The ionisation energy of hydrogen atom is 13.6 eV and the first ionisation energy of helium is 24.6 eV

$\begin{array}{l}\left(\mathrm{I}\right)\mathrm{Hg}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{H}}^{+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-} {\left(\mathrm{IE}\right)}_{\mathrm{H}}=13.6\mathrm{eV}\\ \left(\mathrm{II}\right)\mathrm{Hg}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{He}}^{+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-} {\left(\mathrm{IE}\right)}_{\mathrm{He}}\left(\mathrm{first}\right)=24.6\mathrm{eV}\\ \left(\mathrm{III}\right){\mathrm{He}}^{+}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{He}}^{2+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-} {\left(\mathrm{IE}\right)}_{\mathrm{He}}\left(\mathrm{second}\right)=?=\mathrm{q}\end{array}$

By Bohr's theory of H-atom

$\begin{array}{l}{\left(\mathrm{IE}\right)}_{\mathrm{z}}={\left(\mathrm{IE}\right)}_{\mathrm{H}}=\left(\frac{{\mathrm{Z}}^2}{{\mathrm{n}}^2}\right)\\ {\left(\mathrm{IE}\right)}_{{\mathrm{He}}^{+}}=\left(13.6\right)\left(\frac{2^2}{1^2}\right)\end{array}$

= 54.4 eV 

$\mathrm{He}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{He}}^{2+}\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}$

$\mathrm{\Delta H}=\left(24.6+\mathrm{q}\right)=79.0\mathrm{eV}$

Hence for ${\mathrm{He}}^{2+}\left(\mathrm{g}\right)+2{\mathrm{e}}^{-}\underset{ }{\overset{ }{\to }}\mathrm{He}\left(\mathrm{g}\right)$

$\mathrm{\Delta H}=-79.0\mathrm{eV}$

Note : Second ionisation energy He(g) is the first ionisation of He⁺ (g).

$\mathrm{He}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{He}}^{+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}{\left(\mathrm{IE}\right)}_1 \mathrm{of} \mathrm{He}\left(\mathrm{g}\right)$

${\mathrm{He}}^{+}\left(\mathrm{g}\right)\underset{ }{\overset{ }{\to }}{\mathrm{He}}^{2+}\left(\mathrm{g}\right)+{\mathrm{e}}^{-}{\left(\mathrm{IE}\right)}_2\mathrm{of}\mathrm{He}\left(\mathrm{g}\right)$

${\left(\mathrm{IE}\right)}_1\mathrm{of}{\mathrm{He}}^{+}\left(\mathrm{g}\right)$

Boron has five electrons and +3 means it has now two electron, the concept is mainly applicable to one electron species.