Helium

Helium is the second most abundant element in the universe after hydrogen and one of the rarest on Earth. It is the only element discovered on the Sun before it was found on this planet, and the only substance that will not solidify at ordinary pressure however close to absolute zero it is cooled.

Chemically helium does nothing: its electron shell is full and it forms no compounds in any ordinary sense. Its whole value lies in its physical properties.

On Earth helium is produced by the radioactive decay of heavy elements: an alpha particle is a helium nucleus. It accumulates in some natural gas fields, and that is where it is extracted from; the content is a fraction of a percent, and only a few fields are worth working.

The planet cannot hold on to it. Helium is light and binds to nothing, so it escapes into space irretrievably. That makes the prospect of running out a real one: the helium let out of a balloon does not come back.

During the solar eclipse of 1868 Jules Janssen, and independently Norman Lockyer, saw a yellow line in the spectrum of the corona that matched no known element. Lockyer proposed a new one and named it from the Greek helios, the Sun.

For twenty-seven years the element remained solar. In 1895 William Ramsay treated the uranium mineral cleveite with acid and obtained a gas whose spectrum matched. Helium became the first — and so far the only — element found beyond the Earth before it was found on it.

It does not, and the statement is more precise than it sounds. Helium has the highest ionisation energy of any element and zero electron affinity; it is inclined neither to give electrons up nor to take them on.

No stable helium compounds exist under ordinary conditions. Under extreme ones — hundreds of thousands of atmospheres — compounds such as Na₂He have been made, but this has nothing to do with chemistry in the usual sense.

The physics, though, is remarkable. Cooled below 2.17 K, liquid helium becomes superfluid: viscosity disappears, the liquid creeps up the walls of its container and passes through capillaries no other liquid can enter.

Cooling, above all. Liquid helium boils at 4.2 K, and without it superconducting magnets would not work — which means neither MRI scanners nor the Large Hadron Collider.

Then: protective atmospheres in welding and crystal growth; leak detection, since so small a molecule finds the smallest gap; breathing mixtures for deep diving, in place of nitrogen; and, in the most visible but least important application, balloons.

Helium in a breathing mixture changes the timbre of the voice: the speed of sound in it is nearly three times that in air, which shifts the resonances of the vocal tract. Breathing pure helium is dangerous — not because of the gas but because of the absence of oxygen.