The
chemical element helium exists in a
liquid form only at the extremely low temperature of -269 °C (about 4 K or -452.2 °F). Its
boiling point and
critical point depend on which
isotope of helium is present: the common isotope
helium-4 or the rare isotope
helium-3. These are the only two stable isotopes of helium. See the table below for the values of these physical quantities. The density of liquid helium-4 at its boiling point and a pressure of one atmosphere (101.3
kilopascals) is about 0.125
grams per
cm3, or about 1/8th the density of
liquid water.