Principle quantum number

The principle quantum number n (or m) is an integer used in the Balmer formula for the spectrum of the hydrogen atom denoting the principle energy levels in the Bohr atom model.

It classifies the principle electron shells. They are labelled K,L,M,N,O for n=1,2,3,4,5 between which atomic transitions in the hydrogen atom give the well known spectral series shown below.

(Credit: Olivier Thizy).