Principal Quantum Number Definition

Chemistry Glossary Definition of Principal Quantum Number

Illustration of light
Quantum numbers are associated with electrons. Science Photo Library/MEHAU KULYK/Getty Images

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.