18 Ar 39.95

Argon

Noble gas Argon gas

[Ne] 3s² 3p⁶ · 2 · 8 · 8

← Cl Chlorine K Potassium →

The first noble gas discovered and the cheapest of them: nearly one per cent of air is argon. The name means idle — it reacts with nothing, and that is exactly what it is valued for.

3D model

Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.

Drag to rotate, scroll to zoom

Atomic properties

Atomic number: 18
Atomic mass: 39.95 а.е.м.
Electron configuration: [Ne] 3s² 3p⁶
Electrons per shell: 2 · 8 · 8
Block: p-block
Group: Noble gases
Period: 3
Electronegativity (Pauling): no data
Electronegativity (Allen): 3.24
Atomic radius: 71 pm
Covalent radius: 106 pm
Van der Waals radius: 188 pm
Ionisation energy 1: 1 520.6 kJ/mol
Ionisation energy 2: 2 665.8 kJ/mol
Ionisation energy 3: 3 931 kJ/mol
Electron affinity: 0 kJ/mol
Common oxidation states: 0
Oxidation states: 0

Physical properties

State at 25 °C: gas
Density: 0.00178 g/cm³
Melting point: 83.81 K · -189.3 °C
Boiling point: 87.3 K · -185.8 °C
Speed of sound: 323 m/s

Thermal properties

Heat of fusion: 1.18 kJ/mol
Heat of vaporisation: 6.53 kJ/mol
Specific heat capacity: 0.52 J/(g·K)
Thermal conductivity: 0.01772 W/(m·K)

Mechanical properties

Young's modulus: no data
Shear modulus: no data
Bulk modulus: no data
Poisson ratio: no data
Mohs hardness: no data
Brinell hardness: no data

Electrical and magnetic properties

Electrical resistivity: no data
Magnetic ordering: diamagnetic
Curie point: no data
Néel point: no data
Superconducting point: no data

Crystal structure

Crystal structure: face-centred cubic (fcc)
Lattice constants: a = 525.6 pm

Abundance

In the crust: 3.5 mg/kg
In the ocean: 0.45 mg/L
In the universe: 20 000 mg/kg
In the human body: 0 mg/kg

Isotopes

Isotope	Abundance	Half-life	Decay mode
³⁶Ar	0.334 %	stable	—
³⁸Ar	0.063 %	stable	—
⁴⁰Ar	99.604 %	stable	—

Potassium–argon dating is a mainstay of geochronology: argon-40 accumulates in rock from decaying potassium-40 and dates volcanic layers.

Discovery

Year of discovery: 1894
Discovered by: Lord Rayleigh and William Ramsay
Where: Scotland
Origin of the name: from Greek argos (idle): the gas reacts with nothing

History

Rayleigh noticed that nitrogen from air was half a per cent heavier than nitrogen from compounds. With Ramsay he isolated the residue in 1894 and found a new element. Both took Nobel Prizes for it in 1904.

Where it occurs

0.93 % of the atmosphere by volume — the third gas of air after nitrogen and oxygen. Nearly all terrestrial argon-40 is radiogenic, from decaying potassium-40.

How it is produced

As a by-product of cryogenic air separation. Argon is the cheapest noble gas, which is what makes its bulk uses possible.

Role in living things

No biological role. Under pressure argon is more narcotic than nitrogen, which limits its use in breathing mixtures.

Safety

Non-toxic, but heavier than air, so it pools in pits and holds and displaces oxygen. This is a regular cause of industrial fatalities.