Atoms that only have one electron are called hydrogenoids. They are hydrogenoids atoms: H, He ^{+} , Li ^{2+} , Be ^{3+} .

Hydrogen atoms present a factorable wave function in a radial part and a spherical harmonic, which has the form:

$\Psi(r,\theta,\varphi)=R_{n,l}(r)Y_{l,m}(\theta,\varphi)$

where n,l,m are the quantum numbers that make the wave function acceptable.

The energy of a hydrogen atom is given by the expression:

$E_n=\frac{RhZ^2}{n^2}$

As can be seen, the energy depends exclusively on the principal quantum number, n.