Atoms more complicated than hydrogen have more than one proton in their nucleus. Let Z stands for the number of protons in a nucleus. Also imagine that an atom loses all but one of its electrons so that it changes into a positively charged ion with just one electron. Bohr’s formula for the energy levels of the hydrogen atom can easily be changed to apply to such ions. It becomes ${\mathrm{E}}_{\mathrm{n}}=\frac{-{\mathrm{Z}}^2{\mathrm{e}}^4{\mathrm{m}}_{\mathrm{e}}}{8{\mathrm{\epsilon }}_0^2{\mathrm{h}}^2{\mathrm{n}}^2}$ , where m = mass of electron, e = charge of electron, n = orbit number.