Why can helium not freeze at normal atmospheric pressure?

Helium is the only chemical element that cannot be solidified by cooling alone at normal atmospheric pressure.1

This phenomenon occurs because of zero-point energy, a fundamental principle of quantum mechanics. According to the uncertainty principle, atoms can never come to a complete rest, even at absolute zero. Because helium atoms are extremely light and have very weak interatomic attractions, this residual vibrational energy is sufficient to prevent the atoms from locking into a rigid, solid structure under standard pressure.21

To overcome this energy and force helium into a solid state, it is necessary to apply significant external pressure—specifically 25 atmospheres—at temperatures near 1 K (−272 °C).1