When an electron with charge 'e' and mass 'm' moves with velocity 'v' around the nucleus having nuclear charge 'Z' in a circular orbit of radius 'r', the potential energy of electron is

In the atom the potential energy of an electron is directly proportional to the product of the charge of electron and proton but it is inversely proportion to the radius of orbit.

 $\mathrm{so},\mathrm{P}.\mathrm{E}=\frac{(-\mathrm{e})(+\mathrm{Ze})}{\mathrm{r}}=\frac{-{\mathrm{Ze}}^2}{\mathrm{r}}$

Note: electron has negative charge but the nucleus has positive charge.