Question

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

• A. $Cl,Se$ and $Na$
• B. $Cl,S$ and $Li$
• C. $F,S$ and $Li$
• D. $F,Se$ and $Na$

Answer 1

The correct option is B $Cl,S$ and $Li$

$\begin{array}{cc}\text{Element}& \text{First Electron gain enthalpy(kJ/mol)}\\ \text{Li}& \text{-60}\\ \text{Na}& \text{-53}\\ \text{F}& \text{-320}\\ \text{S}& \text{-200}\\ \text{Cl}& \text{-340}\\ \text{Se}& \text{-195}\end{array}$

Despite $F$ being more electronegative than $Cl$, due to the small size of $F$, $Cl$ would have a more negative value of electron gain enthalpy because of inter-electronic repulsions.

As we go down the group, the negative electron gain enthalpy decreases.