Quantum mechanics divides particles or atoms into two classes: bosons and fermions. One of the most important characteristics of fermions is that no two fermions can be in the same quantum state; this is known as Pauli's exclusion principle. For bosons this restriction is not present, and in principle there is no limit to the number of bosonic particles that can be in the same quantum state. Electrons and protons are examples of fermionic particles. Light quanta (photons) and neutral atoms that contain an even number of neutrons are bosons.