One energy difference between the states n = 2 and n = 3 is E eV, in hydrogen atom. The ionisation potential of H atom is

Ionization energy $=E_{\propto }-E_n$

$\begin{array}{l}=0-\left[-\frac{{\mathrm{Z}}^2}{{\mathrm{n}}^2}\times 13.6\mathrm{eV}\right]\\ =\frac{{\mathrm{Z}}^2}{{\mathrm{n}}^2}\times 13.6\mathrm{eV}\\ =13.6{\mathrm{Z}}_2\left(\frac{1}{{\mathrm{n}}_1^2}-\frac{1}{{\mathrm{n}}_2^2}\right)\\ {\mathrm{n}}_1=2,{\mathrm{n}}_2=3\\ =13.6\times 1\left(\frac{9-4}{4\times 9}\right)\\ =\frac{5}{9}\times 13.6=7.2\mathrm{E}\end{array}$