was first detected on August 18, 1868 as a bright yellow line with a wavelength of 587.49 nanometres in the spectrum of the chromosphere of the Sun, by French astronomer Pierre Janssen during a total solar eclipse in Guntur, India. This line was initially assumed to be sodium. On October 20 of the same year, English astronomer Norman Lockyer observed a yellow line in the solar spectrum, which he named the D3 line, for it was near the known D1 and D2 lines of sodium, and concluded that it was caused by an element in the Sun unknown on Earth. He and English chemist Edward Frankland named the element with the Greek word for the Sun.
On 26 March 1895 British chemist William Ramsay isolated helium
on Earth by treating the mineral cleveite with mineral acids. Ramsay was looking for argon but, after separating nitrogen
from the gas liberated by sulfuric acid, noticed a bright-yellow line that matched the D3 line observed in the spectrum of the Sun. These samples were identified as hhelium
by Lockyer and British physicist William Crookes. It was independently isolated from cleveite the same year by chemists Per Teodor Cleve and Abraham Langlet in Uppsala, Sweden, who collected enough of the gas to accurately determine its atomic weight. helium
was also isolated by the American geochemist William Francis Hillebrand prior to Ramsay’s discovery when he noticed unusual spectral lines while testing a sample of the mineral uraninite. Hillebrand, however, attributed the lines to nitrogen
. His letter of congratulations to Ramsay offers an interesting case of discovery and near-discovery in science.
In 1907, Ernest Rutherford and Thomas Royds demonstrated that an alpha particle is a helium
nucleus. In 1908, helium was first liquefied by Dutch physicist Heike Kamerlingh Onnes by cooling the gas to less than one kelvin. He tried to solidify it by further reducing the temperature but failed because helium
does not have a triple point temperature where the solid, liquid, and gas phases are at equilibrium. It was first solidified in 1926 by his student Willem Hendrik Keesom by subjecting helium to 25 atmospheres of pressure.
In 1938, Russian physicist Pyotr Leonidovich Kapitsa discovered that helium-4 has almost no viscosity at temperatures near absolute zero, a phenomenon now called superfluidity. In 1972, the same phenomenon was observed in helium-3 by American physicists Douglas D. Osheroff, David M. Lee, and Robert C. Richardson.
has the lowest melting and boiling point of any element. Liquid Helium is called a “quantum fluid” as it displays atomic properties on a macroscopic scale. The viscosity of liquid helium
is 25 micropoises (water has a viscosity of 10,000 micropoises). As helium
is cooled below its transition point, it has an unusual property of superfluidity with a viscosity approaching zero micropoises. In addition, liquid helium has extremely high thermal conductivity.
is the second most abundant and second lightest element in the periodic table. It is also the least reactive of all the group 18 (noble gases) elements.
One cubic metre of helium will lift 1kg. helium
is the preferred choice for airships as although it is more expensive it is not flammable and has 92% the lifting power of hydrogen.
The voice of a person who has inhaled helium temporarily sounds high-pitched, resembling those of the cartoon characters “Alvin and the Chipmunks”. This is because the speed of sound in helium
is nearly three times that in air. Although the vocal effect of inhaling helium may be amusing, it can be dangerous if done to excess since helium
is a simple asphyxiant, thus it displaces oxygen
needed for normal respiration. Death by asphyxiation will result within minutes if pure helium is breathed continuously.
is chemically unreactive under all normal conditions due to its valence of zero. Because of this extreme conditions are needed to create the small handful of helium compounds, which are all unstable at standard temperature and pressure.
No useful (with exception to those produced for scientific research) or commercial helium compounds exist.
Reactions of Helium
Helium is chemically unreactive under all normal conditions due to its valence of zero. It is an electrical insulator unless ionized. As with the other noble gases, helium
has metastable energy levels that allow it to remain ionized in an electrical discharge with a voltage below its ionization potential. Helium can form unstable compounds with tungsten, iodine, fluorine, sulfur
and phosphorus when it is subjected to an electric glow discharge, through electron bombardment or is otherwise a plasma. HeNe, HgHe10
and the molecular ions He2+
, and HeD+
have been created this way. This technique has also allowed the production of the neutral molecule He2, which has a large number of band systems, and HgHe, which is apparently only held together by polarization forces. Theoretically, other compounds, like helium fluorohydride (HHeF), may also be possible.
Occurrence and Production of Helium
is the second most abundant element in the known Universe after hydrogen and constitutes 23% of the elemental mass of the universe. It is concentrated in stars, where it is formed from hydrogen by the nuclear fusion of the proton-proton chain reaction and CNO cycle. According to the Big Bang model of the early development of the universe, the vast majority of helium was formed during Big Bang nucleosynthesis, from one to three minutes after the Big Bang. As such, measurements of its abundance contribute to cosmological models.
In the Earth’s atmosphere, the concentration of helium by volume is only 5.2 parts per million, largely because most helium
in the Earth’s atmosphere escapes into space due to its inertness and low mass. In the Earth’s heterosphere, a part of the upper atmosphere, helium and other lighter gases are the most abundant elements.
Nearly all helium on Earth is a result of radioactive decay. The decay product is primarily found in minerals of uranium
and thorium, including cleveites, pitchblende, carnotite, monazite and beryl, because they emit alpha particles, which consist of helium nuclei (He2+
) to which electrons readily combine. In this way an estimated 3.4 litres of helium
per year are generated per cubic kilometer of the Earth’s crust. In the Earth’s crust, the concentration of helium is 8 parts per billion. In seawater, the concentration is only 4 parts per trillion. There are also small amounts in mineral springs, volcanic gas, and meteoric iron. The greatest concentrations on the planet are in natural gas, from which most commercial helium is derived.
For large-scale use, helium is extracted by fractional distillation from natural gas, which contains up to 7% helium. Since helium has a lower boiling point than any other element, low temperature and high pressure are used to liquefy nearly all the other gases (mostly nitrogen
and methane). The resulting crude helium gas is purified by successive exposures to lowering temperatures, in which almost all of the remaining nitrogen
and other gases are precipitated out of the gaseous mixture. Activated charcoal is used as a final purification step, usually resulting in 99.995% pure, Grade-A, helium. The principal impurity in Grade-A helium is neon.
As of 2004, over one hundred and fifty million cubic metres of helium were extracted from natural gas or withdrawn from helium reserves, annually, with approximately 84% of production from the United States, 10% from Algeria, and most of the remainder from Canada, China, Poland, Qatar, and Russia. In the United States, most helium is produced in Kansas and Texas.
Diffusion of crude natural gas through special semi-permeable membranes and other barriers is another method to recover and purify helium. Helium can be synthesized by bombardment of lithium or boron with high-velocity protons, but this is not an economically viable method of production.
Isotopes of Helium
A subset of exotic light nuclei, the exotic helium isotopes have larger atomic masses than helium’s natural isotopes. Although all exotic helium isotopes decay with a half-llife of less than one second, researchers have eagerly created exotic light isotopes through particle accelerator collisions to create unusual atomic nuclei for elements such as helium, lithium, and nitrogen
. The bizarre nuclear structures of such isotopes may offer insight into the isolated properties of neutrons.
The most widely-studied exotic helium isotope, for example, is helium-8. This isotope is thought to consist of a normal helium-4 nucleus surrounded by four neutrons dubbed a “halo” (6He also has a halo of neutrons). Halo nuclei have become an area of intense research. isotopes up to helium-10, with two protons and eight neutrons, have been confirmed. By comparison, the most common He-4 isotope has only two neutrons.
There are eight known isotopes of helium, but only helium-3 and helium-4 are stable. In the Earth’s atmosphere, there is one He-3 atom for every million He-4 atoms.
The most common isotope, helium-4, is produced on Earth by alpha decay of heavier radioactive elements; the alpha particles that emerge are fully ionized helium-4 nuclei. Helium-4 is an unusually stable nucleus because its nucleons are arranged into complete shells. It was also formed in enormous quantities during Big Bang nucleosynthesis.
3He [1 neutrons]
Stable with 1 neutron
Extraplanetary material, such as lunar and asteroid regolith, have trace amounts of helium-3 from being bombarded by solar winds. The Moon’s surface contains helium-3 at concentrations on the order of 0.01 ppm.
4He [2 neutrons]
Stable with 2 neutrons
Liquid helium-4 can be cooled to about 1 kelvin using evaporative cooling.
5He [3 neutrons]
Half life: 7.00(30) x 10-24
Decays to 4
Highly unstable, decays to 4
6He [4 neutrons]
Half life: 806.7(15) ms [ beta- ]
Decays to 6
Produced from 7
He or 11
7He [5 neutrons]
Half life: 2.9(5)-21
Highly unstable, decays to 6
8He [6 neutrons]
Half life: 119.0(15) ms
Produced from 9
He, decomposes to 7
Li through beta decay then emits a delayed neutron.
9He [7 neutrons]
Half life: 7(4) x 10-21
Highly unstable, decays to 8
10He [8 neutrons]
Half life: 2.7(18) x 10-21
Highly unstable, decays to 9He.