Solutions of Periodic Classification Of Elements (Page No - 306) - Chemistry Lakhmir Singh, Class 10 | Extra Documents, Videos & Tests for Class 10 PDF Download

Question 57:
The atomic numbers of the three elements X, Y and Z are 2, 6 and 10 respectively.
 Which two elements belong to the same group ?
 Which two elements belong to the same period ?
 Give reasons for your choice.
 Solution :
(i) X and Z.
X and Z have zero valency hence they belong to same group: noble gases.
(ii) Y and Z.
Y: 2,4 and Z: 2,8 so, both of them belong to second period with two shells filled.

Question 58:
An atom has the electron structure of 2, 7.
 (a) What is the atomic number of this atom ?
 (b) To which of the following would it be chemically similar ?
 7N, 15P, 17CI, isAr
 (c) Why would you expect it to be similar ?
Solution :
(a) 9.
(b) 17 Cl.
(c) Both have the same number of valence electrons (7 electrons each) in their atoms.

Question 59:
Consider the following elements :
 20Ca, gO, isAr, ₁₆S, ₄Be, ₂He Which of the above elements would you expect to be :
 (i) very stable ? (ii) in group 2 of the periodic table ?
 (iii) in group 16 of the periodic table ?
 Solution :
(i) 18 Ar and 2 He (Noble gases) .
(ii) 20 Ca and 4 Be (no. of valence electrons in each = 2).
(iii) 8 O and 16 S (no. of valence electrons in each = 6).

Question 60:
In each of the following pairs, choose the atom having the bigger size :
 (a) Mg (At. No.12) or Cl (At. No. 17)
 (b) Na (At. No. 11) or K (At. No. 19)
 Solution :
(a) Mg since atomic size decreases from left to right in a period.
(b) K since atomic size increases on going down a group .

Question 61:
The atomic numbers of three elements A, B and C are given below :

(i) Which element belongs to group 18 ? (ii) Which element belongs to group 15 ?
 (iii) Which element belongs to group 13 ? (iv) To which period/periods do these elements belong ?
 Solution :
(i) C (2, 8).
(ii) B (2, 5).
(iii) A (2, 3).
(iv) 2nd period (2 shells are filled).

Question 62:
 An element X belongs to 3rd period and group 2 of the periodic table. State :
 (a) number of valence electrons (b) valency (c) metal or non-metal (d) name of the element
 Solution :
(a) 2.
(b) 2 .
(c) Metal .
(d) Magnesium .