Calculating the ionization energy for hydrogen
I assume you refer to Bohr's atomic model.

The energy of an electron in Bohr’s orbit of Hydrogen atom is given by the expression:

$$
\begin{align}
E_{n} &amp;= \frac{2\pi^{2}me^{4}Z^{2}}{n^{2}h^{2}\left(4\pi\epsilon_{0}\right)^{2}} \\
&amp;= \color{\navy}{-13.6\frac{Z^2}{n^2}\pu{ eV}}
\end{align}
$$

Since $Z = 1$ for hydrogen, the above equation can be further simplified. Now just plug in the required values.