All practice pages/Interatomic theory/Elements
Element 18, a noble gas
The third most common gas in the air, far ahead of carbon dioxide, and so unreactive that it was named lazy.
Period 3, group 18. The other noble gases are tinted, and every cell opens its own article.
Argon has 18 electrons, one for every proton, so the atom carries no charge. Below are the same 18 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁶
Short form: [Ne] 3s² 3p⁶, where [Ne] stands for the 10 electrons neon already has.
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14. Shell 2 here is 2s² 2p⁶, and 2 + 6 is 8.
Argon sits in period 3 and has 3 shells. The period number counts the shells.
The outer shell holds 8 electrons. Those are the ones argon can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 18 means 8.
A full valence shell, so it has almost no tendency to gain, lose or share electrons, and it exists as single atoms.
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.
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.
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.