The principal quantum number is the quantum number denoted by *n* and which indirectly describes the size of the electron orbital. It is always assigned an integer value (i.e., *n* = 1,2,3,...), but its value may never be 0. An orbital for which *n* = 2 is larger, for example, than an orbital for which *n* = 1. Energy must be absorbed in order for an electron to be excited from an orbital near the nucleus (*n* = 1) to get to an orbital further from the nucleus (*n* = 2).

The principal quantum number is cited first in the set of four quantum numbers associated with an electron. The principal quantum number has the greatest effect on the energy of the electron. It was first designed to distinguish between different energy levels in the Bohr model of the atom but remains applicable to the modern atomic orbital theory.

### Source

Andrew, A. V. (2006). "2. Schrödinger equation". *Atomic spectroscopy. Introduction of theory to Hyperfine Structure*. p. 274. ISBN 978-0-387-25573-6.