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

Explanation

Recurrence of similar properties of elements at definite regular intervals with increasing atomic number i.e., according to their electronic configurations is known as periodicity. Any property which is periodic in nature is called periodic property.
Electron gain enthalpy: Electron gain enthalpy increase in a period from left to right because the size of the atom decreases and the nature of the element changes from metallic to non -metallic nature when we move from left to right in a period.
Electron gain enthalpy decrease from top to bottom in a group because there is an increase in the atomic size. But the second element has greater electron gain enthalpy than the first element. e.g. : Chlorine has more electron affinity value `(-348 kJ mol^(-1))` than Fluorine `(-333 kJ mol^(-1))`. It is because fluorine atom is smaller in size than chlorine atom. There is repulsion between the incoming electron and electrons already present in fluorine atom i.e., due to stronger inter electronic repulsions.