Question

The ionisation energy of hydrogen atom is $13.6eV$. What will be the ionisation energy of ${He}^{+}$ and ${Li}^{2+}$ ions?

Answer 1

Ionisation energy $=-$ (energy of the $1$st orbit)
Energy of the $1$st orbit of hydrogen $=-13.6eV$
Energy of the $1$st orbit of ${He}^{+}=-13.6\times Z^2(Zfor{He}^{+}=2)$
$=-13.6\times 4eV=-54.4eV$
So, Ionisation energy of ${He}^{+}=-(-54.4)=54.4eV$
Energy of $1$st orbit of ${Li}^{2+}=-13.6\times 9(Zfor{Li}^{2+}=3)$
$=-122.4eV$
Ionisation energy of ${Li}^{2+}=-(-122.4)=122.4eV$