Question

Which of the following will have lowest electron affinity?
A) Nitrogen
B) Oxygen
C) Carbon
D) Sulphur

Answer 1

Electron affinity is the amount of energy released when an electron is added to a neutral gaseous atom in its lowest energy state to form an anion.
In the periodic table generally, electron affinity increases moving from left to right, across the period and decreases moving down the group, from top to bottom. However, there are few exceptions to this trend.

Complete answer:
Determine the group and period number of all given elements.

Element	Group number	Period number
Carbon	IV A	2
Nitrogen	V A	2
Oxygen	VI A	2
Sulphur	VI A	3

Out of the elements given to us, an element carbon is present on the left side of the periodic table and element sulphur is present in the third period while other elements (Carbon, Nitrogen and Oxygen) are present in the second period of the periodic table.
Write the electronic configuration of all electronic configurations of all given atoms.
The atomic number of carbon is 6.
So, the electronic configuration of carbon is
${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{2}}}$
The atomic number of nitrogen is 7
So, the electronic configuration of nitrogen is
${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{3}}}$
The atomic number of oxygen is 8.
So, the electronic configuration of oxygen is
${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{4}}}$
The atomic number of sulphur is 16.
So, the electronic configuration of sulphur is
${\text{1}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{ 3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^4}$
Now, using the position of elements in the periodic table and their electronic configuration we can determine the element having the lowest electron affinity as follows:
As per the trend, electron affinity increases from left to right across the period so carbon would have lower electron affinity among carbon, nitrogen and oxygen as all these three elements are present in the same period. However, even though nitrogen is present in VA group, it has lower electron affinity than IVA group element carbon due to the extra stability of half-filled ${\text{2}}{{\text{p}}^{\text{3}}}$ orbital of nitrogen.
As per the general trend, electron affinity decreases down the group. However in case of p-block elements; the electron affinity of the second element in a group is greater than the first element in the same group. Thus, in case of oxygen and sulphur which are present in the same group, sulphur has greater electron affinity than oxygen.
Thus, decreasing order of electron affinity of given elements is,
Sulphur > Oxygen > Carbon > Nitrogen
So, out of the given elements to us, nitrogen has the lowest electron affinity.

Hence, the correct option is (A) Nitrogen.

Note: As the atomic size decreases from left to right across the period and increases down the group electron affinity increases across the period and decreases down the group. However, there are few exceptions due to the extra stability of half-filled and completely filled orbitals.