Question

In which of the following options the order of arrangement does not agree with the variation of property indicate against it ?

• A. $B < C < N < O$ (increasing first ionisation enthalpy)
• B. $I < Br < Cl < F$ (increasing electron gain enthalpy)
• C. $Li < Na < K < Rb$ (increasing metallic radius)
• D. $Al^{3+} + < Mg^{2 +} < Na^+ + < F-$ (increasing ionic size)

Answer 1

Answer: (A), (B)

Ionisation enthalpy of N is greater than O because N has half filled p subshell. B < C < O < N ($I^{st}$ ionization enthalpy)
$2p^1 \; 2p^2 \; 2p^4 \; 2p^3$
Electron gain enthalpy of F is less than Cl because in F small size of 2p orbital result in high electron density so inter electronic repulsion is high.
I < Br < F < Cl (electron gain enthalpy)