Define the term ‘electron affinity. State its unit.

Hint: It is the quantitative measurement of energy. The energy is released when an electron is added to a neutral atom. The more negative the electron affinity value, the higher an atom’s affinity for electrons. The process is exothermic as there is the release
of a high amount of energy.

Complete answer:
Electron affinity is defined as the amount of energy released when an electron is added to a neutral atom to form an anion. It is the change in the potential energy of an atom when an electron is added to a neutral gaseous atom to form a negative ion. The unit of electron affinity is KJ per mole $\left( {\dfrac{{KJ}}{{mol}}} \right)$.
Not all elements form a stable negative ion. In the periodic table, the electron affinity increases from going left to right across the period, because of the increased nuclear attraction. And electron affinity decreases going down the group because the electrons are being added up which increases the size of the atom. The smaller the size of an atom higher is the electron affinity and vice-versa, this is because as the size of the atom decreases the nuclear force increases hence the electron gain enthalpy increases. With the increases in effective nuclear charge, the electron affinity also increases. Halogens are considered to have the highest electron affinity. This is because of their smaller size. Also, halogens have a high effective nuclear charge and almost full outer shell electrons. Whenever an electron is added to a neutral halogen atom, the energy released is very high because of its high electron affinity.

Note:
Metals usually have a lower value of electron affinity. This is because it is easier for them to lose an electron from their valence shell and form a cation rather than gaining electrons to form an anion. The first electron affinity is negative and the second electron affinity is positive. Electron affinities of inert gases are zero. This is because their atoms have a stable electronic configuration in their shell.