Atomic number 10

Neon

Ne
+ phase
− phase

Shells Orbitals 2s 2p

Drag to rotate; pinch or Ctrl + scroll to zoom. Arrow keys rotate, + − zoom, R resets.

10 electrons in 2 shells: 2, 8.

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

Category: Noble gas
Period: 2
Group: 18
Block: p
Standard atomic weight: 20.180(1) Da
Electron configuration: [He] 2s² 2p⁶
Phase at 25 °C: gas
Melting point: −248.59 °C
Boiling point: −246.046 °C
Density: 0.8999 g/L
Discovered: 1898, William Ramsay and Morris Travers, in liquefied air

Neighbours in the table

F Na

In configuration notation sodium is written [Ne] 3s¹: an atom with one electron more than neon, whose arrangement stands in for the core of the period 3 elements. Neon’s own configuration, [He] 2s² 2p⁶, leaves every electron paired and both of its shells full. The same ten-electron pattern turns up in ions. OpenStax lists N³⁻, O²⁻, F⁻, Ne, Na⁺, Mg²⁺ and Al³⁺ as isoelectronic, meaning they share one electron configuration. Along that series the radius shrinks as nuclear charge grows, so neon, with ten protons, falls in the middle for size.

Taking an electron away from a free neon atom costs 21.565 eV, its first ionisation energy. Nor does a neon atom readily take an extra electron. The newcomer would need a higher shell, which OpenStax calls more difficult, and PubChem’s record carries two electron affinities for neon, 0 eV and −0.3 eV. PubChem defines electron affinity as the energy released when a neutral atom gains an electron, so zero or less means none is given out.

Between neighbouring neon atoms the only attraction is the London dispersion force, a weak pull that arises from electrons shifting briefly to one side of an atom. It can hold the atoms together only when they barely move, so neon stays a gas down to −246.046 °C and freezes at −248.59 °C.

Where it is found

Neon is a trace gas of the atmosphere. Los Alamos puts it at 1 part in 65,000 of air, while Jefferson Lab, in PubChem’s record, gives 0.0018 per cent. The crust holds about 5 × 10⁻³ milligrams per kilogram. William Ramsay and Morris Travers picked it out of liquefied air in 1898 by its bright orange spectral lines, shortly after they had found krypton. Supply still comes from liquefied air, by fractional distillation, the splitting of a liquid mixture by the temperatures at which its parts boil.

Three stable isotopes make up natural neon: neon-20 at 90.48 per cent of atoms, neon-22 at 9.25 per cent and neon-21 at 0.27 per cent. Not all of it is primordial, the neon the planet began with. IUPAC notes that some neon-21 and neon-22 forms in the crust when neutrons and alpha particles from uranium and thorium decay strike oxygen-18 and fluorine-19 in minerals. Cosmic rays add more at Earth’s surface and on other bodies in space, by knocking fragments off magnesium, silicon, aluminium and sodium nuclei. Measuring all three isotopes tells the three origins apart.

Why it matters

Signs take most of it. Los Alamos says advertising signs account for the bulk of neon’s use, and Jefferson Lab calls them the largest. OpenStax describes the effect: an electric spark through neon at low pressure produces a red glow, which Los Alamos calls reddish orange. Los Alamos adds that, of the rare gases, neon gives the brightest discharge at everyday voltages and currents.

The same discharge does quieter jobs. Los Alamos lists lightning arrestors, indicators for high voltage, television tubes and the tubes of wave meters. Combined with helium, neon makes the helium-neon laser, which competes with gallium arsenide laser diodes where visible light is needed, according to the US Geological Survey. Low-pressure sodium lamps carry a little neon, argon or both as a starter gas. RP Photonics explains that the discharge begins in that gas and the heat it produces evaporates the sodium.

Neon’s isotopes have work of their own. Striking neon-22 with protons produces the radioisotope sodium-22, and neon-20 has been used to make fluorine-18. Neon-21 records how long a rock surface has faced cosmic rays, which makes it a clock for meteorites and for boulders uncovered by melting glacial ice.

Properties

Properties of neon 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	10
Standard atomic weight	20.180(1) Da	Da	IUPAC/CIAAW 2024
Melting point	−248.59 °C	°C	PubChem: 24.56 K
Boiling point	−246.046 °C	°C	Los Alamos: −246.046 °C
Density	0.8999 g/L	g/L
Electron configuration	1s² 2s² 2p⁶
Phase at 25 °C	gas

Safety

Neon is essentially harmless in its everyday forms: the CAMEO hazards database, quoted in PubChem, calls it nontoxic, nonflammable and noncombustible. The cautions on its safety record are physical. Cylinders hold the gas under pressure, the refrigerated liquid causes cold burns, and a large release leaves less oxygen in a confined space. The PubChem Laboratory Chemical Safety Summary for neon is the sheet for people who store or move it in quantity.

Questions

Why does neon glow red-orange in signs?

A sign tube holds neon at low pressure, and an electric discharge through the gas makes it give off light. OpenStax calls the colour red, Los Alamos reddish orange, and CIAAW, quoted in PubChem, describes neon's spectral lines as bright orange. Tubes of other colours get their light from argon or mercury vapour mixed into the neon, or from glass of a special colour, according to OpenStax.

Is neon gas dangerous to breathe?

Neon is not poisonous; CAMEO, the US chemical-hazards database quoted in PubChem, calls it nontoxic and notes that it can asphyxiate. In a confined space a large release lowers the oxygen content of the air. Vapour boiling off the cold liquid at first spreads along the ground, although the gas at room temperature is lighter than air. The PubChem Laboratory Chemical Safety Summary for neon is the authoritative sheet.

Does neon form any compounds?

Stable compounds of neon are not known, according to OpenStax. Los Alamos mentions a reported compound with fluorine and calls true neon compounds questionable, though it adds that evidence for them is mounting. Other species are known: the ions Ne⁺, (NeAr)⁺, (NeH)⁺ and (HeNe⁺) have been seen in optical and mass spectrometric studies, neon forms an unstable hydrate, and an electrical discharge joins mercury and neon into HgNe, held together by van der Waals forces.

What is liquid neon used for?

Los Alamos describes liquid neon as a commercially available cryogenic refrigerant, a coolant for very low temperatures. Per unit volume it has over 40 times the refrigerating capacity of liquid helium and over three times what liquid hydrogen offers, and where it is cold enough for the job it costs less than helium. Its liquid range is narrow: it boils at −246.046 °C and freezes at −248.59 °C.

Appears in these articles

Intermolecular forces: why water boils at 100 °C and methane at −161 °C Methane's molar mass is 16 g/mol and water's 18, yet methane boils at −161 °C and water at 100 °C.