Argon

The third most common gas in the air, far ahead of carbon dioxide, and so unreactive that it was named lazy.

At a glance

Symbol: Ar
Atomic number: 18
Average atomic mass: 39.95
Group: 18
Period: 3
Family: noble gas
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶, or [Ne] 3s² 3p⁶
Valence electrons: 8
Electronegativity: none; it forms no stable compounds under ordinary conditions

On the table

Period 3, group 18. The other noble gases are tinted, and every cell opens its own article.

Electrons and bonding

A full valence shell, so it has almost no tendency to gain, lose or share electrons, and it exists as single atoms.

Properties

State at 25 °C: gas
What it looks like: colorless, odorless gas
Melting point: −189.3 °C
Boiling point: −185.8 °C
Density: 1.78 g/L at 0 °C and 1 atm

Where it is found

0.93% of dry air. Nearly all of it is argon-40, made inside rocks over billions of years by the radioactive decay of potassium-40 and slowly released into the atmosphere.

Discovery and name

Lord Rayleigh noticed in 1892 that nitrogen taken from air was slightly denser than nitrogen made from chemicals. With William Ramsay he tracked down the reason in 1894: an unknown gas making up about 1% of air. They named it from the Greek argos, idle or lazy, because it would not react with anything. It was the first noble gas to be isolated.

What it is used for

A shielding gas for welding and for growing silicon crystals, since it keeps air away from hot metal. It fills incandescent light bulbs and the gap between panes of energy-efficient windows, and protects old documents: the original Declaration of Independence is displayed in argon.