Elements of Group 16 generally show lower value of first ionisation enthalpy compared to the corresponding periods of group 15. Why?

Explanation

(a) Elements of group `16` generally show lower value of first ionisation enthalpy as compared to the corresponding elements of group `15`. This is due to the extra stability of half-filled electronic configuration of group `15` elements `(ns^2 np^3)`, hence more energy is required to remove an electron as compared to those of group `16` elements `(ns^2 np^4)`.
(b) Down the group `(darr)`, as the size of the element increases, `E-H` bond length increases and hence `E-H` bond strength decreases and bond dissociation energy decreases. In other words, `H-S` bond dissociation energy is more than that of `H-Te` bond dissociation energy and hence `H-S` bond ionises less readily than `H-Te` bond. Therefore, `H_2 S` is a weaker acid than `H_2 Te`.