Helium exists in liquid form only at extremely low temperatures.
The boiling point and critical point depend on the isotope of the
helium; see the table below for values. The density of liquid
helium-4 at its boiling point and 1 atm is approximately 0.125
g/mL. Helium-4 was first liquefied on 10 July 1908 by Dutch
physicist Heike Kamerlingh Onnes. Liquid helium-4 is used as a
cryogenic refrigerant; it is produced commercially for use in
superconducting magnets such as those used in MRI or NMR. It is
liquefied using the Hampson-Linde cycle. The temperatures required
to liquefy helium are low because of the weakness of the attraction
between helium atoms. The interatomic forces are weak in the first
place because helium is a noble gas, but the interatomic attraction
is reduced even further by quantum effects, which are important in
helium because of its low atomic mass. The zero point energy of the
liquid is less if the atoms are less confined by their neighbors;
thus the liquid can lower its ground state energy by increasing the
interatomic distance. But at this greater distance, the effect of
interatomic forces is even weaker
