Helium Isotopes

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Helium has two stable, naturally occurring isotopes. The defining characteristic of helium is two protons in the nucleus, for an atomic number Z = 2. However, the helium nucleus can contain different numbers of neutrons. On the left we have 3He, with two protons, one neutron, and an atomic mass number A = 3. On the right we have 4He, with two protons, two neutrons, and A = 4. As both neutrons and protons are spin-1/2 fermions, the A = 3 nucleus is a fermion, and the A = 4 nucleus is a boson. In each case, the electron density cloud of a neutral helium atom contains a pair of spin-1/2 electrons. All possible arrangements of the two electrons in both isotopes result in a total electron spin of either 0 or 1, so the electrons form a bosonic pair in either case. Thus, when we add electronic and nuclear spins, we find that atomic 4He is a boson, and 3He, a fermion. (Unit: 6)