All practice pages/Interatomic theory/Elements
Element 17, a halogen
The halogen of table salt, bleach and swimming pools: a greenish, choking gas as an element, and a harmless ion in the sea.
Period 3, group 17: the second rung down the stem of the 7. The other halogens are tinted, and every cell opens its own article.
Chlorine has 17 electrons, one for every proton, so the atom carries no charge. Below are the same 17 electrons written two ways.
1s² 2s² 2p⁶ 3s² 3p⁵
Short form: [Ne] 3s² 3p⁵, 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.
Chlorine sits in period 3 and has 3 shells. The period number counts the shells.
The outer shell holds 7 electrons. Those are the ones chlorine 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 17 means 7.
Seven valence electrons, one short of a full shell, so it gains one to form the Cl⁻ ion or shares one in a covalent bond.
Seven valence electrons, one short of eight. Two chlorine atoms share one pair, a single bond.
About 1.9% of seawater by mass is chloride, and beds of rock salt (halite, NaCl) mark the places where ancient seas dried up. It is about 0.015% of the crust, and never free in nature.
Scheele made the gas in 1774 from hydrochloric acid and manganese dioxide, but thought it contained oxygen. Humphry Davy showed in 1810 that it was an element and named it from the Greek chloros, pale green.
Chlorine gas disinfects drinking water and pools and makes bleach. Chlorine compounds make PVC pipe and flooring, solvents, pesticides and many medicines, and the chloride in table salt is essential to life.