Helium – facts and properties

Position in the periodic table

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Where helium is found and how it is extracted

After hydrogen, helium is the second most abundant element in the universe. Much of it formed in the first minutes after the Big Bang, and stars have been making more ever since by fusing hydrogen nuclei. On Earth it is a different story. Helium is present in the air only at a few parts per million, and because the atoms are so light and so unreactive, any helium released into the atmosphere eventually escapes into space.

The helium we use comes from underground. Radioactive elements such as uranium and thorium in rocks emit alpha particles as they decay, and an alpha particle is simply a helium nucleus. Over millions of years the gas accumulates in porous rock and migrates into natural gas reservoirs. Some gas fields in the United States contain several per cent helium; others, such as those in Qatar, contain far less but are so large that they still yield substantial amounts. Helium is separated from natural gas by cooling the gas until every other component has liquefied, leaving helium behind as the only gas. The element was identified in 1868, when astronomers studying the spectrum of the Sun during an eclipse noticed a yellow line that matched no known element. It was named after helios, the Greek word for the Sun, and was not isolated on Earth until nearly three decades later.

Properties and behaviour

Helium is a colourless, odourless, tasteless and non-toxic gas. Its electron configuration is 1s²: the first shell holds its two electrons and is completely full. A full shell means helium has nothing to gain by sharing, giving or taking electrons, so it forms no stable compounds under ordinary conditions and is usually listed without an electronegativity value. Unlike hydrogen, oxygen or nitrogen, helium exists as single atoms rather than molecules. It is placed in period 1 and group 18 with the other noble gases, even though it has only two valence electrons rather than eight; what matters is that the outer shell is complete.

The remarkable properties of helium show up at low temperatures. It boils at -268.93 °C, lower than any other substance, and at atmospheric pressure it never solidifies, not even at absolute zero. Only under pressures of roughly twenty-five times atmospheric pressure does it freeze, reaching a melting point of -272.2 °C. Cooled below about two kelvin, liquid helium becomes a superfluid: it flows without any viscosity, climbs the walls of its container and conducts heat better than copper. With a density of 0.179 g/L helium is the second-lightest gas after hydrogen, and with an atomic radius of 31 pm its atoms are the smallest of any element. Sound travels through helium almost three times faster than through air, which is why a voice sounds high-pitched after a breath of it.

Uses in everyday life and technology

Balloons and airships are the most visible uses of helium. It lifts because it is lighter than air, and unlike hydrogen it cannot burn, so it is the safe choice for anything that flies. Weather balloons carry instruments into the stratosphere with it. Yet these applications account for only a small share of demand.

The largest single use is cooling. The superconducting magnets inside MRI scanners work only at a few kelvin, and liquid helium is the only coolant that reaches those temperatures; hospital scanners hold large volumes of it. Particle accelerators such as the Large Hadron Collider cool their magnets the same way, as do the instruments of many physics experiments. The semiconductor industry uses helium as an inert atmosphere and a heat-transfer gas when making chips and drawing optical fibres. Welders shield aluminium and stainless steel with it. Deep-sea divers breathe mixtures of helium and oxygen because helium, unlike nitrogen, does not cause narcosis at depth. Engineers use it to find leaks: the tiny atoms pass through the smallest cracks and are detected by mass spectrometers. Because helium cannot be made in any practical way and escapes the planet once released, it is treated as a finite resource, and many laboratories now recover and reliquefy it rather than letting it vent.

Quick question

Why is an electronegativity value often missing for many noble gases in tables?

FAQ