Question

In which of the following arrangements the order is NOT according to the property indicated against it?
(a) Al³⁺ < Mg²⁺ < Na⁺ < F increasing ionic size
(b) B < C < N < O increasing first ionisation enthalpy
(c) I < Br < F < Cl increasing electron gain enthalpy (with negative sign)
(d) Li < Na < K < Rb increasing metallic radius

Answer 1

(b) : As we move from left to right across a period, ionisation enthalpy increases with increasing atomic number. So the order of increasing ionisation enthalpy should be B < C < N < O. 

But N(1s² 2s² 2p³ ) has a stable half filled electronic configuration. So, ionization enthalpy of nitrogen is greater than oxygen. 

So, the correct order of increasing the first ionization enthalpy is B < C < O < N.