Helium

hydrogen ← helium → lithium – ↑ He ↓ Ne

Atomic properties Atomic number 2 Standard atomic weight 4.002 602(2) Electron configuration 1s2 Physical properties Melting point see text Boiling point 4.22 K (−268.93 °C) Critical point 5.2 K, 2.274 bar Density 0.169 kg m−3 (1 atm, 15 °C) 16.891 kg m−3 (1 atm, 4.2 K) 0.12496 g cm−3 (l, 4.2 K) Chemical properties Solubility in water 8.9 cm3 kg−1 (1 atm, 20 °C) Ionization energies 1st 24.587 3985(14) eV 2360.634 56(12) kJ mol−1 2nd 54.416 eV 5250.5 kJ mol−1 Total 79.003 eV 7622.8 kJ mol−1 Thermodynamic properties Standard entropy 126.153(2) J K−1 mol−1 Enthalpy change of vaporization 0.08 kJ mol−1 Molar heat capacity (Cp) 20 J K−1 mol−1 Miscellaneous CAS number 7440-59-7 EC number 231-168-5 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Helium (symbol: He) is a chemical element, the lightest of the noble gases.

History

Helium is the only element to have been discovered extraterrestrially before being found on Earth, specifically in the Sun. A bright yellow spectral line (λ = 587.49 nm) was first observed by French astronomer Jules Janssen during the eclipse of 18 August 1868, which Janssen observed from Guntur in India, and independently by Norman Lockyer in London on 20 October 1868. Lockyer, together with English chemist Edward Frankland, showed that the line could not be explained by any known element, and proposed the name helium, from the Greek ἥλιος (helios; the Sun).

The first isolation of helium from a terrestrial source is usually credited to the British chemist William Ramsey, who isolated the helium that was occluded in the mineral cleveite (a uranium-containing mineral) and identified it by its spectrum. The occluded gas had been noticed previously by American geochemist William Francis Hillebrand, but had been misidentified as nitrogen. Helium had been identified (through its spectrum) on Earth as early as 1881 by Italian physicist Luigi Palmieri, but Palmieri could not isolate a sample of the element. Finally, helium was isolated independently in 1895 by Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. Nevertheless, it was Ramsey who was awarded the Nobel Prize in Chemistry (in 1904), and who is usually credited with the discovery.

The first significant quantities of helium on Earth were discovered in the natural gas from a well in Kansas, USA, in 1903–6: the discovery was designated a National Historic Chemical Landmark by the American Chemical Society in 2000. At the same time, New Zealander Ernest Rutherford was speculating that one type of radioactivity, known as α-decay, was caused by the emission of a form of helium. Rutherford was able to prove that α-particles are He²⁺ ions, for which he was awarded the Nobel Prize In Chemistry in 1908. Rutherford's elegant proof, coupled with many more circumstantial results from other workers, explained why helium was found in association with uranium and thorium minerals.

Helium was first liquified by Dutch physicist Heike Kamerlingh Onnes in 1908, a feat for which he would receive the Nobel Prize in Physics (1913), but was not solidified until 1926 (by Kamerlingh Onnes' student Willem Hendrik Keesom). Yet another Nobel Prize (again in physics) was awarded to Lev Landau in 1962 for his explanation of the extraordinary proporties of liquid helium at low temperatures, known as superfluidity.

Occurrence and production

Country	Production 106 m3
United States	122*
Algeria	24
Qatar	15
Russia	7
Poland	2.5
Estimates for 2009 from the U.S. Geological Survey
*U.S. figure includes 42 million cubic metres withdrawn from the federal government stockpile.

Helium is the second most common element in the Universe (after hydrogen), accounting for 23% of all atoms. However, the Earth's gravitational field is not strong enough to retain helium in the atmosphere for long periods, and almost all the Earth's primordial helium is believed to have escaped. The helium currently present on Earth has been formed from the α-decay of radioactive nuclides: most of this helium escapes to the atmosphere, where the current volume fraction is 5.24 ppm, and then into space: some of it, however, can be trapped underground by impermeable rock formations, often associated with natural gas deposits.

The commercial production of helium is based around its extraction from natural gas, which is economically viable when the helium fraction is greater than about 0.3%. The United States has historically been the predominant producer, although its de novo production is declining and Algeria and Qatar are gaining importance.

The U.S. Bureau of Land Management operates a stockpile of crude (~80%) helium, the National Helium Reserve at Cliffside Field, Potter County, Texas, and a crude-helium pipeline from Bushton, Kansas, passing through the Reichel Field (Kansas) and the Keyes Field (Oklahoma) to the Cliffside Field. The National Helium Reserve reached its peak in 1980, with 194,000 tonnes (1150 million cubic metres) stored, but it is being run down under the terms of the Helium Privatization Act of 1996 (Pub. L. 104–273), and will eventually be just 600 million cubic feet (16.6 million cubic metres). About one third of the helium produced in the United States is refined from stored helium rather than being extracted from natural gas, and the National Helium Reserve held 561 million cubic metres of crude helium at the end of 2008.

The refinement of crude helium to the normal commercial grade ("Grade I", about 99.995%) requires cooling with liquid hydrogen to below 27 K (−246 °C, −411 °F) to condense out the neon present: liquid hydrogen is required, as helium gas cannot be cooled by the Joule–Thomson effect until its temperature is below about 40 K. Commercial helium is usually shipped in bulk as a liquid.

Use

Total U.S. consumpton (2009) 52.1 million cubic metres
Cryogenics	32%
Pressurizing and purging	18%
Controlled atmospheres	18%
Welding cover gas	13%
Leak detection	4%
Breathing mixtures	2%
Other uses	13%
Estimates for 2009 from the U.S. Geological Survey The breakdown of usage in other countries may be significantly different: see text.

The main use of helium is in cryogenics, where helium is essential for temperatures below −256 °C (−429 °F), the approximate boiling point of liquid hydrogen. Cooling with liquid helium allows an operating temperature of around −269 °C (−452 °F), just four kelvins. Such low temperatures are required for superconducting magnets to operate; these are used in a variety of applications, including NMR spectrometers and magnetic resonance imaging (MRI) scanners.

Uses for pressurizing and purging, in controlled atmospheres (e.g., glove boxes) and as a welding cover gas are only important in the United States, where helium is relatively cheap, and are declining even there along with a general fall in U.S. helium consumption (−42% over the period 2000–2009). In other countries, argon is used for these purposes.

There are several niche uses of helium, including:

• as a means of detecting small leaks of gas in otherwise closed systems using the helium mass spectrometer approach, usually for leaks of 10⁻⁵–10⁻¹³ Pa m³ s⁻¹ (from about 1 ml/min to less than 3 ml per century)
• as a replacement for nitrogen in breathing mixtures which are to be delivered to humans at high pressure, such as in deep-sea diving, and also in some medical uses;
• as a flow gas in gas chromatography, particularly when using a katharometer, a helium ionization detector (HID) or in GC–MS;
• as a lifting gas for meteorological balloons and airships;
• as a heat transfer agent, particularly in VHTR nuclear reactors.

Physical properties

Helium has a number of unique physical properties. It is the chemical substance with the lowest boiling point (4.22 K) and the only substance that cannot be solidified by cooling alone at atmospheric pressure. It is also the only substance without a triple point, a temperature and pressure at which solid, liquid and gas are all in equilibrium.

Helium has an extremely high specific heat capacity (C_p = 5.19 kJ K⁻¹ kg⁻¹ at 25 °C) and, for a gas, a relatively high thermal conductivity (0.1430 W m⁻¹ K⁻¹ at 0 °C): both of these are due to the low mass of the atoms which make up helium gas (H₂ is even more exceptional on both counts), and are important in its uses in gas chromatography and as a heat transfer agent.

Helium at low pressures and ambient temperatures is the nearest practical model to an ideal gas, one in which the attractive forces between atoms are negligible, as is the volume of the atoms compared with the volume occupied by the gas. It has an extremely low Joule–Thomson inversion temperature of about 40–45 K: above this temperature, the Joule–Thomson coefficient is negative and so helium will warm up as it expands.

Liquid helium

	4He		3He
	T	p	T	p
Critical point	5.2 K		3.3 K
Boiling point	4.2 K	1 atm	3.2 K	1 atm
Melting point at minimum freezing pressure	1.0 K	25 bar	0.3 K	29 bar
λ point at saturated vapour pressure	2.17K		1 mK

Liquid helium-4 can be condensed at 4.22 K to a liquid phase now known as helium I. If cooling is continued (by allowing some of the liquid to evaporate), the physical properties of the liquid change drastically at about 2.2 K (the exact value depends on the pressure). For a start, the visible bubbling of a boiling liquid disappears, although evaporation continues: the heat capacity increases ten-fold, the thermal conductivity increases a million-fold and the viscosity drops to virtually zero.

The low-temperature liquid phase is known as helium II, and is a superfluid similar in nature to a Bose–Einstein condensate, first discovered in 1938. The transition between the two liquid phases is a second-order phase transition, and the transition temperature is known as the λ point after the shape of the plot of heat capacity vs. temperature, which resembles a Greek letter lambda. Perhaps the best known property of helium II is its propensity to form Rollin films, layers approximately 30 nm thick that will climb up surfaces cool enough to support helium II even against the force of gravity: as such, the confinement of liquid helium below the λ point is particularly difficult.

Helium-3 also forms a superfluid phase, discovered in 1972, but only at temperatures of one millikelvin or less. The difference in behaviour between the two isotopes is due to their nuclear spin: helium-4 atoms are bosons, with zero spin, while helium-3 atoms are spin-½ fermions, and the two types of particle obey different exclusion principles. For similar reasons, liquid helium-4 and liquid helium-3 are no longer miscible below about 0.9 K.

Neither isotope can be solidified at pressures of 1 atm. By raising the pressure to 25 atm or higher, helium can be can be solidified at 1–1.5 K. The solid form is unusually compressible, with a bulk modulus of around 50 MPa (fifty-times more compressible than liquid water). Liquid and solid phases of helium have extremely low refractive indexes, which makes it difficult to distinguish phase changes by visual means.

Chemical properties

Helium has the highest first ionization energy of any element (2372.3 kJ mol⁻¹) and has almost unprecedented chemical inertness. No chemical compounds of helium are known in condensed phases, although the [HeH]⁺ cation (the protonation product of a helium atom) was first observed in 1925, and its neutral analogue is also known in the gas phase.

The low solubility of helium in water (8.61 cm³ kg⁻¹ at 101.325 kPa, 20 °C) is important in its use as a component of breathing mixtures: helium, unlike nitrogen at high pressure, will not dissolve in blood serum, and so poses fewer problems of degassing of the blood stream during decompression.

External links

• WebElements