Question

The formation of the oxide ion O^2–_(g) requires first an exothermic and then an endothermic step as shown below.
O_(g) + e^– = O^–_(g) ; DH° = –142kJ mol^–1

O^–_(g) + e^– = O^2–_(g) ; DH° = 844kJ mol^–1

This is because
(a) oxygen is more electronegative
(b) oxygen has high electron affinity
(c) O^– ion will tend to resist the addition of another electron
(d) O^– ion has comparatively larger size than oxygen atom.

Answer 1

(c) :- The addition of second electron in an atom or ion is always endothermic.