Question

The first ionisation enthalpies of S, Cl and Ar are 1000, 1256 and 1520 kJ/mol^-1, respectively. Explain the observed trend.

Answer 1

(i) The atomic number increases as, ₁₆S < ₁₇Cl < ₁₈Ar₁.

(ii) Due to decrease in atomic size and increase in effective nuclear charge, Cl binds valence electrons strongly.

(iii) Hence ionisation enthalpy of Cl (1256 kJ mol^-1) is higher than that of S(1000 kJ mol^-1)

(iv) Ar has electronic configuration 3s²3p⁶. Since all electrons are paired and the octet is complete, it has the highest ionisation enthalpy, (1520 kJ mol^-1)