Q3 / 170 · Manipal (MET) 2015 · Chemistry
In which of the following arrangement the order is not according to the property indicated against it?
- A$\mathrm{Li} \lt \mathrm{Na} \lt \mathrm{K} \lt \mathrm{Rb}$ (increasing metallic radius)
- B$1 \lt \mathrm{Br} \lt \mathrm{F} \lt \mathrm{Cl}$ (increasing electron gain enthalpy, with negative sign)
- Correct answerC$B \lt C \lt N \lt O$ (increasing . first ionisation enthalpy)
- D$\mathrm{Al}^{3+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Na}^{+} \lt \mathrm{F}^{-}$(increasing ionic size)
Solution
(a) Metallic radii increase in a group from top to bottom. Thus, $\mathrm{Li} \lt \mathrm{Na} \lt \mathrm{K} \lt \mathrm{Rb}$ is true.
(b) Electron gain enthalpy of $\mathrm{Cl}\gt\mathrm{F}$ and afterwards it decreases along a group.
(c) Ionisation enthalpy increase along a period left to right but due to presence of half-filled orbital in N , ionisation enthalpy of $\mathrm{N}\gt\mathrm{O}$. Thus $\mathrm{B} \lt \mathrm{C} \lt \mathrm{N} \lt \mathrm{O}$ is incorrect.
(d) For isoelectronic species, ionic size $\propto \frac{1}{\text { atomic number }}$
Thus , $\mathrm{Al}^{3+} \lt \mathrm{Mg}^{2+} \lt \mathrm{Na}^{+} \lt \mathrm{F}^{-}$is true.
Hence, the correct answer is option: 'C'.