Pauli Exclusion Principle
The rule that no two electrons in an atom may have the same set of all four quantum numbers.
Formula / equation
n, l, mₗ, mₛ must differ
Unit
Dimensionless
In depth
The principle limits any orbital to two electrons with opposed spins and is what gives atoms their layered shell structure and finite size. Without it every electron would collapse into the 1s orbital and all elements would be chemically identical. It follows from the antisymmetry of fermion wavefunctions.
Examples in the real world
Helium's 1s orbital is full at two electrons, so lithium's third electron must begin the 2s orbital.