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Helium is the second chemical element in the periodic table. Helium has an atomic number of 2 and is given the symbol He. Helium is the first of the noble gases and is completely unreactive chemically. Under normal conditions it exists only as a monatomic gas. It was first discovered by spectroscopy, when an emission line of a previously unknown element was discovered in the light from the Sun. Hence the name, from Greek Helios (sun), with the suffix -ium because until its discovery on earth it was expected to be a metal.

The most common isotope of Helium is Helium-4, where the nucleus has two protons and two neutrons. This is an unusually stable nuclear arrangement since it has a magic number of nucleons, that is, a number where they are arranged into complete shells. Many heavier nuclei decay by the emission of Helium-4 nuclei, a process called alpha decay, and heium nuclei are thus called alpha particles. Most of the helium on earth is generated by this process.

Helium also has a second isotope, Helium-3, where the nucleus only has a single neutron, as well as several heavier isotopes that are radioactive. Both helium-3 and helium-4 were produced in the Big Bang, and after hydrogen it is the second most abundant element in the universe. Additional helium is produced by the fusion of hydrogen inside stellar cores, via a process called the proton-proton chain.

Helium atoms have very little attraction to one another, so helium condenses only under fairly extreme conditions. The critical temperature, above which there is no difference between the liquid and gaseous phases, is only 5.19 K. As it is cooled further at normal pressures, it becomes a superfluid, which has zero viscosity due to quantum effects and was one of the first examples of their operation on a macroscopic scale. This transition takes place at much lower temperatures in Helium-3 than it does in Helium-4, since the effect relies on condensation of bosons but the nuclei of the former are fermions, so can't condense individually but must do so in bosonic pairs.

Solid helium only exists at great pressures, around 100 MPa at 15 K, and at roughly this temperature helium undergoes a transition between high temperature and low temperature forms, in which the atoms have cubic and hexagonal close packings, respectively. At a fraction of the temperature and pressure a third form occurs where the atoms have a body-centered cubic arrangement. All these arrangements are fairly similar in energy and density, and the reasons for the changes have to do with the details of how the atoms interact.