# What is the Pauli Exclusion Principle?

The Pauli Exclusion Principle states that no two electrons can have the same four quantum numbers. The fourth quantum number is the electron spin quantum number ${m}_{s} = \pm \frac{1}{2}$. An orbital can contain a maximum of two electrons, which can have three quantum numbers in common, but not the same spin quantum number. One must spin up, $+ \frac{1}{2}$, and the other must spin down, $- \frac{1}{2}$. We represent the electron and its spin with up arrows and down arrows, $\uparrow$ and $\downarrow$, in an orbital diagram.
The $1 \text{s}$ orbital can contain a maximum of two electrons. In the diagram below, notice that hydrogen's single electron is represented by an up arrow. Helium has two $1 \text{s}$ electrons. The first is represented as an up arrow and the second is represented by a down arrow. The up and down arrows represent opposite spins.