In which of the following options order of arrangement does not agree with the variation of property…

In which of the following options order of arrangement does not agree with the variation of property indicated against it?
  1. $\mathrm{Al}^{3+} < \mathrm{Mg}^{2+} < \mathrm{Na}^{+} < \mathrm{F}$ (increasing ionic size)
  2. $\mathrm{B} < \mathrm{C} < \mathrm{O} < \mathrm{N}$ (increasing first ionisation enthalpy)
  3. l $ < \mathrm{Br} < \mathrm{Cl} < \mathrm{F}$ (increasing electron gain enthalpy)
  4. $\mathrm{Li} < \mathrm{Na} < \mathrm{K} < \mathrm{Rb}$ (increasing metallic radius)

Solution

Electron gain enthalpy becomes less negative on moving down a group due to increase in atomic size. However, the electron gain enthalpy of $\mathrm{F}$ is less negative than $\mathrm{CI}$ due to electron-electron repulsion in small-sized $\mathrm{F}$ atom. Thus, the correct order is $\mathrm{F} < \mathrm{CI}>\mathrm{Br}>\mathrm{l}$. $\mathrm{F}, \mathrm{CI}, \mathrm{Br}$ and $\mathrm{I}$ have $\Delta e_g H$ values (in $\mathrm{kJ}^{\mathrm{Jmol}} \mathrm{I}^{-1}$ ) $-328,-349,-325$ and -295 respectively. That order of arrangement does not agree with the variation of property. Hence, the correct option is (3).

Asked in: AP EAMCET 2020 (22 Sep Shift 1)

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