According to Bohr's model, the energy of an electron in the $n$-th orbit is given by $E_n=-\frac{m{e}^4{Z}^2}{8{ϵ}_0^2{h}^2{n}^2}$. For hydrogen-like ions, this can be simplified to $E_n=-13.6\frac{Z^2}{n^2}\text{eV}$. The negative sign indicates that the electron is bound to the nucleus. The energy is directly proportional to the square of the atomic number ($Z^2$) and inversely proportional to the square of the principal quantum number ($n^2$).