A colourless, odourless, inert monatomic noble gas with two stable isotopes, helium-3 and helium-4. The helium-4 isotope, which has an extremely stable nucleus identical to an alpha particle, liquefies at temperatures below 4K and undergoes a phase change to a superfluid form known as liquid helium II below 2.2K which is impossible to solidify on cooling, even with temperatures down to absolute zero. This is due to the quantum mechanical effect of the zero point energy, or energy of motion at absolute zero temperature, which is present for all substances, but plays a dominant role in helium-4 and helium-3 because they are so small; the average fluctuation of the atoms from their mean position is 40% of their nearest neighbour distance, which prevents solidification. But it is possible to solidify it under a combination of high pressure and low temperatures.

Helium-4 has an even number of nucleons (four), and the spins of each pair off anti-parallel with each other to produce a nucleus of zero-spin. Any object with integral or zero-spin is a Boson. All Bosons can occupy exactly the same energy level, whereas the opposite is true for the half-integral spin Fermions. Liquid helium-4 (because it has an even number of nucleons) is a Bosonic liquid below the superfluid critical temperature, obeying Bose-Einstein statistics, where all the atoms are in the same ground state energy (which is non-zero) and with all their (atomic) spins parallel. The de-Broglie wavelength (arising from matter waves of the slowly moving atoms) is large and spreads out to encompass the whole mass, at which point all the atoms behave as one, with each atom being indistinguishable from the next. Move one atom and they will all move identically, without friction; it is a superfluid. This is the Bose-Einstein condensate. The thermal conductivity in this superfluid state is extremely high, whilst viscosity is near zero. Unlike other gases, it expands on cooling!

Superfluid liquids do not like to be spun. If a bucket of superfluid helium is spun slowly on a turntable, then the superfluid liquid within remains stationary. In fact, the superfluid should respond to absolute rotation with reference to the stars as the inertial frame. It should even detect the slow rotation of the Earth, remaining still as the Earth spins. If spun faster, contra-rotating vortices form, the number of which is dependant upon the rotational speed. The spin of superfluids is quantised, and jumps in discrete steps. If a toroidal container is filled with superfluid, and a physical barrier which would act as a stirrer is placed within the torroid then the fluid can be forced to rotate. If a pin-hole is now put in that barrier, some superfluid will flow through the pin-hole against the overall flow, such as to make the total momentum of the superfluid zero. In ordrr to do this, the fluid flowing back through the pin-hole has to be flowing much faster. When spun faster than a highly critical speed, the superfluid flowing through the pin-hole becomes a swhirling vortex, and this criticality makes the device suitable as a highly sensitive gyroscope able to detect miniscule variations in the rotational speed of the Earth, such as those caused by changes in ocean current by the flow of molten rock in the Earths core.

Solid helium-3 and helium-4 are unusual in that the volume can be decreased by 30% by the application of pressure. Helium vapour exhibits great expansion when heated to room temperature from 4 Kelvin.

Helium is almost completely inert, but the compound HeNe forms in the electrical discharge in helium-neon lasers. Clathrate compounds are known where helium is mechanically trapped in a cage, for instance, in a football shaped sphere of carbon atoms called Buckminster Fullerene, He@C₆₀.

Molecular helium, He₂, once thought impossible and discovered only recently - can form only at near absolute zero because its' Van der Waals force bond is 1000 times weaker than the next weakest bond, (the hydrogen bond). Molecular helium is a huge molecule because its two helium atoms move so slowly and hence, by Heisenbergs Uncertainty Principle, have low momentum and thus large positional uncertainty, which varies from 3 to 100 Angstroms.

Liquid helium is the standard coolant for devices working at cryogenic temperatures. As the next lightest gas to hydrogen, it has replaced hydrogen as a safe buoyant gas to fill airships, dirigibles and toy balloons (which are made of aluminised plastic instead of rubber because helium atoms are so small they can escape through the pores in rubber).

Helium, as helium-4, is continually formed inside the Earth by the radioactive alpha decay of numerous radioisotopes, especially uranium-238. The emitted alpha particles are helium-4 nuclei. Helium is present in the atmosphere at 5.2 parts per million where, being so light, it is continually lost to space. In the 1900's, before they discovered the high concentrations in natural gas, helium was obtained fron green monazite sand, which, being radioactive, continually produces helium-4 as alpha particles. The chief source of helium is now from natural gas, which can contain up to 8% helium (so high, it prevents combustion of the gas unless removed), produced by radioactive alpha decay deep underground. Helium also escapes at concentrations up to 350ppm at some vents along fault zones. Only a few natural gas reservoirs contain helium, most are near Texas. The atmosphere is the largest reservoir of helium, but at this very low concentration extraction from air is uneconomic. Economic extraction from air would require a concentration of 3000ppm. The use of helium is rising every year, and supplies of the gas are expected to last only up to 2060, and possibly just 2012. The price, in 2002, of pure helium-4 is £16350/cubic metre, which makes it costlier than gold. When supplies run out, extraction from the atmosphere is the only possible course, but this will increase the price a hundred fold. Being continuously lost and replenished, the atmospheric residence time of helium atoms is about a million years.

Uncommon on Earth, helium is the second most common element in the Universe, present at 23% by weight. All the helium was produced from the nuclear fusion of hydrogen during the first few minutes of the Big Bang. Helium is also present in the sun at 25%, the rest being mostly hydrogen.

Showmen demonstrate that sound travels much faster in helium by breathing in the gas and talking in a higher pitch voice like Donald Duck. Oxygen diluted by helium instead of nitrogen (as in air) is breathed in by deep-sea divers so as to avoid the potentially lethal nitrogen-narcosis commonly called 'the bends'. Helium is used with neon in a 10:1 ratio as the gas in helium-neon lasers, which produce a beam of coherent monochromatic red light. NASA uses huge amounts of helium to pressurise rocket engines.

A lighter isotope of helium exists, helium-3, which is used in cryogenics when temperatures between 1K and 1mK are required. Helium-3 has virtually no natural occurrence on Earth, but occurs locked up within the Earth. Whereas most of the Earths' helium-4 is produced by the alpha decay of radioactive elements, the helium-3 is primordial, having been created in the Big Bang. Man made helium-3 is produced as a by-product of tritium production, and is very expensive. The tritium, hydrogen-3, halflife 12.3 years, radioactively decays by beta decay into stable helium-3. However, the Moons' surface layer of rocks, unlike Earths, contains more titanium metal primarily within the mineral ilmenite (FeTiO₃), which absorbs the helium-3 ions streaming through space driven by the solar wind. The helium-3 ions are spallogenic in origin, that is produced by spallation reactions where high energy cosmic rays chip bits off other atoms when they crash into them.

The nuclear fusion reaction possible between helium-3 and deuterium holds out greater promise for fusion power than the currently investigated reaction between tritium and deuterium. This is because the latter produces energetic neutrons which are without electrical charge and therefore cannot be directed and which irradiate the reactors' metal parts making them radioactive, whilst the former produces energetic protons which, being charged, can be magnetically focused through a coil of wire, which decelerates them generating electricity directly.

Helium-3, with an odd number of nucleons, must have half-integral spin, and hence is a Fermi gas, obeying Fermi-Dirac statistics, where the Pauli exclusion principle applies and no two atoms can occupy the same energy level. The boiling point of helium-3 is 3.2 Kelvin at atmospheric pressure. Below the boiling point, helium-3 like helium-4, remains liquid right down to absolute zero temperatures at normal pressures because the binding energy between atoms is so weak that it is easily overcome by zero point motions. Only under a pressure greater than 29 atmospheres and a temperature below 0.5 Kelvin can helium-3 be solidified. Strangely, however, solid helium-3 is more disordered than liquid helium-3 because in the solid state, its' spins are totally disordered. Thus by cooling liquid helium-3 to 0.3 Kelvin and then pressurising it into a solid, latent heat is consumed which reduces the temperature to as low as 2 milliKelvin, and this provides a good method of cryogenic refrigeration.

Helium-3 can also exist in superfluid form; but only below 3 milliKelvin, much lower than for helium-4: Here, because liquid helium-3 is a Fermi liquid with half integral spin, Bose-Einsten condensation can only occur when two atoms of helium-3 pair up to produce a unit having integral spin, but not a spin-zero particle as helium-4, rather a pair with spin-one. Although both are bosons, there is a crucial difference: having spin, it can be aligned by a magnetic field; it is magnetic. In superfluid helium-3 the spins of the nuclei align themselves in one direction, as do the spins of the pairs of atoms, but not necessarily in the same direction as the nuclear spins, so superfluid helium-3, unlike superfluid helium-4, can exist in 3 different phases, and is an anisotropic superfluid, with differing properties in differing directions. The velocity of sound and the superfluid flow velocity are different in differing directions.

Normally, sound cannot propagate in a gas when the pressure of the gas is reduced such that the mean free path of the atoms is greater than the wavelength of sound. But within supercold liquid helium, a new type of sound can propagate, called zero sound, where the collision-less collective motion of atoms conveys the sound. see Bose-Einstein Condensate.

Nothing will dissolve in superfluid helium-4, except liquid helium-3, which naturally exists in the ratio 1 : 0.2 ppm. Even this can be removed by using a plug made out of a superleak material, which will allow only the superfluid components to pass. By this means, helium-4 can be purified to extraordinary purity, possibly purer than anything else on Earth. By this means also, further refrigeration can be effected, the non superfluid helium-3 becomes hotter (because it is not superfluid and cannot transport heat rapidly like a superfluid can) whilst the superfluid helium-4 becomes colder. This is the so-called 'dilution refrigerator'.
A solid-state device that reaches sub-cryogenic temperatures has now been constructed. See Cryogenic Peltier Cooler

Liquid helium-4, obtained by cryogenic cooling, is used to cool superconductors to a temperature below their superconducting transition temperature, when they start to conduct electricity with zero resistance. In this way superconducting coils can generate huge magnetic fields, used for medical MRI scanners, in bending magnets for high-energy particle accelerators, and to support magnetically levitated trains.

Ultracold neutrons, those with an energy less than 10^-7eV, are totally reflected by most substances, but are able to travel several hundred kilometres through superfluid liquid helium-4 unhindered; but they are absorbed by the nuclei of any helium-3 atoms (producing helium-4 nuclei).

A superfluid has zero viscosity, it is able to flow unhindered. All the atoms follow each other, since they are all truly identical. If a beaker of superfluid helium containing a central straw is gently heated, the helium will flow up the straw defying gravity, and emerges from the top in a fountain. If the beaker is instead spun on its axis, at first the helium remains stationary within the rotating beaker. If it is spun faster, several columns of rotating fluid form, and the faster the beaker is spun, the more vortex columns form. This is a large scale manifestation of the quantisation of spin.

Helium-4 nuclei, or alpha particles, are extremely stable units consisting of two protons bound together with two neutrons. Many of the most stable isotopes of the light elements are based on multiples of helium nuclei, such as carbon-12, oxygen-16, neon-20, calcium-24, magnesium-24, silicon-28 and sulphur-32, except for beryllium-8 which escapes being stable by a whisker.

Four other isotopes of helium are known, all unstable with halflives less than a second, and thus do not occur on Earth. Helium-5 and helium-7 have halflives much less than 1 attosecond, whilst helium-6 and helium-8 have much longer halflives, in the region of hundreds of milliseconds.

Claim to fame: Helium has the lowest melting point (0.95K at 26atm), lowest boiling point (4.2K), lowest latent heat of fusion (0.02KJ/mol), lowest liquid range (2.6ºC), lowest heat of vaporisation (0.082KJ/mol), the lowest entropy (gas) (126 J K^-1 mol^-1), the lowest polarizability (0.2 m³??), and the highest first ionization energy (24.6eV) of any element.