Helium is the only element that refuses to freeze solid under ordinary atmospheric pressure.

The longer version

Helium atoms are extremely light and attract one another very weakly, so thermal motion alone keeps them from settling into an ordered solid. Even at the coldest temperatures reached in ordinary laboratories the atoms keep moving enough to stay fluid. Squeezing the gas hard changes that, and under strong pressure it does solidify.

Quantum effects also play a part. The atoms are so light that their zero-point motion, the irreducible jitter that never fully stops, is large compared with the weak forces trying to hold a lattice together. Without extra pressure there is simply nothing strong enough to pin them in place.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.