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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?

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    $\begingroup$ en.wikipedia.org/wiki/Zero-point_energy $\endgroup$ – Mithoron May 2 '15 at 12:25
  • $\begingroup$ 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. $\endgroup$ – MaxW Sep 29 '16 at 5:29
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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.

enter image description here

For more details, please click here. Pressure is required to produce solid helium (25 atmospheres or more), because of the small mass and extremely weak forces between the helium atoms.

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  • $\begingroup$ Great answer, the properties of helium are indeed remarkably unique - are there any other elements/compounds having 2 liquid phases? $\endgroup$ – user15489 May 4 '15 at 8:42
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    $\begingroup$ @santiago helium-3 is even stranger than helium-4: en.wikipedia.org/wiki/Helium-3#Cryogenics $\endgroup$ – Mithoron May 5 '15 at 0:03
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    $\begingroup$ It should be noted that helium is the only substance whose isotopes have radically different phase diagrams. $\endgroup$ – Nicolau Saker Neto May 9 '15 at 11:18
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    $\begingroup$ @santiago Liquid crystalline materials often have even three liquid phases. $\endgroup$ – Karl Sep 13 '15 at 23:44
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    $\begingroup$ There are a numer of liquid sulphur modifications with varying viscosity. $\endgroup$ – Karl Apr 10 '16 at 16:36

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