For hydrogen-like species (single-electron systems), the ionization energy is: $IE=(13.6\text{ eV})\times Z^2$

1. Hydrogen ($H$): $Z=1⟹IE=13.6\text{ eV}$.
2. Helium Ion ($H{e}^{+}$): $Z=2$. $I{E}_{H{e}^{+}}=13.6\times 2^2=13.6\times 4=54.4\text{ eV}$

• The higher charge of the helium nucleus ($+2$) attracts the single electron much more strongly than the hydrogen nucleus ($+1$), requiring 4 times more energy to remove it.