Atomic number 2

Helium

He
+ phase
− phase

Shells Orbitals 1s

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2 electrons in 1 shell: 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Noble gas
Period: 1
Group: 18
Block: s
Standard atomic weight: 4.0026(1) Da
Electron configuration: 1s²
Phase at 25 °C: gas
Melting point: −272.2 °C
Boiling point: −268.93 °C
Density: 0.1785 g/L
Discovered: 1868, Pierre Janssen and Norman Lockyer, in the spectrum of the Sun

Neighbours in the table

H Li

Helium’s two electrons fill the only shell it has, the configuration written 1s². A full shell has nothing to offer: no electron loosely held, no gap for another atom’s electron to occupy. Filled valence shells, the outer shells that take part in bonding, made group 18 look unreactive, hence the name noble gases. For helium the first ionisation energy, the energy that must be supplied before a free atom gives up one electron, is 24.587 eV.

No stable compound of helium is known. Argon forms unstable compounds at low temperatures and xenon forms stable fluorides, but nothing keeps hold of helium. PubChem’s element record says so flatly: no compound containing helium is known, though chemists have kept trying to make helium difluoride, HeF₂. PubChem’s safety datasheet files helium among the substances that undergo no chemical reaction under any known circumstances.

That leaves one helium atom nothing to grip its neighbour with except weak London dispersion forces, the fleeting attraction between atoms whose electrons sit briefly off-centre. It is why helium boils at −268.93 °C, far below other substances of similar mass. Helium is colourless, odourless and barely soluble in water. At 0.1785 g per litre it is about a seventh as dense as the air around it, which is what lifts a balloon.

Where it is found

After hydrogen, helium is the most abundant element in the universe. Stars also make it, in the fusion reactions that power them, the proton-proton chain and the carbon cycle. Helium was found there before it was found here. During the total solar eclipse of 1868 the French astronomer Pierre Janssen recorded a yellow line in the spectrum of the Sun.

Norman Lockyer showed that the line, at 587.49 nanometres, could not come from any element then known, and named it helium, after helios, the Greek sun. No one caught it on Earth until 1895, when William Ramsay treated the uranium mineral cleveite with mineral acids and had the gas identified. Rutherford and Royds supplied the reason in 1907: the alpha particles thrown out by radioactive decay are helium nuclei.

An alpha particle becomes a helium atom once it picks up two electrons, and that helium works its way up through cracks in the crust. Some natural gas holds 1–2% helium by mass; it is separated by liquefying everything else, leaving helium as the only gas.

Why it matters

Helium’s usefulness begins where its boiling point ends. Liquid helium is the coolant that reaches the temperatures at which traditional superconductors work, materials that carry a current with no resistance once they are cold enough. Those temperatures are what the magnet inside a magnetic resonance imaging scanner needs. Liquid nitrogen, the other common coolant, is significantly cheaper, but only helium goes that cold. The Department of Energy’s Jefferson Lab runs its superconducting electron accelerator on large amounts of liquid helium.

Inertness does the rest of the work. Welders use helium as a shield, so metals that oxidise easily can be melted and joined out of contact with air. The same atmosphere protects silicon and germanium crystals while they are grown. Divers breathing helium instead of nitrogen avoid nitrogen narcosis, the disorientation known as rapture of the deep, with the helium-to-oxygen ratio set by the working depth.

Because it does not burn, helium is the safer lifting gas for Goodyear’s advertising blimps and for scientific balloons. NASA flew helium balloons over Antarctica to sample the air for whatever was depleting the ozone layer. Rockets use it to pressurise liquid fuel. A Saturn booster of the kind flown on the Apollo lunar missions took about 13 million cubic feet of helium for one firing, plus more for checkouts.

Properties

Properties of helium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	2
Standard atomic weight	4.0026(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−272.2 °C	°C	Los Alamos: −272.2 °C
Boiling point	−268.93 °C	°C	Los Alamos: −268.93 °C
Density	0.1785 g/L	g/L
Electron configuration	1s²
Phase at 25 °C	gas

Safety

Helium is non-toxic, non-flammable and chemically inert. Its safety sheet gives it the signal word Warning, no health hazard class, and two statements about the container: gas under pressure, and a refrigerated liquid that causes cold burns. The one further caution on that sheet is that helium escaping into a confined space lowers the oxygen content of the air. Helium in quantity is a workplace substance, and the PubChem Laboratory Chemical Safety Summary is the authoritative sheet for the people who keep and move it.

Questions

Why is helium used in balloons instead of hydrogen?

Both gases are lighter than air, so both give lift. The difference is that hydrogen burns and helium does not, which is why blimps and party balloons are filled with helium. The lift comes from displacement: helium has 0.14 times the density of air, and a litre of it weighs 0.1785 g. The heavier air sinks around the envelope and presses it up from below.

Can helium be frozen solid?

Only under pressure. No other known substance stays liquid when cooled at ordinary pressure: helium remains a liquid down to about a thousandth of a degree above absolute zero, and it solidifies only under applied pressure. A melting point for helium, such as the −272.20 °C above, therefore belongs to helium held under pressure. PubChem's safety datasheet notes that helium has no triple point, no temperature and pressure where solid, liquid and gas meet.

Is it dangerous to breathe helium from a balloon?

Helium is not poisonous, and the squeaky voice it produces is noted on the same datasheet that gives the reason for caution. Helium carries no oxygen of its own, so breathing it in place of air lowers the oxygen reaching the lungs. That datasheet warns that helium vapour can cause dizziness or asphyxiation without warning, especially in a closed space. The PubChem safety summary is the authoritative sheet.

Is the world running out of helium?

Helium that escapes is gone: the atmosphere holds about 0.0005% helium, which Earth's gravity cannot keep, so it drains away to space. In the United States, extracting helium from natural gas pays only for gas holding more than about 0.3% by volume. Helium is classed there as a strategic resource, stored underground at Cliffside Field under a conservation programme begun in 1960 and sold off under the 1996 Helium Privatization Act.

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