Question

Electron affinity is positive, when:
(a) $O$ changes to $O^{-}$
(b) $O^{-}$ changes into $O^{2-}$
(c) $O$ changes into $O^{+}$
(d) electron affinity is always negative

Answer 1

Hint: The electron affinity of an atom or molecule is defined as the amount of energy released when an electron is attached to a neutral atom or molecule in the gaseous state to form a negative ion.

Complete step by step answer:
We need to tell when electron affinity is positive,
We know that: The electron affinity is a measure of the attraction between the incoming electron and the nucleus - the stronger the attraction, the more energy is released. The factors which affect this attraction are exactly the same as those relating to ionization energies - nuclear charge, distance and screening.
Now, if a reaction is exothermic, the change in energy is negative. This means that the electron affinity is positive.
As we know, the first electron affinities can be negative, positive, or zero. Second electron affinities are always positive.
When $O^{-}$ changes to $O^{2-}$ change is endothermic because $O^{-}$ repels the incoming electron due to similar charge, hence, it needs energy to accept the electron. The second electron affinity of oxygen is particularly high because the electron is being forced into a small, very electron-dense space. So, electron affinity is positive.
So, we can say that electron affinity is positive when $O^{-}$changes into $O^{2-}$,
So, the correct answer is “Option B”.

Note: In general, exceptions arise when new subshells are being filled/half-filled, or in cases where the atom is too small. In the first case, Be and Mg are interesting examples; they have a positive electron affinity because of the energy difference between the s and p subshells.