Given that the ionisation energy of the hydrogen atom is 13.6 eV, what is the ionisation energy of a

Question

Given that the ionisation energy of the hydrogen atom is 13.6 eV, what is the ionisation energy of a singly ionised helium atom ($He^+$)?

Professor PC Thomas Classes · Answer

For hydrogenic atoms, energy $E \propto Z^2$. Since Helium has $Z=2$, its ionisation energy is $13.6 	imes 2^2 = 54.4$ eV.