# Why are He-4 nuclei considered bosons, and He-3 nuclei considered fermions?

Why are helium-4 nuclei considered bosons, while helium-3 nuclei are considered fermions?

Examples of bosons are photons, gluons, phonons, helium-4 nuclei and all mesons. Examples of fermions are electrons, neutrinos, quarks, protons, neutrons, and helium-3 nuclei.

How does the difference of one neutron change whether it is a boson or fermion?

• Protons and neutrons are, individually, spin 1/2 particles. So, an odd number of them will be a non-integer spin. An even number will be integer spin. So, He-4 is a boson (integer spin) and He-3 is a fermion (half-integer spin). Jan 6, 2017 at 3:34

Bosons by definition have integer spin, while Fermions have half integer spin. Neutrons and protons are Fermions, but $He^4$ has both neutron and proton spins oppositely aligned so it's total spin is 0 (hence a Boson). $He^3$ on the other hand has the proton spins oppositely aligned (total spin 0) but the remaining neutron has spin 1/2 . Thus the spin of $He^3$ is 1/2 (Fermion).