Question

Which of the following has maximum value of EA:
A.$1{s^2},2{s^2},2{p^5}$
B.$1{s^2},2{s^2},2{p^6},3{s^2},3{p^5}$
C.$1{s^2}$
D.$1{s^2},2{s^2}$

Answer 1

To answer this question, you must recall the concept of electron affinity. The element which tends to release the maximum amount of energy in addition to an electron in the valence shell of its isolated atom in the gaseous state would have the highest value of electron affinity. In other words, electron affinity is the measure of how easily an atom can gain an electron.

Complete answer:
The amount of energy released in addition to an electron in the valence shell of an isolated atom in the gaseous state to form an anion is defined as electron affinity. It is also known as electron gain enthalpy of the atom. The electron affinity of an atom depends on the atomic number, electronegativity and size of the atom.
In the given question the elements given in options A and B need one more electron in their valence shell to get a noble gas-like electronic configuration. Thus, they clearly have a higher electron affinity than options C and D.
Option A represents fluorine and option B represents chlorine.
Based on the electronegativities, we can say that fluorine must have a higher electron affinity than chlorine. But that is not the case. Due to the extremely small size of fluorine, the incoming electron faces repulsion from the valence shell electrons and thus it is difficult to gain the electron, whereas in chlorine the repulsions are quite less due to larger size.

Thus, the correct answer is B.

Note:
In the modern periodic table, as we move from left to right across a period, the electron affinity of elements increases as the size decreases and the electronegative character increases. When we move from top to bottom in a group, the electron affinity decreases as the electronegative character of elements decreases.