Argon (Ar)

Argon is the third most abundant gas in the atmosphere at 0.93%, more common than carbon dioxide, and almost all of it is argon-40 produced by potassium-40 decaying inside rocks over billions of years — the atmosphere of a planet with less potassium would have very little argon. Rayleigh found it by noticing that nitrogen from the air was consistently 0.5% denser than nitrogen made chemically, a discrepancy he refused to ignore, and the pair got a Nobel Prize for chasing it down. The name means "the lazy one".

Properties

Atomic number

18

Symbol

Ar

Standard atomic weight

39.95 u

Category

Noble gas

Group

18

Period

3

Block

p-block

Electron configuration

[Ne] 3s² 3p⁶

Electrons per shell

2, 8, 8

Melting point

83.8 K — -189.3 °C — -308.8 °F

Boiling point

87.3 K — -185.8 °C — -302.5 °F

Density

0.0017837 g/cm³

Oxidation states

0

Discovered

1894 by Lord Rayleigh and William Ramsay

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶

Noble gas shorthand

[Ne] 3s² 3p⁶

Shells

K: 2 · L: 8 · M: 8

What it is used for

• Shielding gas for MIG and TIG welding
• The fill gas between double-glazing panes
• Blanketing molten metal and reactive chemicals
• Filling incandescent and halogen bulbs
• The carrier gas in gas chromatography and plasma spectrometry

Hazards

• Asphyxiant; denser than air and collects in pits, tanks and ship holds

IUPAC publishes an interval, [39.792, 39.963]; 39.95 is the conventional value, and the spread is unusually wide because argon from different geological sources carries different amounts of radiogenic argon-40. Density is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 18

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Argon sits in group 18 with helium, neon, krypton, xenon, radon, oganesson.