# Fermions vs Bosons

The Standard Model
MissMJ CC BY 3.0

Fermions are elementary particles that have a half integer spin – leptons and quarks, and combinations of quarks such as a proton. See image.

Quarks and leptons are grouped together as fermions as they obey the Pauli Exclusion Principle —fermions cannot occupy the same quantum state.

Bosons do not obey the Pauli Exclusion Principle — they can be placed in the same quantum state.

This is why it possible to make lasers in which huge numbers of photons (bosonic particles) can be produced in the same quantum state.