Principal Quantum Number
The quantum number n that specifies the main energy level and average distance of an electron from the nucleus.
Formula / equation
E_n = −13.6 Z² / n² eV
Unit
Dimensionless
In depth
Taking positive integer values 1, 2, 3…, n defines the shell (K, L, M…) and sets the maximum electron capacity as 2n². As n increases, orbitals become larger and closer in energy, which is why the outermost electrons are the ones lost in ionisation.
Examples in the real world
An electron with n = 3 lies in the M shell, which can hold up to 18 electrons across the 3s, 3p and 3d subshells.