Quantum Mechanics – Why He-4 Nuclei Are Bosons and He-3 Nuclei Are Fermions

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

From the Wikipedia page on Identical Particles:

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?

Best Answer

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).