The second period element 'F' has electrons gain enthaly than the third period elements of same group 'Cl ' .Why ?

Explanation

In a group of 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 Fluorine atom is smaller in size than chloride atom .`F_(2) ` also has strong inter electronic repulsions . In the addition of an electron to Fluorine atom , the electronic repulsions overcome overcome at the expense of a part of the energy liberated . hence the overall liberated is less than that of chlorine atom .