Which element among the following is most probable to have the following values? $\begin{aligned} \Delta_i H_1 & =419 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \Delta_i H_2 & =305 \mathrm{~kJ} \mathrm{~mol}^{-1} \\ \Delta_{\mathrm{eg}} H & =-48 \mathrm{~kJ} \mathrm{~mol}^{-1}\end{aligned}$

Atomic number of $\mathrm{K}=19$ Electronic configuration $[\mathrm{Ar}] 4 s^1$ $\mathrm{K} \longrightarrow \mathrm{K}^{+}+\mathrm{l} e^{-} ; \quad \Delta_i H_1=419 \mathrm{~kJ} / \mathrm{mol}$ Potassium has only one electron in outermost shell which looses easily to attain noble gas configuration. $\mathrm{K}^{+} \longrightarrow \mathrm{K}^{2+}+1 e^{-} ; \quad \Delta_i H_2=3051 \mathrm{~kJ} / \mathrm{mol}$ Potassium ion has stable noble gas configuration similar to that of argon, from which the removal of electron becomes difficult resulting into increase in $\mathrm{IE}_2$. Potassium being a metal is electropositive in nature, which has low electron affinity resulting into low electron gain enthalpy.