Helium He

Discovery

Helium is unusual in having been found in the Sun before it was found on Earth. During the solar eclipse of 18 August 1868, the French astronomer Jules Janssen observed a bright yellow line in the spectrum of the solar prominences. Later that year the English astronomer Norman Lockyer studied the same line and concluded that it could not come from any known element. Together with the chemist Edward Frankland he proposed a new solar element.

For almost three decades many chemists were sceptical. Then in 1895 the Scottish chemist William Ramsay treated the uranium mineral cleveite with acid and collected a gas whose spectrum showed the same yellow line. Per Teodor Cleve and Nils Abraham Langlet in Sweden isolated it independently at about the same time and measured its atomic weight.

In 1903 large amounts of helium were discovered in natural gas in Kansas, turning a laboratory curiosity into an industrial resource. Heike Kamerlingh Onnes first liquefied helium in 1908, and the extreme cold it provided led him to discover superconductivity in 1911.

Origin of the name

The name comes from the Greek helios, meaning Sun, because the element was first detected in sunlight. The ending '-ium' is normally used for metals; Lockyer chose it because he assumed the new element was a metal. The symbol He is the first two letters of the name.

Main uses

• Cryogenics: liquid helium cools the superconducting magnets of MRI scanners, NMR spectrometers and particle accelerators.
• Lifting gas: it fills weather balloons, research balloons and party balloons, and replaced hydrogen in airships because it cannot burn.
• Breathing mixtures: deep-sea divers use helium–oxygen blends to avoid nitrogen narcosis.
• Manufacturing: it provides an inert shield for welding and for growing silicon crystals, and is used for leak detection because its small atoms slip through tiny gaps.
• Space and science: it pressurises rocket fuel tanks and cools sensitive detectors.

Isotopes

Helium has two stable isotopes. Helium-4 makes up almost all natural helium; its nucleus is identical to the alpha particle emitted in radioactive decay, and most of the helium in Earth's crust was produced this way over billions of years. Helium-3 is extremely rare on Earth, roughly one atom per million in air-derived helium. It is used in neutron detectors and in dilution refrigerators that reach temperatures within a few thousandths of a degree of absolute zero. Much of the world's helium-3 comes from the decay of tritium.

In everyday life

Helium is non-toxic, but it displaces oxygen, so breathing it straight from a tank can cause loss of consciousness; inhaling it from balloons for the squeaky-voice trick is not risk-free. The voice change happens because sound travels much faster in helium than in air, shifting the resonances of the vocal tract.

On Earth helium is a finite resource. Once released, it is light enough to escape the atmosphere into space, so it is extracted from certain natural gas fields and recycled where possible.