Helium

Helium is the chemical element with atomic number 2 and the symbol He. It is a colorless, odorless gas that is almost completely unreactive, and it is the first member of the noble gas family in group 18 of the periodic table. After hydrogen it is the second most abundant element in the universe, yet on Earth it is scarce enough that people recover it from underground natural gas rather than from the air around them.

History of helium

Helium was seen in the Sun before anyone found it on Earth. In 1868 the French astronomer Pierre Janssen traveled to India to study the solar spectrum during a total eclipse and noticed a bright yellow line that matched no known substance. The English astronomer Joseph Norman Lockyer recorded the same line from London and proposed that it came from a new element. He named it helium after helios, the Greek word for the Sun.

Hints of the same line turned up later in terrestrial samples, including gas from Mount Vesuvius in 1882 and gas released when the uranium mineral uraninite dissolved in acid in 1889. The identification was settled in 1895, when the Scottish chemist William Ramsay found helium in a uranium mineral. Per Teodor Cleve and Nils Abraham Langlet in Sweden worked on the same mineral independently and measured the atomic weight of the gas. Their results confirmed that the element seen in the solar spectrum was real and present on Earth.

Properties of helium

Helium has two protons and, in its common form, two neutrons and two electrons. Its two electrons fill the first electron shell completely, which is why the atom has almost no tendency to share or trade electrons with other atoms. Chemists therefore call it inert, and it forms compounds only under unusual laboratory conditions.

The element has a standard atomic weight of about 4.0026. At room temperature its density is only about 0.000164 grams per cubic centimeter, far lighter than air, which is why a helium balloon rises. Helium boils at 4.22 kelvin, about minus 268.9 degrees Celsius, one of the lowest boiling points known for any substance. That extreme cold makes liquid helium valuable as a coolant, because very few other materials stay liquid at such temperatures.

Measurements compiled by PubChem give helium a very small atom, with a van der Waals radius of 140 picometers and a covalent radius of 28 picometers. Its first ionization energy, the energy needed to pull one electron away, is 24.587 electronvolts. That figure is high compared with most elements and again reflects how tightly the filled shell holds its electrons.

Occurrence and production of helium

Helium makes up a large share of the mass of stars, where it forms from hydrogen through nuclear fusion. On Earth the supply comes from a different process: alpha particles released when radioactive elements such as uranium and thorium decay become helium atoms. Some of this gas collects underground in the same rock formations that trap natural gas, and some escapes into the atmosphere.

The atmosphere holds only about 5 parts per million of helium by volume. Because the gas is light, it drifts upward and gradually leaks into space, so the air is a balance between slow production and slow loss. Extracting helium from air is not economical. Instead, producers separate it from natural gas, which can contain up to roughly 7 percent helium, and the main commercial sources in the United States have been gas fields in Texas, Oklahoma and Kansas.

Reference sources also note that every sample of natural gas holds at least trace amounts of helium, and that helium takes part in the fusion reactions, including the proton-proton reaction, that supply the energy of the Sun and other stars.

Uses of helium

Cooling is the largest scientific use. Liquid helium keeps the superconducting magnets in the Large Hadron Collider at CERN, in magnetic resonance imaging (MRI) scanners and in nuclear magnetic resonance (NMR) spectrometers at their working temperature. A superconductor is a material that carries electricity without resistance once it is cold enough, and helium supplies that cold. Helium has also been used to cool instruments on satellites and the liquid fuels of the Apollo launch vehicles.

Other applications depend on its low density or its lack of reactivity. It fills weather balloons, airships and decorative balloons, and it replaced flammable hydrogen for these purposes because it cannot burn. As an inert shielding gas it protects materials during arc welding and during the manufacture of optical fibers and semiconductors. It is used to pressurize the fuel tanks of liquid-fueled rockets and to detect leaks in systems such as car air conditioners, since its small atoms slip through the tiniest openings.

Divers and others working under high pressure breathe a mixture of about 80 percent helium and 20 percent oxygen, which avoids the nitrogen narcosis that can affect people breathing compressed air at depth. Helium-neon lasers have been used in supermarket barcode scanners, and helium-ion microscopes can produce sharper images than some scanning electron microscopes.

Isotopes of helium

Helium has two stable isotopes, helium-3 and helium-4. An isotope is a version of an element with the same number of protons but a different number of neutrons. Helium-4 makes up almost all natural helium, with an abundance of about 99.99987 percent. Helium-3 is extremely rare in comparison, at roughly 1.3 parts per million in the IUPAC data, so ordinary tanks of the gas are essentially pure helium-4.

Health and environmental effects of helium

Helium has no known biological role and is not toxic. Its main hazard is that, like any gas that displaces air, it can reduce the oxygen available to breathe in a confined space, and compressed gas cylinders carry the usual handling risks. Because helium escapes to space once released, it is treated as a resource that is not replaced on human timescales, which is one reason recovery and recycling matter for laboratories and hospitals that rely on it.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue