The Hydrogen atom consists of an electron bound to a proton by the Coulomb potential.
Since the potential is spherically symmetric, the problem separates and the solutions
will be a product of a radial wavefunction and one of the spherical harmonics.
The radial wavefunction satisfies the differential equation that depends on the angular
momentum quantum number
,
The differential equation can be solved using techniques similar to those used to solve the 1D harmonic oscillator equation. We find the eigen-energies
The principle quantum number
is an integer from 1 to infinity.
This unusual way of labeling the states comes about because a radial excitation has the same energy as an angular excitation for Hydrogen. This is often referred to as an accidental degeneracy.