Question

The electron affinity of sulphur is -200kJ/mol. Then the electron affinity of oxygen is:
(A) -142kJ/mol
(B) -702kJ/mol
(C) -332kJ/mol
(D) -348kJ/mol

Answer 1

Hint: The amount of energy released when an electron is added to a neutral atom to form an anion is known as the electron affinity of an element. It is the potential energy required to change a neutral atom into anion. More negative the electron affinity of an element, the more favourable the addition process is.

Complete step by step answer:
-The electron affinity is also known as electron gain enthalpy. It is represented as EA.
$A(g)+{{e}^{-}}\to {{A}^{-}}(g)$

-Electron affinity depends on the atomic size as well as the nuclear charge.
-The energy value of electron gain enthalpy can be endothermic or exothermic depending on the nature of the element.
-The electron gain enthalpy is measured in kJ/mol or kcal/mol.
-The electron affinity increases down the group due to a decrease in the effective nuclear charge and decrease in electronegativity.
-Group 16 has a high electron affinity. As we move down the group from Oxygen to sulphur, the electron affinity value increases and then decreases from S to Po. The oxygen has lower electron affinity and this is an exception.
-The electron density in the 2p energy shell in oxygen is high due to the small size of oxygen. Due to this, there is some resistance to the incoming electron and thus the electron affinity value of Oxygen is less.
-Therefore the electron affinity of oxygen is -142 kJ/mol,
So, the correct answer is “Option A”.

Note: Electron affinity will be low when the atomic size will be very less. Electron affinity will be low when the last orbital is too far from the nucleus due to less effective nuclear charge.