Question

Within each pair of elements$\mathrm{F}\&\mathrm{Cl},\mathrm{S}\&\mathrm{Se}\mathrm{and}\mathrm{Li}\&\mathrm{Na}$, respectively, the elements that release more energy upon an electron gain are:

• A. $\mathrm{Cl},\mathrm{Se}\mathrm{and}\mathrm{Na}$
• B. $\mathrm{Cl},\mathrm{S}\mathrm{and}\mathrm{Li}$
• C. $\mathrm{F},\mathrm{S}\mathrm{and}\mathrm{Li}$
• D. $\mathrm{F},\mathrm{Se}\mathrm{and}\mathrm{Na}$

Answer 1

The correct option is B

$\mathrm{Cl},\mathrm{S}\mathrm{and}\mathrm{Li}$

Electron gain enthalpy:

• The amount of energy released when an electron is injected into an isolated gaseous atom.
• Let us first find out the first electron gain enthalpy for all the given elements:

Elements	First electron gain enthalpy
$\mathrm{Li}$	-60
$\mathrm{Na}$	-53
$\mathrm{F}$	-320
$\mathrm{S}$	-200
$\mathrm{Cl}$	-340
$\mathrm{Se}$	-195

• Due to the increase in size, the electron affinity decreases down the group.
• The electron affinity of $\mathrm{Li}>\mathrm{Na}$ and $\mathrm{S}>\mathrm{Se}$. But in case of $\mathrm{F}$ and $\mathrm{Cl}$, $\mathrm{Cl}>\mathrm{F}$.
• This is because of the size of $\mathrm{F}$ atom, the incoming electron faces more repulsion, and energy release is more in $\mathrm{Cl}$ as compared to $\mathrm{F}$ atom.