Question

Arrange Sulphur (S), Oxygen (O) and Selenium (Se) in ascending order of their electron affinity.
A) Se < S < O
B) O < Se < S
C) S < O < Se
D) S < Se < O

Answer 1

Hint: Electron affinity is the potential energy change of the atom when electron is added to a neutral atom to form a negative ion. It is the energy released when an electron is added in an atom. If it is difficult to add electrons in an atom then the value of electron affinity will be smaller.

Complete answer:
Electron affinity becomes more negative when electron gets added easily, that is the conditions are favorable for the addition of electron then the electron affinity will be negative.
Conditions on which electron affinity depends are as following:
> Nuclear size: If the nuclear charge increases, the attraction for the incoming electron increases. Hence the value of electron affinity increases.
> Atomic Size: If the size of the atom is more, greater will be the distance between the nucleus and incoming electron. Hence the incoming electron experiences less forces of attraction towards the nucleus of an atom. Therefore, the value of electron affinity will be smaller.
> Electronic Configuration: Atoms with stable electronic configuration have a less tendency to accept an electron. Hence they will have smaller electron affinity values.
> Electron Affinity behavior in a period: In a period from left to right electron affinity increases as addition of electron becomes easy as we move from left to right in a period.
> Electron Affinity behavior in a group: Down the group electron affinity decreases as addition of electron becomes difficult down the group.
And in the above case the elements lie in group 16 so the order of their electron affinity should be Se < S < O but there is exception in the case of Oxygen (O) as oxygen have very less electron affinity as compared to S and Se as in Oxygen it is difficult to add electrons in oxygen as size of oxygen is very small due to which when electron is added in it then it results high electronic repulsion in it. So the order according to the above exception will be O < Se < S.

So the correct option is (B) in this question.

Note: It is to be remembered that if the size of the atom is small then electron affinity will be less as adding electrons in such atoms will result in more electron repulsions. So due to the same reason oxygen will have very less electron affinity than expected.Electron affinities are low or almost zero in elements having stable electron configurations (half filled and completely filled valence subshells) because of the small tendency to accept additional electrons.