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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 almost completely unreactive chemically (under the influence of electric glow discharge or electron bombardment helium forms compounds with tungsten, iodine, sulfur and phosphorus). Under normal conditions it exists only as a monatomic gas. It was first discovered by Lockyer in 1868 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. It was first isolated on Earth by Ramsay in 1895, from the mineral clevite.

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 helium nuclei are thus called alpha particles. Most of the helium on earth is generated by this process. Helium has a second isotope, Helium-3, where the nucleus only has a single neutron, as well as several heavier isotopes that are radioactive. Helium-3 is virtually unknown on the Earth's surface, as the only internal sources of helium produce the helium-4 isotope as alpha particles and atmospheric helium escapes into space over relatively short geological timescales.

Both helium-3 and helium-4 were produced in the Big Bang, and after hydrogen helium 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 past 2.189 K at normal pressures, the so-called lambda point, it becomes a superfluid known as liquid Helium II (as opposed to "normal" liquid Helium I) which has many unusual characteristics 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. Since the transformation is one of higher order, without latent heat at the lambda point, the two liquid forms never coexist.

Helium II has zero viscosity and has a heat conductivity much higher than any other substance. Furthermore, helium II exhibits a thermomechanical (fountain) effect; if two vessels containing helium II are connected by a narrow capiliary and one of the two is heated a flow of helium toward the heated vessel will occur. Conversely, in the mechanocaloric effect, a forced flow of helium II through a capiliary will result in cooling of the helium II leaving the capiliary. Pulses of heat introduced into helium II will propagate through the liquid in the same manner as the density pulses of sound, a phenomenon which has been dubbed "second sound." Solid surfaces in contact with helium II are covered with a film 50 to 100 atoms thick, along which frictionless flow of the liquid can occur; as a result it is impossible to contain helium II in an open vessel without it flowing out over the edge. Mass transport through the helium II film takes place at a constant rate which only depends on temperature. Finally, a mass of helium II will not rotate as a unit; instead, attempts to set it rotating will induce small frictionless vortices throughout the liquid.

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.

Helium is found in minerals of uranium and thorium, such as clevites, pitchblende, carnotite, monazite and beryl; it is produced from these elements by radioactive decay in the form of alpha particles. It is also found in some mineral waters (1 part helium per thousand water in some Iceland springs), in volcanic gasses, and in certain natural gas deposits in the United States (from which most of the commercial helium on Earth is derived). It is also found in the atmosphere at a concentration of about 1 part in 200,000. Helium can be synthesized by bombardment of lithium or boron by high-velocity protons.

Helium is often used as a lifting gas in lighter-than-air vessels; it has 92.64% of the lifting power of hydrogen but is not flammable and is therefore considered safer. Helium-oxygen atmospheres are used in high-pressure breathing work such as deep diving suits or submersibles because helium is inert, less soluble in blood than nitrogen, and diffuses 2.5 times faster than nitrogen. This reduces the time required for degassing on decompression, eliminates the danger of nitrogen narcosis, and is less likely to collect as bubbles in joints. Liquid helium is used as a coolant for many extremely low-temperature applications. Helium is also used as an inert carrier gas, such as in gas chromatography.

Helium compounds

These are some references to papers on helium compounds.

• M. W. Wong. Prediction of a Metastable Helium Compound: HHeF.
• G.M.Chaban, J.Lundell, R.B.Gerber, J.Chem.Phys. 115 (2001) 7341. Lifetime and decomposition pathways of a chemically bound helium compound
• http://www.theorie.physik.uni-goettingen.de/~koehler/HeKoeI.ps.gz -- On the other hand, for the helium dimer He₂, discovered a few years ago,the extremely low binding energy of 20.11 meV is much smaller than the incident kinetic energy of typical experiments.
• http://pearl1.lanl.gov/periodic/elements/2.html -- Los Alamos National Laboratory's Chemistry Division: Periodic Table - Helium

See also: Helium-3