$(i)$ What is the energy of an electron for hydrogen in $J \ atom^{-1}$ and $J \ mol^{-1}$?
$(ii)$ Calculate the energy of this electron per mole for the first transition ($n=1$ to $n=2$).
$(iii)$ Calculate the ionization energy of hydrogen.

(N/A) $(i)$ The energy of an electron in the ground state $(n=1)$ of hydrogen is given by $E_n = -2.18 \times 10^{-18} \ J \ atom^{-1}$.
In $J \ mol^{-1}$,$E = (-2.18 \times 10^{-18} \ J \ atom^{-1}) \times (6.022 \times 10^{23} \ atom \ mol^{-1}) = -1.312 \times 10^6 \ J \ mol^{-1} = -1312 \ kJ \ mol^{-1}$.
$(ii)$ For the first transition ($n=1$ to $n=2$),$\Delta E = E_2 - E_1 = -2.18 \times 10^{-18} \times (\frac{1}{2^2} - \frac{1}{1^2}) = 1.635 \times 10^{-18} \ J \ atom^{-1}$.
Per mole,$\Delta E = 1.635 \times 10^{-18} \times 6.022 \times 10^{23} = 9.846 \times 10^5 \ J \ mol^{-1} = 984.6 \ kJ \ mol^{-1}$.
$(iii)$ Ionization energy is the energy required to move an electron from $n=1$ to $n=\infty$. $\Delta E = E_{\infty} - E_1 = 0 - (-1312 \ kJ \ mol^{-1}) = 1312 \ kJ \ mol^{-1}$.