Atomic number 18

Argon

Ar
+ phase
− phase

Shells Orbitals 3s 3p

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18 electrons in 3 shells: 2, 8, 8.

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

Category: Noble gas
Period: 3
Group: 18
Block: p
Standard atomic weight: 39.95(16) Da
Electron configuration: [Ne] 3s² 3p⁶
Phase at 25 °C: gas
Melting point: −189.34 °C
Boiling point: −185.85 °C
Density: 1.7837 g/L
Discovered: 1894, William Ramsay and Lord Rayleigh, in liquefied air

Neighbours in the table

Cl K

Argon was found by subtraction. Henry Cavendish set the puzzle in 1785: once oxygen and nitrogen had been taken out of air, some unidentified gas still remained. In 1894 William Ramsay and Lord Rayleigh found it in liquefied air. The name comes from the Greek argos, lazy or inactive. CIAAW, the atomic-weights commission of the International Union of Pure and Applied Chemistry, or IUPAC, ties it to the gas’s refusal to combine with other elements.

Argon’s configuration, [Ne] 3s² 3p⁶, is shared by a whole family of ions. OpenStax groups P³⁻, S²⁻, Cl⁻, K⁺, Ca²⁺ and Sc³⁺ with the argon atom as an isoelectronic series, particles with the same electron arrangement. Along it the nucleus grows from 15 protons to 21. The greater that nuclear charge, the smaller the particle, so the phosphide ion is the largest member and Sc³⁺ the smallest.

Between argon atoms act only weak London dispersion forces, brief attractions set up when an atom’s electron cloud shifts to one side for an instant. OpenStax gives those forces as the reason the noble gases melt and boil so low for their mass. For argon the two points sit about 3.5 degrees apart, at −189.34 °C and −185.85 °C, a narrow window for the liquid.

Where it is found

Jefferson Lab puts argon at 0.93 per cent of Earth’s atmosphere, and Los Alamos gives 0.94 per cent. Among the gases of dry air, OpenStax places it third by abundance. By Jefferson Lab’s estimates the crust holds about 3.5 milligrams per kilogram and seawater 0.45 milligrams per litre. Los Alamos adds that the atmosphere of Mars contains 1.6 per cent argon-40 and 5 parts per million argon-36.

Nearly every argon atom on Earth is argon-40. NIST, the US National Institute of Standards and Technology, gives representative shares of about 99.6 per cent argon-40, 0.33 per cent argon-36 and 0.063 per cent argon-38. All three are stable. Those shares shift from one sample to another, and CIAAW allows argon-40 anywhere from 93.6 to 100 per cent. CIAAW gives the standard atomic weight as an interval; 39.95 Da is the conventional value used in its place.

Argon comes from the fractional distillation of liquid air, in which air is cooled until it condenses and then separated as each component boils off at its own temperature. Jefferson Lab describes the gas as a byproduct of producing oxygen and nitrogen.

Why it matters

Inside an incandescent bulb, argon slows the vaporisation of the tungsten filament. OpenStax explains why it was preferred to nitrogen for that job: it conducts heat less well and is chemically inert, so the filament lasts longer. Jefferson Lab names a second cause, keeping oxygen from corroding the hot filament. Fluorescent tubes hold argon with mercury vapour, at about 400 pascals, Los Alamos says. Low-pressure sodium vapour lamps carry a little neon or argon as a starter gas, while high-pressure ones usually use xenon, RP Photonics notes.

Argon’s isotopes have jobs of their own. Argon-41, whose half-life is 109.61 minutes in the Atomic Mass Data Center’s table, traces industrial gas flows; IUPAC credits the role to its inertness, half-life and gamma radiation. Potassium-38, a widely used blood-flow tracer with a half-life of 7.6 minutes, is made from argon targets. Usually these are argon-40, which IUPAC says is cheaper than argon-38 and has other advantages as a target.

Groundwater studies measure argon-37 (half-life 35 days), argon-39 (268 years) and argon-40 together. The ⁴⁰Ar/³⁹Ar method of dating rock, used especially on volcanic samples, irradiates a sample in a nuclear reactor to turn some potassium-39 into argon-39. It then measures the ratio of argon-40 to argon-39. Cosmic-ray argon-38 in surface rock records how long it has been exposed and how fast it erodes.

Properties

Properties of argon 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	18
Standard atomic weight	39.95(16) Da	Da	IUPAC/CIAAW 2024
Melting point	−189.34 °C	°C	Los Alamos: −189.34 °C
Boiling point	−185.85 °C	°C	Los Alamos: −185.85 °C
Density	1.7837 g/L	g/L
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶
Phase at 25 °C	gas

The standard atomic weight of this element is an interval in nature, from 39.792 to 39.963 Da; the value shown is IUPAC's conventional abridged value.

Safety

Argon is nontoxic and does not burn, and in air or a sealed lamp it is essentially harmless. Its safety sheet lists three cautions for the bulk product. In a confined space the gas can crowd out oxygen and cause suffocation, the evaporating liquid causes frostbite, and a heated cylinder under pressure may explode. The PubChem Laboratory Chemical Safety Summary for argon is the authoritative sheet for anyone who stores or handles it.

Questions

Does argon form any compounds?

Jefferson Lab says argon forms at least one compound: argon fluorohydride, HArF, reported in August 2000. It begins to break down once it warms above −246 °C, so it has no use outside basic research. OpenStax also notes that unstable argon compounds form at low temperatures.

How is argon used to date rocks?

Potassium-40 has a half-life of 1.248 billion years in the International Atomic Energy Agency (IAEA) evaluation. Its decays split 89.28 per cent beta-minus and 10.72 per cent electron capture or positron emission. Only the second yields argon-40: both turn a proton into a neutron, lowering the atomic number from 19 to 18. After lava crystallises, the argon-40 it makes stays trapped, and IUPAC puts ages measured this way at about 10⁶ to 10⁹ years.

Is argon safe to breathe?

Argon is nontoxic, and roughly one part in a hundred of ordinary air is argon already. The danger is what it replaces. The International Chemical Safety Cards (ICSC) note that the gas is heavier than air and can gather in low spaces, leaving too little oxygen there. No threshold limit value, the workplace exposure ceiling, is assigned to it, because the limiting factor is the oxygen available.

Why do welders use argon?

Los Alamos names arc welding and cutting among argon's uses, with the inert gas acting as a shield; Jefferson Lab says it forms an inert atmosphere for arc welding. The same inertness earns argon other protective roles. Titanium is produced under an argon blanket, and silicon and germanium crystals are grown in an argon atmosphere. Argon is also nonflammable and noncombustible.

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