Helium-3 - Physical Properties

Physical Properties

Because of its lower atomic mass of about 3.00 atomic mass units, helium-3 has some physical properties different from those of from helium-4, with a mass of about 4.00 atomic mass units. Because of the weak, induced dipole-dipole interaction between helium atoms, their macroscopic physical properties are mainly determined by their zero point energy ("ground state kinetic energy"). Also, the microscopic properties of helium-3 cause it to have a higher zero point energy than helium-4. This implies that helium-3 can overcome dipole-dipole interaction with less thermal energy than helium-4 can.

The quantum mechanical effects on helium-3 and helium-4 are significantly different because with two protons, two neutrons, and two electrons, helium-4 has an overall "spin" of zero, making it a boson, but with one fewer neutron, helium-3 has an overall spin of plus or minus one half, making it a fermion.

Helium-3 boils at 3.19 kelvin compared with helium-4 at 4.23 kelvin, and its critical point is also lower at 3.35 kelvin, compared with helium-4 at 5.2 kelvin. Helium-3 has less than one-half of the density when it is at its boiling point: 59 gram per liter compared to the 125 gram per liter of helium-4 -- at a pressure of one atmosphere. Its latent heat of vaporization is also considerably lower at 0.026 kilojoule per mole compared with the 0.0829 kilojoule per mole of helium-4.