The second period element , for example , for example 'F' has less electron gain enthyalpy than the third period element of the same group of example'Cl' . Why ?

Explanation

In a group elements , the electron gain enthalpy decreases from top to bottom ,But in general the second element in a group I .e `3 ^(rd ) ` period element has greater electron gain enthalpy than the first element i.e `2^(nd ) ` period element . Ex : E.A of F lt E.A of Cl.
This is because .Flourine atom is simller in size than chlorine atom `F_(2 ) ` is also having strong inter electronic repulsions overcome at the experence of a part of the energy liberated . Hence the overall enegy liberated is less than that of chlorine atom .