Ch-5 Periodic Classification of Elements NCERT Solutions
NCERT Book, Page 81
Q.1. Did Dobereiner’s triads also exist in the columns of Newlands’ octaves ? Compare and find out. Ans. Yes, Dobereiner’s triads also exist in the columns of Newlands’ classification of elements based on the law of octaves. Consider the elements lithium (Li), sodium (Na) and potassium (K) which are present in the second column of Newlands’ classification of elements. Now, if we start with lithium as the 1st element, then the 8th element from it is sodium, and according to Newlands’ law of octaves, the properties of 8th element sodium should be similar to those of the first element lithium. Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newlands’ law of octaves, the properties of 8th element potassium should be similar to those of the 1st element sodium. This means that according to Newlands’ law of octaves, the elements lithium, sodium and potassium should have similar chemical properties. We also know that lithium, sodium and potassium form a Dobereiner’s triad having similar chemical properties. From this we conclude that Dobereiner’s triads also exist in the columns of Newlands’ octaves.
Q.2. What were the limitations of Dobereiner’s classification of elements ? Ans. The main limitation of Dobereiner’s classification of elements was that it failed to arrange all the then known elements in the form of triads of elements having similar chemical properties. Dobereiner could identify only three triads from the elements known at that time. So, his classification of elements was not much successful. Another limitation was that Dobereiner failed to explain the relation between atomic masses of elements and their chemical properties.
Q.3. What were the limitations of Newlands’ law of octaves ? Ans. Newlands’ law of octaves for the classification of elements had the following limitations : (i) Newlands’ law of octaves was applicable to the classification of elements up to calcium only. After calcium, every eighth element did not possess the properties similar to that of the first element. Thus, Newlands’ law of octaves worked well with lighter elements only. (ii) Newlands assumed that only 56 elements existed in nature and no more elements would be discovered in the future. But later on, several new elements were discovered whose properties did not fit into Newlands’ law of octaves. (iii) In order to fit elements into his table, Newlands put even two elements together in one slot and that too in the column of unlike elements having very different properties. For example, the two elements cobalt (Co) and nickel (Ni) were put together in just one slot, and that too in the column of elements like fluorine, chlorine and bromine which have very different properties from these elements.
NCERT Book, Page 85
Q.1. Use Mendeleev’s periodic table to predict the formulae for the oxides of the following elements : K, C, Al, Si, Ba Ans.
(i) The element K (potassium) is in group I of Mendeleev’s periodic table in which the general formula of the oxides of elements is R2 O. So, the formula of oxide of K will be K2 O.
(ii) The element C (carbon) is in group IV of Mendeleev’s periodic table in which the general formula of the oxides of elements is RO2. So, the formula of oxide of C will be CO2.
(iii) The element Al (aluminium) is in group III of Mendeleev’s periodic table in which the general formula of the oxides of elements is R2 O3. So, the formula of oxide of Al will be Al2 O3.
(iv) The element Si (silicon) is in group IV of Mendeleev’s periodic table in which the general formula for the oxides of elements is RO2. So, the formula of the oxide of Si will be SiO2.
(v) The element Ba (barium) is in group II of Mendeleev’s periodic table in which the general formula for the oxides of elements is RO. So, the formula of oxide of Ba will be BaO.
Q.2. Besides gallium, which other elements have since been discovered for which gaps were left by Mendeleev in his periodic table ? (any two) Ans. Scandium (Sc) and Germanium (Ge).
Q.3. What were the criteria used by Mendeleev in creating his periodic table ? Ans. Mendeleev used two criteria in creating his periodic table : (i) increasing atomic masses, and (ii) grouping together of elements having similar chemical properties. Mendeleev took the formulae of the oxides and hydrides formed by the elements as the basic chemical properties of elements for their classification in the form of a periodic table.
Q.4. Why do you think the noble gases are placed in a separate group ? Ans. The noble gases are placed in a separate group because they are chemically very inert or unreactive (having completely filled outermost electron shells).
NCERT Book, Page 90
Q.1. How could the Modern periodic table remove various anomalies of Mendeleev’s periodic table ? Ans. When the elements are arranged according to their atomic numbers on the basis of modern periodic law, then all the anomalies (or defects) of Mendeleev’s classification disappear. This is discussed below.
(i) Explanation for the Position of Isotopes. All the isotopes of an element have the same number of protons, so their atomic number is also the same. Since all the isotopes of an element have the same atomic number, they can be put at one place in the same group of the periodic table. For example, both the isotopes of chlorine, Cl-35 and Cl-37, have the same atomic number of 17, so both of them can be put at one place in the same group of the periodic table.
(ii) Explanation for the Position of Cobalt and Nickel. The atomic number of cobalt is 27 and that of nickel is 28. Now, according to modern periodic law, the elements are arranged in the order of increasing atomic numbers. So, cobalt with lower atomic number (27) should come first and nickel with higher atomic number (28) should come later, even if their atomic masses are in the wrong order.
(iii) Explanation for the Position of Hydrogen. Hydrogen element has been placed at the top of group 1, above the alkali metals because the electronic configuration of hydrogen is similar to those of alkali metals. Both, hydrogen as well as alkali metals have 1 valence electron each.
Q.2. Name two elements you would expect to show chemical reactions similar to magnesium. What is the basis for your choice ? Ans. The two elements which will show chemical reactions similar to magnesium are beryllium (Be) and calcium (Ca). This is because beryllium and calcium belong to the same group of periodic table as magnesium (which is group 2). All of them have similar electronic configurations with 2 valence electrons each.
Q.3. Name : (a) three elements that have a single electron in their outermost shells. (b) two elements that have two electrons in their outermost shells. © three elements with filled outermost shells.
Ans. (a) Three elements that have a single electron in their outermost shells are : Lithium, Sodium and Potassium. (b) Two elements that have two electrons in their outermost shells are : Magnesium and Calcium. © Three elements with completely filled outermost shells are : Helium, Neon and Argon.
Q.4. (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) Helium is an unreactive gas and neon is a gas of extremely low reactivity. What, if anything, do their atoms have in common ? Ans. (a) Yes, there is a similarity in the atoms of lithium, sodium and potassium elements. All these elements have similar electronic configurations having 1 electron each in their valence shells. The electronic configurations of lithium, sodium and potassium are given below : Lithium : 2, 1 Sodium : 2, 8, 1 Potassium : 2, 8, 8, 1
(b) Helium and neon are noble gases. The common thing between helium and neon is that both of them have the valence shells (outermost shells) completely filled with electrons. Helium has 2 electrons in its K valence shell (K shell can accommodate only a maximum of 2 electrons), whereas neon has 8 electrons in its valence shell.
Q.5. In the modern periodic table, which are the metals among the first ten elements ? Ans. In the modern periodic table, the first ten elements (having atomic numbers from 1 to 10) are : H, He, Li, Be, B, C, N, O, F, Ne Out of these 10 elements only 2 elements, Lithium (Li) and Beryllium (Be) are metals.
Q.6. By considering their position in the periodic table, which one of the following elements would you expect to have maximum metallic character ? Ga, Ge, As, Se, Be Ans. The maximum metallic character is found in elements on the extreme left side of the periodic table. Out of the above given elements, Be (Beryllium) will have the maximum metallic character because it is on the extreme left side in the periodic table (in group 2)
NCERT Book, Pages 91 and 92
Q.1. 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 © The atoms lose their electrons more easily (d) The oxides become more acidic Ans. © The atoms lose their electrons more easily.
Q.2. Element X forms a chloride with the formula XCl2 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 © Al (d) Si Ans. (b) Mg
Since element X forms a chloride XCl2, so the valency of X is 2. Now, out of Na, Mg, Al and Si, the element of valency 2 is Mg. So, X would be in the same group as that of Mg (because all the elements of the same group have equal valency).
Q.3. Which element has : (a) two shells, both of which are completely filled with electrons ? (b) the electron configuration 2, 8, 2 ? © 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 ?
Ans. (a) Neon (2, 8) (b) Magnesium (2, 8, 2) © Silicon (2, 8, 4) (d) Boron (2, 3) (e) Carbon (2, 4)
Q.4. (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 ? Ans. (a) The element boron is in column (or group) 13 of the periodic table and has a valency of 3. So, all the elements in the same column of periodic table as boron will have valency of 3. (b) The element fluorine is in column (or group) 17 of the periodic table and has a valency of 1. So, all the elements in the same column of periodic table as fluorine will have a valency of 1.
Q.5. 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 ? (Atomic numbers are given in parentheses) N (7) F (9) P (15) Ar (18) Ans. (a) 17 (b) F(9)
(a) The atomic number of this element can be obtained by adding all the electrons present in its electronic configuration. So, the atomic number of the given element having electronic configuration 2, 8, 7 is 2 + 8 + 7 = 17.
(b) The electronic configuration of the given element 2, 8, 7 shows that this element has 7 valence electrons in its atoms. This element will be chemically similar to that element which has the same number of valence electrons (7 valence electrons) in its atoms. To know the number of valence electrons in the elements N, F, P and Ar we have to write their electronic configurations by using their atomic numbers.
(i) The atomic number of N is 7, so its electronic configuration is 2, 5. It has 5 valence electrons (and not 7).
(ii) The atomic number of F is 9, so its electronic configuration is 2, 7. It has 7 valence electrons just like that of the given element. So, the given element of atomic number 17 will be chemically similar to the element fluorine (F) of atomic number 9. This is because both of them have similar electronic configurations, each having the same number of (7) valence electrons.
The atomic number of P is 15, so its electronic configuration is 2, 8, 5 (it has 5 valence electrons). The atomic number of Ar is 18, so its electronic configuration is 2, 8, 8 (it has 8 valence electrons). Thus, neither element P (phosphorus) nor Ar (argon) have 7 valence electrons in their atoms.
Q.6. The positions of three elements A, B and C in the periodic table are shown below :
(a) State whether A is a metal or non-metal. (b) State whether C is more reactive or less reactive than A. © Will C be larger or smaller in size than B ? (d) Which type of ion, cation or anion, will be formed by element A ? Ans.
(a) Element A is in group 17. Now, group 17 is on the right side of the periodic table where non-metals (called halogens) are placed. So, element A is a non-metal. It is a halogen.
(b) In group 17 of halogens, the chemical reactivity decreases on going down in a group. Thus, element C will be less reactive than element A.
© On going from left to right in a period, the size of atoms decreases. So, the atom of C will be smaller in size than an atom of B.
(d) Element A of group 17 has 7 valence electrons. So, it will accept 1 electron to form a negatively charged ion, A– . The negatively charged ion is called an anion. Thus, element A will form an anion.
Q.7. Nitrogen (atomic number 7) and phosphorus (atomic number 15) belong to group 15 of the periodic table. Write the electronic configurations of these two elements. Which of these will be more electronegative ? Why ?
Ans. The atomic number of nitrogen is 7 so its electronic configuration is 2, 5. The atomic number of phosphorus is 15 so its electronic configuration is 2, 8, 5. We can see that nitrogen atom has two electron shells whereas phosphorus atom has three electron shells. Due to the presence of only two electron shells in it, a nitrogen atom is smaller than a phosphorus atom. Nitrogen will be more electronegative because its atom has a small size due to which the attraction of its nucleus for the incoming electron is more.
Q.8. How does the electronic configuration of an element relate to its position in the modern periodic table ? Ans. The electronic configuration of an element tells us two things : ‘the number of electron shells’ and ‘the number of valence electrons’ in one atom of the element.
(i) The ‘number of electron shells’ in an atom of the element gives us the ‘period number’ of the element in the periodic table. If an element has 2 electron shells in its atom, it belongs to 2nd period, if an element has 3 electron shells, it belongs to 3rd period, and so on. For example, the electronic configuration of carbon element is K L (2,4). Since the carbon atom has two electron shells (K and L), therefore, it belongs to 2nd period of the periodic table.
(ii) The ‘group number’ of an element in the periodic table having up to two valence electrons is ‘equal to the number of valence electrons’. If an element has 1 valence electron, it belongs to group 1 and if the element has 2 valence electrons, then it will be in group 2 of the periodic table. For example, the electronic configuration of sodium element is K L M (2, 8 1). Since sodium has 1 valence electron (in M shell), so it belongs to group 1 of the periodic table.
(iii) The ‘group number’ of an element having more than 2 valence electrons is equal to the ‘number of valence electrons plus 10’. If an element has 3 valence electrons, its group number will be 3 + 10 = 13, if the element has 4 valence electrons, then its group number will be 4 + 10 = 14, and so on. For example, the electronic configuration of aluminium is K L M (2 8 3). Since aluminium has 3 valence electrons (in M shell), therefore, it belongs to group number 3 + 10 = 13 of the periodic table.
Q.9. 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 ?
Ans. The atomic number of calcium is 20, so its electronic configuration is 2, 8, 8, 2. Thus, calcium has 2 valence electrons (in its outermost shell). Now, that element which has 2 valence electrons will have physical and chemical properties resembling that of calcium. We have now to write the electronic configurations of all the elements one by one. The electronic configuration of element having atomic number 12 is 2, 8, 2. It has 2 valence electrons just like calcium. So, the element having atomic number 12 will have physical and chemical properties resembling that of calcium.
Q.10. Compare and contrast the arrangement of elements in Mendeleev’s periodic table and the Modern periodic table. Ans.
