The Principal quantum number (usually written n) is the first quantum number of an atomic orbital. It represents the relative overall energy, and hence also the average distance from the nucleus, of an orbital. The sets of orbitals with the same n-value are often referred to as electron shells or energy levels.

• The atomic orbitals with n=1 are said to be in the 'K'-shell
• The atomic orbitals with n=2 are said to be in the 'L'-shell
• The atomic orbitals with n=3 are said to be in the 'M'-shell
• The atomic orbitals with n=4 are said to be in the 'N'-shell