Rydberg's formula

To calculate the energy change resulting from electron changing energy levels, use:

$$\Delta E=-R\cdot Z^2\left(\frac{1}{n_f^2}-\frac{1}{n_i^2}\right)$$

where $R=2.178\cdot 10^{-18}\mathrm{J}$ (Rydberg’s constant) and $Z$ is the atomic number.

To calculate the energy of an electron at a certain energy level, use:

$$E=-\frac{R\cdot Z^2}{n^2}$$