 is on the right-end of the periodic table. So, elements in Group 17 are nonmetals. Therefore, C is a nonmetal. (b) In a group of nonmetals, the reactivity decreases as we go down the group. Therefore, C is less reactive than A. (c) B and C lie in the same period. In a period, the atomic size decreases in going from left to right. Therefore, C is smaller than B. (d) The element C is a nonmetal. Therefore, it forms an anion. EXAMPLE. An atom has the electronic configuration of 2, 8, 7. (a) What is the atomic number of this element? (b) To which of the following element would it be chemically similar? (Atomic numbers are given in parentheses) N(7), F(9), P(15), Ar(18). SOLUTION, (a) The electronic configuration 2, 8, 7 shows that there are 17 electrons ( = 17) in the given atom. In an atom, the number of electrons is equal to the number of protons, and the atomic number of an element is equal to the number of protons inside its nucleus. Therefore, Atomic number of the element = 17 (b) The electronic configurations of the given elements are 5 P a g e Periodic Classification of Elements by deepak sir

6 The electronic configuration of F, that is 2, 7 is similar to that of the element (2, 8, 7) because both contain 7 valence electrons. Thus, the element having electronic configuration 2, 8, 7 is chemically similar to fluorine (F). Q.l. In the modern periodic table, calcium element (atomic number 20) is surrounded by elements with atomic number 12, 19, 21 and 38 respectively. Which of these have physical and chemical properties resembling with calcium? Ans. Elements having numbers 12 and 38 will show properties resembling with calcium because all these belong to the same group (Group no. 2). Q.2. By considering their position in the periodic table, which one of the following elements would you expect to have the most metallic characteristics?ga, Ge, As, Se, Be Ans. Metals occupy the left side of the periodic table. Be (beryllium) is expected to have the most metallic characteristics. Q.3. in the modern periodic table, of the first ten elements, which are metals? Ans. Of the first ten elements the elements which are metals are Li and Be Q.4. (a) What are the two properties that all elements in the same column of the periodic table as boron have in common? (b) What are the two properties that all elements in the same column of the periodic table as fluorine have in common? Ans. (a) (i) All elements belonging to boron group will have the same, K 2, M 3 electronic configuration, (ii) All elements belonging to boron group will show a valency of + 3, and are metallic in nature. (b) (i) All elements belonging to the fluorine group will have the same, 2 5 / outer electronic configuration, (ii) All the elements belonging to the fluorine group are nonmetals. Q.5. Name: (a) three elements that have only a single electron in their outermost shell. (b) two elements that have two electrons in their outermost shell. (c) three elements with filled outermost shell. (d) one element that has a single electron in its outermost shell. Ans. (a) Lithium, sodium and potassium have only a single electron in their outermost shell. (b) Magnesium and calcium have two electrons in their outermost shell. (c) Helium, neon and krypton have completely filled outermost shell. (d) Hydrogen has a single electron in its outermost shell. Q.6. In how many groups and periods, the modern periodic table of elements is divided? Ans. Modern periodic table (long form IUPAC 1984) has 18 groups, and 7 periods. Q.7. An element of group 14 has the atomic number 14. Examine if this element will have metallic properties or not. Ans. The electronic configuration of the element having atomic number 14 is 2, 8, 4. So, it lies in middle of the 3rd period. This element, therefore, has no tendency to lose its valence electrons. As a result, this element does not have metallic properties. Q.8. Explain why the properties of the 8th element are repeated in case of elements arranged in 2nd and 3rd period of the long form of the periodic table. Ans. This is because every 8th element in 2nd and 3rd period have similar electronic configurations and lie in the same group of the periodic table. That is why, every 8th element is 2nd and 3rd period of the periodic table has similar properties. Q.9. Indicate the atomic numbers of elements of period 3 of the modern periodic table having the following characteristics: (a) nonmetals (b) elements forming negative ions (c) elements with high melting points (d) elements forming positive ions (e) metals (f) elements with low boiling points Mention the atomic numbers only. Ans. Atomic numbers of the elements of period 3 of the moden periodic table (from left to right) are 11, 12, 13, 14, 15, 16, 17 and 18. (a) Nonmetals. Elements having atomic numbers 15, 16 and 17 are nonmetals. (b) Elements forming negative ions. Elements having atomic numbers 15, 16 and 17 form negative ions. (c) Elements with high melting points. Elements having atomic numbers 12, 13 and 14 have high melting points. (d) Elements forming positive ion. Elements having atomic numbers 11, 12 and 13 form positive ions. (e) Metals. Elements having atomic numbers 11, 12 and 13 are metals. (f) Elements with low boiling point. Elements having atomic numbers 17 and 18 have low boiling points. Q.10. Sodium and aluminium havee atomic numbers of 11 and 13 respectively. These are separated by one element in the periodic table, and luwe valencies of one and three respectively. Chlorine (at. no. 17) and potassium (at. no. 19) are also separated by one element in the periodic table, but both show valency of one. Why is it so? 6 P a g e Periodic Classification of Elements by deepak sir

7 Ans. Writing the electronic configurations of the various elements, we have This difference is because sodium and aluminium belong to the same period. These elements would lose respectively 1 and 3 electrons to attain the stable configuration of the nearest noble gas and show different valencies. On the other hand, chlorine and potassium lie in different periods. These elements are separated by a noble gas element. So, CI attains the nearest noble gas configuration by gaining one electron, and potassium does so by losing one electron. As a result, both chlorine and potassium show valencies equal to 1. Q.11. Why is silicon tetravalent, while chlorine is monovalent? Ans. Silicon belongs to group 14 of the periodic table. It has four electrons in its valence shell (2, 8, 4). Thus, it can attain a stable configuration either by losing, gaining or by sharing four electrons. Thus, silicon is tetravalent. On the other hand, chlorine lies in group 17 of the periodic table. It has seven electrons in its valence shell (2, 8, 7). So, it can attain a stable configuration by gaining one electron. Thus, chlorine is monovalent. Q.12. A part of the periodic table is shown below. Please note, here the groups are numbered on the basis of short form periodic table Lithium Carbon Oxygen L Neon X E G Q Y Z Answer the following: (a) Give the letter of the most reactive metal. (b) Give the letter of the most reactive nonmetal. (c) Name the family of elements represented by L, Q, R, T. (d) Name one element in each case occurring in groups 2, 3 and 5. Ans. (a) The element at the bottom of the group on the extreme left is the most reactive element. So, Z is the most reactive metal. (b) The element at the top of the group on the extreme right is the most reactive nonmetal. So, the element L is the most reactive nonmetal. (c) L, Q, R, T belong to group 7. These elements are the members of the halogen family. (d) The first element in each group 2, 3, and 5 are, Beryllium, Boron and Nitrogen respectively. Q.13. Elements having atomic numbers 3 to 18 are shown in the form of a table by using certain letters of the alphabet: (these letters are not the usual symbols of these elements) A B C D F (a) Which of these is (a) a noble gas (b) a halogen (c) an alkali metal (d) au element with valency 4? (b) Write the formula of the compound formed when A reacts with I. (c) Write the electronic configuration of element G. Ans. (a) (i) The element G (atomic no. 10) has the electronic configuration 2, 8. Thus, its outermost shell is completely filled. So, it is a noble gas. (ii) The element F (at. no. 17) is a member of halogen family. (iii) The elements A and B both are alkali metals. (iv) The element D has the electronic configuration 2, 8, 4. So, its valency is 4. Therefore, the element D has a valency of 4. (b) The element A (at. no. 3) has the electronic configuration 2, 1. The element F (at. no. 17) has the electronic configuration 2, 8, 7. So, the element A has to lose one electron, and the element F has to gain one electron to attain stable electronic configurations. By doing so, A becomes a positive ion, and F becomes a negative ion. A A + + e~ F + e~ -» F" These positive and negative ions combine to give a compound AF. A + + F - -> A + F- -> AF So, the formula of the compound formed is AF. 7 P a g e Periodic Classification of Elements by deepak sir R T 10 C

8 (c) G has atomic number 10. So, the electronic arrangement in G is 2, 8. Q.14. Which of the following statements is not a correct statement about the trends going from left to right across the 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. Ans. The statement that the atoms lose their electrons more easily is not correct. So, the correct answer is (c). Q.15. Fill in the blanks: In its modified form_is an organisation of all the known elements, arrangedin order of increasing. Elements that lie in the same_have similar, have the same number of. The table is particularly useful for predicting the outcome of chemical reactions between since an atom of any element tends to react so as to convert its electronic structure to that of a nearby. Ans. The completed statement is In its modified form the periodic table is an organisation of all the known elements, arranged in order of increasing atomic number. Elements that lie in the same group have similar electronic configuration, have the same number of valence electrons. The table is particularly useful for predicting the outcome of chemical reactions between the same or different elements since an atom of any element tends to react so as to convert its electronic configuration to that of a nearest noble gas. Q.16. Element Atomic Properties Reaction with Number water Li 3 Metal Fast Na 11 Metal Fast K 19 Metal Vigorous Rb 37 Metal Vigorous Cs 55 Metal Vigorous 1. Using the atomic number of the elements given in the table write the electronic configuration of each element. 2. What will you infer from their electronic configuration and the properties given Ans. The electronic configuration of the given elements are Li Na K Rb Cs , 1 2, 8, 1 2, 8, 8, 1 2, 8, 18, 8, 1 2, 8, 18, 18, 8, 1 All these elements are metals. All of these contain only one electron in their outermost shell, and all the inner shells are completely filled. Thus, all these elements belong to first group of the periodic table. The reactivity increases as we go down the group Q.17. Which element has (a) an atomic mass of 14. (b) an atomic number of 16. (c) two energy shells, both of which are filled with electrons. (d) the electronic structure 2, 8, 2. (e) a total of three energy shells, with four electrons in its valence shell, (f) a total of two energy shells, with three electrons in its valence shell, (g) twice as many electrons in its second shell as in its first. Ans. (a) The element having atomic mass of 14 is nitrogen. (b) The element having atomic number of 16 is sulphur. (c) Neon has both of its electron /energy shells completely filled. (d) The element having electronic structure 2, 8, 2 is magnesium (atomic no. 12). (e) The element having three electronic shells and four electrons in its valence shell belongs to 3rd period and 14th group of the long form periodic table. Thus, the element is silicon. Silicon has electronic configuration of 2, 8, 4. (f) The element having two energy shells and three electrons in its valence shell belongs to 2nd period and 13th group of the long form periodic table. Thus, the element is boron, (2, 3). (g) The element has two shells. The first shell can accommodate 2 electrons. So, the second shell has 4 electrons. So, the electronic configuration of the element is 2, 4 (atomic number = = 6). From the periodic table, this element is carbon. Try yourself 1. Which modern discovery led to a change in the Mendeleefs periodic law? State the modern periodic law. 8 P a g e Periodic Classification of Elements by deepak sir

9 2. How are the anomalies of Mendeleefs classification removed in the modern classification? 3. How are the groups and periods in the long form periodic table numbered? How many elements are there in each period? 4. Why is the group consisting of noble gases labelled as zero group? Name an element of this group which is used in the advertisement sign board tubes. 5. In which part of the periodic table are (a) metals (b) nonmetals located? PERIODICITY IN PROPERTIES The properties of elements depend on their electronic configurations. Electronic configurations change regularly along the period and down the group of the periodic table. Therefore, the properties of elements change across the period and down the group of the periodic table. There are quite a few atomic/physical properties of elements. At this stage, we describe the following properties of elements. Atomic number Atomic size (or atomic radius) Valence electrons Valency Tendency to lose electrons Tendency to gain electrons Metallic and nonmetallic character Chemical reactivity Nature of oxides ATOMIC NUMBER The number of protons inside the nucleus of an atom of any element is called its atomic number. Element Li Be B C N ' O F Ne Atomic number No. of protons inside the nucleus Thus, the atomic number increases as we go down the group. the atomic number increases by one at each element as we go from left to right in a period. ATOMIC SIZE The atomic size of an element can be expressed as the radius of its atom (assumed to be spherical). The atomic radius may be defined "The distance from the centre of the nucleus to the outermost shell of electrons in an isolated atom is called its atomic radius." For nonmetallic elements, atomic radius is defined as follows: "Half the distance between the nuclei of the two similar atoms covalently bonded to each other by a single bond is called its atomic (or covalent) radius." For example, the H - H covalent bond has a length of 74 pm (pm is called picometre; 1 pm = 1 x m). So, Atomic radius of hydrogen atom = 37 pm For metals, atomic radius is defined as follows: "Half of the distance between the nuclei of the two adjacent metal atoms in the metallic crystal is called its atomic radius." How does atomic size vary in a group of the periodic table In any group of elements, the atomic size increases while going from top to the bottom in any group. This can be explained as follows. When we;go down the group, at each new element, a new shell of electrons is added to the atom. This increases the size of the atom at each new element down the group. For example, the atomic size of group 1 and group 17 elements follows the order: The values given in brackets are atomic radii in picometres (pm). Thus, atom of the last element in any group is the largest atom in that group. 9 P a g e Periodic Classification of Elements by deepak sir

10 The atomic radius of metals is also called metallic radius How does atomic size vary in a period of the periodic table The atomic size (or atomic radius) decreases in going from left to right in a period of the periodic table, (except for He and Ne). For example, for the elements of second and third periods, the atomic size decreases in going from left to right. Element Li Be B C N F Atomic radius / pm o Element Na Mg Al Si P S CI Atomic radius / pm Atomic size decreases This can be explained as follows. The number of protons and electrons in an atom increases in going from left to right in a period. The added electrons enter the same shell. Due to the increase in nuclear charge, the electrons are pulled towards the nucleus with greater attractive force. As a result, the electronic shell shrinks, and the atomic size decreases.. group--> Atomic volume also increases while going from top to bottom in any group of the periodic table. atomic Atomic volume also decreases in going from left to right in a period of the periodic table. radius period--> atomic radius Decreases increases VALENCE ELECTRONS In an atom, electrons are distributed in various shells (or orbits). The electrons in the outermost shell of an atom are called valence electrons. To calculate the number of valence electrons in the atom of any element, write down its electronic configuration and count the number of electrons in the outermost shell. For the elements of first three periods of the periodic table, we write as follows: First period Second period Third period H He 1 2 Li Be B C N O F Ne 2, 1 2, 2 2, 3 2, 4 2, 5 2, 6 2, 7 2, 8 Na Mg Al Si P S CI Ar 2, 8,1 2, 8,2 2,8,3 2,8,4 2, 8,5 2,8,6 2,8,7 2, 8,8 No. of valence electrons: (For He: 2) Thus, we can say that "The number of valence electrons increases from 1 to 8 in going from left to right in a period (except in the first period where valence electrons increase from 1 to 2)." Elements having different number of valence electrons show different chemical properties. From the electronic configurations given above, we can see that all elements in a group have the same number of valence electrons. For example, all elements of group 1 contain only one valence electrons, those belonging to group 2, contain only two valence electrons and so on. Elements having the same number of valence electrons show similar chemical properties. The number of valence electrons increases by one at each element in going from left to right in a period. 10 P a g e Periodic Classification of Elements by deepak sir

11 period group valance electron same increses VALENCY Combining capacity of an element is expressed in terms of its valency. There are many ways to define the valency. Here we define it in terms of number of valence electrons and location of the element in the periodic table. According to the electronic structure of the atom, the valencies of first twenty (20) elements are either equal to the number of valence electrons and / or equal to eight (8) minus the number of valence electrons, i.e., Valency of an element = No. of valence electrons or Valency of an element = 8 - No. of valence electrons In terms of the location of an element in the long form periodic table, valency is given by the following formulae: Valency of any Group 1 and 2 element = Group number, i.e., 1 or 2 Valency of any element from Group 13 to 17 = Group number - 10 The application of these rules is illustrated below: IN A GROUP All elements in a group show the same valency. For example, all group 1 elements called (alkali metals) show a valency of 1+. All elements of group 2 (called alkaline earth metals) show a valency of 2+. This is because all elements of a group have the same number of valence electrons. IN A PERIOD The valency of the elements in a period first increases from 1 to 4 and then decreases to zero. For example, for the elements of Group 2, Thus, the elements in a period show different valencies. TENDENCY TO LOSE ELECTRONS In any group of elements, the tendency to lose its valence electrons increases in going from top to the bottom. This is because, the atomic size increases as we go down the group. The electron-losing tendency increases in going from Li to Cs. In any period, the tendency to lose an electron decreases in going from left to right. This is because, the atomic size decreases (or nuclear charge increases) when we go from left to right in a period. For example, the electron-losing tendency decreases in going from Li to F in second period. Be B C N O F Electron-losing tendency decreases in this direction > IN A GROUP In a group of the periodic table, the electron-attracting tendency decreases in going from top to the bottom. This is because, the atomic size increases down the group. IN A PERIOD In a period, the electron-attracting tendency increases in going from left to right. This is because in going from left to right in a period, the positive charge in the nucleus increases tendency to attract electrons increases in this direction P a g e Periodic Classification of Elements by deepak sir

12 How does the metallic character and nonmetallic character vary in a group Metallic character of elements in a group increases as we go down the group. Thus, in any group, the elements which appears at the bottom of the group is the most metallic. This can be explained as follows. Metallic character of an element is related to its electron-losing tendency. The element which has a greater tendency to lose electrons is more metallic. For example, in group 14, carbon (C), its first member is a nonmetal. The metallic character increases in going from top to bottom. As a result, the last member of this group, i.e., lead (Pb) is a typical metal. On the same logic, cesium (Cs) is the most metallic and lithium (Li) is the least of all the alkali metals. Nonmetallic character of elements in a group decreases as we go down the group. Thus, in any group, the element at the top is the most nonmetallic and that the bottom the least nonmetallic. For example, in the halogen group (Group 17) fluorine (F) is the most nonmetallic and iodine (I) is the least nonmetallic. Nonmetallic character of an element is related to its tendency to accept / gain electron. The tendency to gain electron decreases with an increase in the atomic size. Therefore, the nonmetallic character of elements in a group decreases down the group. METALLIC AND NONMETALLIC CHARACTERS In the periodic table, metals and nonmetals occupy different locations. Metals and nonmetals are separated by a few elements which are called metalloids or semimetals. How do the metallic character and nonmetallic character vary in a period The tendency of an atom to lose electrons decreases in a period from left to right. Therefore, the metallic character decreases in going from left to right in a period. Thus, the elements on the left side of the periodic table are metals. For the same reason, nonmetallic character increases in going from left to right in a period. Therefore, the elements on the right side of the periodic table are nonmetals. CHEMICAL REACTIVITY Chemical properties of an element are largely governed by its valence electrons. That is why, all elements of a group show similar chemical properties. However, there is a slight variation in the reactivity of the elements in a group. For metallic elements, the reactivity increases in going from top to the bottom in a group. Thus, the reactivity of alkali metals (elements of Group 1) follows the order, Cs > Rb > K > Na > Li -Reactivity decreases in this direction-> For non-metallic elements, the reactivity decreases in going from top to the bottom in a group. The reactivity of the halogen group elements (Group 17) varies as follows: F > CI > Br > I - Reactivity decreases -> All group 0 elements (noble gases) are non-reactive. This is because these elements have their outermost shell completely filled. As a result, atoms of these elements show no tendency to lose or gain electrons. NATURE OF OXIDES The elements at the beginning of a period form basic oxides. The elements at the end of a period form acidic oxides. Thus, the basic nature of oxides of the elements in a period decreases in going from left to right. For example, sodium forms a basic oxide, while sulphur and phosphorus form acidic oxides. 3rd period Na Mg Al Si P S Cl sodium forms these elements form basic oxide acidic oxides Oxides become more acidic in this direction > How do the atomic properties of the elements vary in a group and in a period 12 P a g e Periodic Classification of Elements by deepak sir

13 Variation in the properties of elements in a group, and in a period of the periodic table are summarized as follows: A) Very short answer type questions 1. Write a typical Dobereiner's triad. 2. State the law of octaves as given by Newlands. 3. What was the basis of Mendeleefs classification of elements? 4. Why did Mendeleef leave a few gaps in his periodic table? 5. State the modern periodic law. ft. What is the basis of the classification of elements in the modern periodic table? 7. Bv which common name are the elements of Croup I and Croup 17 called? 8. Name an element which is (a) most metallic (b) most nonmetallic 9. How is the position of an element in the periodic table related to the number of valence electrons? (B) Short answer type questions 1. What is meant by horizontal relationship between the elements in the periodic table? 2. To which electronic shell would an incoming electron go in the case of elements belonging to, (a) first period, (b) second period, (c) third period? Based upon this, predict the maximum number of elements which could be placed in these periods. 3. On the basis of electronic configuration, how will you identify, (a) the first element of a period, (b) the last element of a period, (c) an element of the oxygen group, and (d) a noble gas element? 4. Describe the basic character of the oxides of 3rd period elements across the period. (C) Long answer type questions 1. Name the properties of the elements which, (a) increase in going from left to right in a period. (b) decrease in going from left to right in a period. (c) increase in going from top to bottom in a group. (d) decrease in going from top to bottom in a group. 2. By giving suitable examples show that all the elements of a group show similar properties, while the properties of the elements in a period show a gradual change. 3. Given below are the electronic configurations of some elements: (a) 2, 8, 3 (b) 2, 8, 18, 5 (c) 2, 8, 7 (d) 2, 8, 18, 8, 1 (e) 2, 8, 2 (f) 2, 5 (g) 2, 8, 18, 8 Answer the following: (i) to which group does each belong? (ii) to which period does each belong? (iii) which is an inert gas? (iv) which is an alkali metal? (v) which is a halogen? (vi) which elements belong to the same group? (vii) which is the first element of a period? (viii) which elements belong to the same period? (D) Multiple choice questions (Tick (/) the correct answer) 1. All the elements in a group of the periodic table have the same, (a) atomic number (b) mass number (c) atomic mass (d) number of electrons (c) number of valence electrons 2. The long form of the periodic table is based on (a) molecular masses (b) atomic masses (c) atomic numbers (d) mass numbers (c) atomic volumes 13 P a g e Periodic Classification of Elements by deepak sir