What is a periodic property? How the following properties vary in a group and in a period? Expain
(b) Electron gain enthalpy.

Explanation

The repetition of similar chemical properties of elements at regular intervals with increasing atomic number is called periodic property or periodicity.
(b) Electron gain enthalpy or Electron affinity: In a period, as we go from left to right, Electron affinity gradually in creases.
Reason: As we go from left to right in a period, the atomic radius gradually decreases. As a result of which, the attraction between the added electron and the nucleus increases. Hence, electron affinity increases in a period.
In a group as we go from top to bottom the electron affinity values gradually decrease.
Reason: As we go down a group, the atomic radius gradually increases. As a result of which, the attraction between the added electron and the nucleas decreases. Hence, electron affinity decreases in a group.