The atom that bonds with nothing
A helium atom is two protons, two neutrons and two electrons that fill the first shell completely. That closed shell is its whole chemistry: helium has the highest ionisation energy of any element, forms no stable compounds under ordinary conditions and exists as single atoms. It is colourless, odourless and, after hydrogenElement: Hydrogen. The lightest element, and most of the universe, the lightest gas there is.
Its atoms attract one another so little that it is the hardest substance to condense. It boils at 4.22 K, lower than anything else, and at atmospheric pressure it never freezes: the motion of its light atoms keeps it liquid down to absolute zero, and about 25 atmospheres are needed to make it solid. Below 2.17 K the liquid becomes a superfluid, as Pyotr Kapitsa found in 1938: it flows without friction, creeps up the walls of its vessel and conducts heat almost perfectly. Nearly all of it is ⁴He; the lighter ³He, about one atom in a million, is a superfluid too, below 0.0025 K.
Where it comes from
About 24 % of the ordinary matter in the universe by mass is helium. Most of it was made in the first minutes after the Big Bang; the rest is made in stars, which fuse hydrogen into helium.
On Earth it is rare. The air holds 5.24 parts per million by volume, and the planet cannot keep even that: helium atoms are too light for gravity to hold, so they drift upwards and are lost to space. The helium we use comes from the ground, where it has been made over hundreds of millions of years by the alpha decay of uranium and thorium — an alpha particle is simply a helium nucleus — and has collected in the same traps that hold natural gas. The richest fields are in the United States, Qatar, Algeria and Russia. It is separated by cooling the gas until everything else has liquefied and helium alone remains a gas.
What it is used for
Helium is the coldest liquid there is, and that is its main job. It cools the superconducting magnets of MRI scanners, of laboratory spectrometers and of particle accelerators — the Large Hadron Collider holds over a hundred tonnes. Because it will not react, it shields the arc in welding and pressurises the fuel tanks of rockets. Because it is light, it lifts balloons and airships in place of the hydrogen that burnt the Hindenburg. Deep divers breathe it mixed with oxygen, since, unlike nitrogen, it does not cloud the mind under pressure; and sound travels almost three times faster through it than through air, which is why a lungful raises the voice.
It is not renewable on any human timescale. Every balloon let go is helium leaving the planet.
Discovery and name
Helium is the only element found in the sky before it was found on Earth. On 18 August 1868, watching a total eclipse from Guntur in India, Pierre Janssen saw a bright yellow line in the spectrum of the Sun's prominences that matched nothing known. Two months later Norman Lockyer in London found he could see the line without an eclipse; he and the chemist Edward Frankland proposed a new element and called it helium, from the Greek helios, the sun — with the ending of a metal, which Lockyer supposed it must be.
In 1895 William Ramsay, looking for argon in the uranium mineral cleveite, released a gas that showed the same yellow line; Lockyer and William Crookes confirmed it, and Per Teodor Cleve and Nils Langlet found it independently in Uppsala. Heike Kamerlingh Onnes liquefied it in Leiden in 1908 and, with it, discovered superconductivity three years later.