Helium is a colourless, odourless gas and the second lightest of the chemical elements. Around 23% of the mass of the universe is helium, so it is also the second most abundant element. The nucleus of its most common isotope consists of two protons and two neutrons.
Helium is an inert, or noble, gas and so does not react well with other elements. The table below shows that helium has no melting point because, under normal conditions, it will not solidify. Very high pressures are required for solidification to take place.
Essential Facts
| Atomic number | 2 |
| Name | Helium |
| Symbol | He |
| Atomic mass | 4.003 |
| Classification | Noble Gas |
| State at 20°C | Gas |
| Melting point | None |
| Boiling point | -268.9°C |
History
Helium was first observed during an eclipse in 1868 by Pierre Janssen. He saw a yellow line in the spectrum of the sun’s chromosphere very close to that of sodium and so assumed that this was what he had detected. Joseph Norman Lockyer also observed this yellow line in the same year but saw that it did not correspond to the know sodium lines.
The new element was unknown on Earth and for many years was assumed to exist only in the sun. Lockyer and Edward Frankland coined the name helium by combining hēlios (Greek for sun) with the New Latin suffix -ium.
In 1895 Sir William Ramsay discovered helium on Earth. He heated a sample of the uranium-bearing mineral cleveite and collected the gas produced. The spectrum of this gas contained the same yellow line observed by Janssen and Lockyer.
Isotopes
Helium has six known isotopes but four of these are unstable and decay rapidly. Of the two stable isotopes, helium-3 and helium-4, helium-4 is by far the most abundant.
Both stable isotopes are produced by radioactive decay. Helium-3 is produced by the decay of the hydrogen-3 isotope tritium, also very rare. The nucleus of a helium-4 atom is an alpha particle ejected from a radioactive element.
The stable isotopes of helium are unique in having multiple liquid forms. As well as the standard liquid form (known as helium I) the helium-4 isotope becomes a superfluid at temperatures below -271°C. The properties of superfluidity are a low viscosity, allowing the liquid to flow freely with little friction, and high conductivity. The superfluid form of helium-4 (known as helium II) has a conductivity around 1000 times greater than that of copper. Helium-3 also has a normal liquid form and two superfluids.
Production
Helium is generated in stars from hydrogen by nuclear fusion. On Earth it is produced naturally by radioactive decay. Very little is present in the Earth’s atmosphere because it is light enough to escape the pull of gravity and it does not bond with other elements.
Natural gas fields in North America, South Africa and the Sahara region contain large amounts of helium. Smaller quantities can be obtained from springs, radioactive minerals or iron-bearing meteorites.
Uses
Helium’s low mass makes it an ideal gas for lifting balloons. Although hydrogen is lighter, helium is a safer alternative as it is non-flammable. This non-flammability also makes it useful in creating an inert atmosphere for the welding of metals.
Helium is used to pressurize fuel tanks in rockets because it is still a gas at the low temperatures where the hydrogen fuel is a liquid. When mixed with oxygen, helium can be used in the high pressure breathing apparatus used in scuba diving.
Cooling applications also use helium. For instance, the Large Hadron Collider (LHC) at CERN in Switzerland uses helium as a coolant. It was a leakage of several tonnes of helium, and the resultant damage to the magnets, that caused the closure of the LHC just after it was switched on in September 2008.
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Related Elements
Hydrogen (H), Lithium (Li), Oxygen (O), Sodium (Na), Iron (Fe), Copper (Cu), Uranium (U)
