# Why can't helium be solidified at 'ordinary' pressures?

According to the UC Davis ChemWiki Chemistry of Helium, helium has a comparatively unusual property, specifically:

Helium is the only element that cannot be solidified by lowering the temperature at ordinary pressures.

'Ordinary' referring to standard air pressure (1 atmosphere). In order to solidify, there needs to be a corresponding pressure increase, with a projected density of:

$0.187 \pm 0.009~\mathrm{g~mL^{-1}}$ at $0~\mathrm{K}$ and $25~\mathrm{bar}$.

So why can't helium be solidified at 'ordinary' pressures?

• en.wikipedia.org/wiki/Zero-point_energy – Mithoron May 2 '15 at 12:25
• The short answer is that even the London dispersion forces between helium atoms (molecules) are extremely weak. So helium boils at a lower temperature than any other element as well. – MaxW Sep 29 '16 at 5:29

When the temperature of helium gas is decreased to about 5.2 K, a phase transition to ordinary liquid helium ($\ce{He}$ I) occurs. The behavior of this liquid phase is normal and identical to any other liquid phase.
As temperature is decreased more (at moderate pressures), helium does not solidify. In fact, it undergoes a phase change to a second liquid phase known as $\ce{He}$ II at a temperature of approximately 2.17 K. As shown in the phase diagram below, the $\ce{He}$ II phase persists until absolute zero. Due to quantum effects, $\ce{He}$ II has remarkably unique properties. It is considered a superfluid.