Isotopes of Helium
In nature the inert gas helium exists in two forms, isotopes, with fundamentally different properties. Helium-4 is the commonest while helium-3 occurs only as a very small fraction. Helium-4 has a nucleus with two protons and two neutrons (the 4 stands for the total number of nucleons, i.e. protons and neutrons). The nucleus is surrounded by an electron shell with two electrons. The fact that the number of particles constituting the atom is even makes helium-4 what is termed a boson. The nucleus of helium-3 also has two protons, but only one neutron. Since its electron shell also has two electrons, helium-3 consists of an odd number of particles, which makes it what is termed a fermion. Since the two isotopes of helium are built up of different numbers of particles, dramatic differences in their behaviour arise when they are cooled to temperatures near absolute zero.