Question

In which of the following arrangements, the order is not according to the property indicated against it?

• A. $Al ^{3+}< Mg ^{2+}< Na ^{+}< F ^{-}$ : Increasing ionic size
• B. $B < C < N < O$ : Increasing first ionisation enthalpy
• C. $I< B r< F< C l$ : Increasing electron gain enthalpy with negative sign
• D. $Li < Na < K < Rb$ : Increasing metallic radius

Answer 1

Answer: (B)

(a) All of these are isoelectronic ions with decreasing nuclear charge and increasing ionic size.

$Al ^{3+}(Z=13)< Mg ^{2+}(Z=12)< Na ^{+}(Z=11)< F ^{-}(Z=9)$

(b) $I E_{1}$ of these elements are in the order:

$B< C< O< N$

$I E_{1}$ of $N \left(1 s^{2} 2 s^{2} 2 p^{3}\right)$ is more than $O \left(1 s^{2} 2 s^{2} 2 p^{4}\right),$ due to

extra stable half filled $2 p$ -subshell in the valence shell. Thus, the given order is not correct.

(c) Electron gain enthalpies of these elements are

$ I \left(-295 kJ mol ^{-1}\right)< Br \left(-325\, kJ\, mol ^{-1}\right)< F \left(-328 \,kJ\, mol ^{-1}\right) < Cl \left(-349\, kJ\, mol ^{-1)}\right.$

(d) Metallic radius increases down the group.