Helium is the 2nd lightest and 2nd most abundant element, right behind hydrogen. The two could not be more different. Helium’s stability is the opposite of hydrogen’s volatility.
Helium is a gas except under extreme conditions. Helium has the lowest boiling and melting points. Helium becomes a liquid at 4.2 K. Below 4 K, helium fiercely boils.
Below 2.172 K, the boiling stops; helium becomes a superfluid. A superfluid exhibits zero viscosity and zero entropy. Superfluid helium flows without friction; through tiny holes as small as a molecule, up and out of a tube, over the edge of a cup.
More particularly, helium-4, the ubiquitous isotope comprising 2 neutrons with 2 protons, becomes a superfluid more readily than helium-3, which is a rare isotope, with only 1 neutron per helium atom. Helium-3 only becomes a superfluid when chilled to 2 millikelvins.
The reason is that helium-4 atoms are bosons, whereas helium-3 atoms are fermions. Hence helium-4 corresponds to the characteristics of Bose-Einstein condensation, whereas helium-3 becomes a fermionic condensate.
A fermionic condensate interacts by Cooper pairing between atoms, as contrasted to the electron Cooper pairs that facilitate superconductivity.
Helium-4 is bosonic via the subatomic components (protons, neutrons, and electrons) canceling each other’s complementary spins, resulting in zero spin for the helium-4 atom as a whole. This facilitates the stability of helium-4.
Only neon is less reactive than helium, and it needs a cluster of buddies to be so. Neon is most commonly molecular as 20Ne, whereas helium is monoatomic (1He).
Helium is not the only element capable of superfluidity. Rubidium, a highly reactive silvery-white metal, becomes a superfluid at 500 nanokelvins (500 10–9 Kelvin).