Chapter 3: Classification of Elements and Periodicity in Properties

The formation of the oxide ion, O2– (g), from oxygen atom requires first an exothermic and then an endothermic step as shown below: 

O (g) + e– → O– (g) ; ∆ H = – 141 kJ mol–1 

O– (g) + e– → O2– (g); ∆ H = + 780 kJ mol–1 

Thus process of formation of O2– in gas phase is unfavourable even though O2– is isoelectronic with neon. It is due to the fact that, 

(i) oxygen is more electronegative. 

(ii) addition of electron in oxygen results in larger size of the ion. 

(iii) electron repulsion outweighs the stability gained by achieving noble gas configuration. 

(iv) O– ion has comparatively smaller size than oxygen atom.

Comprehension given below is followed by some multiple choice questions. Each question has one correct option. Choose the correct option. In the modern periodic table, elements are arranged in order of increasing atomic numbers which is related to the electronic configuration. Depending upon the type of orbitals receiving the last electron, the elements in the periodic table have been divided into four blocks, viz, s, p, d and f. The modern periodic table consists of 7 periods and 18 groups. Each period begins with the filling of a new energy shell. In accordance with the Aufbau principle, the seven periods (1 to 7) have 2, 8, 8, 18, 18, 32 and 32 elements respectively. The seventh period is still incomplete. To avoid the periodic table being too long, the two series of f-block elements, called lanthanoids and actinoids are placed at the bottom of the main body of the periodic table.

(a) The element with atomic number 57 belongs to 

(i) s-block 

(ii) p-block 

(iii) d-block 

(iv) f-block

(iv) 4f145d106s26p4

(iv) 102

(iv) 1s2 2s2 2p6 3s2 3p6 3d7 4s2

(iv) light metals

Electronic configurations of four elements A, B, C and D are given below : (A) 1s2 2s2 2p6 (B) 1s2 2s2 2p4 (C) 1s2 2s2 2p6 3s1 (D) 1s2 2s2 2p5 

Which of the following is the correct order of increasing tendency to gain electron : 

(i) A < C < B < D 

(ii) A < B < C < D 

(iii) D < B < C < A 

(iv) D < A < B < C