According to the Pauli exclusion principle, a single atomic orbital can hold a maximum of two electrons. Those two electrons must have opposite spins. No two identical fermions, including electrons, can occupy the same quantum state simultaneously.
An orbital is described by a set of quantum numbers. Two electrons can share one orbital because they have the same spatial orbital quantum numbers but differ in their spin quantum number. This rule explains why electron configurations fill in structured patterns and why the periodic table has its characteristic arrangement.
The principle was formulated by Austrian physicist Wolfgang Pauli in 1925. It is not the same as the Aufbau principle, which describes the general order in which orbitals are filled, or Hund’s rule, which says that electrons occupy equal-energy orbitals singly before pairing. Together, these ideas help predict configurations such as hydrogen’s 1s¹ and helium’s 1s².
A common mistake is to say that a shell holds only two electrons. A single orbital holds two, but a principal shell can contain several orbitals. The second shell, for example, has one s orbital and three p orbitals, allowing up to eight electrons.