Ionization energy of Cl^– is 349 KJ/mole. Approximate heat liberated when an electron is added to gaseous chlorine atom.

$$\begin{gathered} \mathrm{Given} {\mathrm{Cl}}^{-}\left(\mathrm{g}\right) \to \mathrm{Cl}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}, \mathrm{\Delta H} = +349 \mathrm{KJ} \\ \mathrm{Cl}\left(\mathrm{g}\right) + {\mathrm{e}}^{-} \to {\mathrm{Cl}}^{-}\left(\mathrm{g}\right), \mathrm{\Delta H} = -349 \mathrm{KJ} \end{gathered}$$

6.023x10²³ atoms of Cl(g) →Cl^-(g) releases 349 Kj
One atom of Cl(g)→Cl^- releases  = ?

$=\frac{\left(349\times 1000\right)\mathrm{J}}{6.023\times {10}^{23}}=5.9\times {10}^{-19}\mathrm{J}$