Pauli stated in 1925 that two electrons can occupy the same orbital only if they have opposite spins. This is known as the Pauli Exclusion Principle.

Here's a breakdown:

* Orbitals: These are regions around the nucleus of an atom where electrons are likely to be found.

* Spin: Electrons behave as if they are spinning, creating a magnetic field. This spin can be either up (↑) or down (↓).

The Pauli Exclusion Principle states that no two electrons in the same atom can have the same set of four quantum numbers. This means that if two electrons are in the same orbital (same n, l, and ml), they must have opposite spins to satisfy the principle.

This principle is fundamental to understanding the behavior of electrons in atoms and molecules, and it explains why the periodic table is structured the way it is.