Helium Is the Only Element That Cannot Be Solidified by Pressure Alone

The reason helium stands alone as the only element that resists solidification through pressure at standard temperatures lies within the realm of quantum mechanics. Due to its very low atomic mass and extremely weak forces between its atoms, helium possesses a high zero-point energy. This is the lowest possible energy that a quantum mechanical system may have, and for helium, it is substantial enough to overcome the weak attractions that would typically cause particles to lock into a solid structure. Even as the temperature approaches absolute zero, the atoms continue to vibrate with enough energy to prevent them from settling into a fixed, solid state at normal atmospheric pressure.

This peculiar behavior puzzled scientists for years. Dutch physicist Heike Kamerlingh Onnes was the first to liquefy helium in 1908 by cooling it to less than one kelvin. However, his attempts to solidify it by further reducing the temperature failed. It wasn't until 1926 that his student, Willem Hendrik Keesom, finally succeeded by applying an external pressure of about 25 atmospheres to the frigid liquid helium. This demonstrated that both immense pressure and extreme cold, near -272°C or -457°F, are necessary to force helium's restless atoms into a crystalline solid.

Even in its solid form, helium continues to exhibit strange properties. The solid is surprisingly compressible; its volume can be reduced by more than 30% with the application of pressure. Under specific conditions of extreme pressure and low temperature, solid helium is believed to enter a "supersolid" state. In this exotic phase of matter, a portion of the helium atoms can flow without any friction through the otherwise rigid crystalline structure, a bizarre and counterintuitive quantum effect.