A chlorine atom has 17 electrons around its nucleus. There are 2 in the K shell, 8 in the L shell and 7 in the M shell.
With 7 outer electrons, a chlorine atom that gains one more electron has the same arrangement of electrons as argon. So chlorine readily forms the singly charged negative ion \mathrm{Cl}^{-}.
The elements of group 17 of the periodic table are called the halogens, and chlorine is one of them.
Chlorine as an element is made of molecules, \mathrm{Cl_2}, in which two atoms are bonded together. It is a poisonous yellow-green gas with a sharp, choking smell.
Its relative molecular mass is about 71, about 2.5 times the average for air (about 29), so chlorine is denser than air. For this reason, it is collected by downward delivery: the gas is led to the bottom of a gas jar and pushes the air out upwards.
In the laboratory, chlorine is made by adding concentrated hydrochloric acid to manganese(IV) oxide and heating the mixture.
\mathrm{MnO_2} + 4\mathrm{HCl} \longrightarrow \mathrm{MnCl_2} + 2\mathrm{H_2O} + \mathrm{Cl_2}
The gas that comes off also contains hydrogen chloride and water vapour, so it is passed first through water and then through concentrated sulfuric acid to remove them.
In industry, chlorine is made together with sodium hydroxide by the electrolysis of brine, a solution of sodium chloride.
Chlorine dissolves a little in water, and part of it reacts with the water to form hydrogen chloride and hypochlorous acid, \mathrm{HClO}.
\mathrm{Cl_2} + \mathrm{H_2O} \longrightarrow \mathrm{HCl} + \mathrm{HClO}
Hypochlorous acid is a strong oxidizing agent: it breaks down dyes, removing their colour, and kills bacteria. This action is used to disinfect tap water and the water in swimming pools.
A dry petal placed in dry chlorine keeps its colour. A wet petal loses its colour because chlorine dissolves in the water on the petal and forms hypochlorous acid.
Chlorine bleaches contain sodium hypochlorite, and mixing them with acidic cleaners is dangerous because chlorine is given off.
When light falls on a mixture of chlorine and hydrogen, they react explosively to form hydrogen chloride.
\mathrm{H_2} + \mathrm{Cl_2} \longrightarrow 2\mathrm{HCl}
Hydrogen chloride is extremely soluble in water, and its solution is hydrochloric acid.
When hydrogen chloride and ammonia meet, they form a white smoke of ammonium chloride, so this reaction is used to test for either gas.
\mathrm{NH_3} + \mathrm{HCl} \longrightarrow \mathrm{NH_4Cl}