Fluorine, \mathrm{F}, chlorine, \mathrm{Cl}, bromine, \mathrm{Br}, and iodine, \mathrm{I}, are all elements of group 17 of the periodic table, and they are called the halogens.
Each has 7 electrons in its outer shell, so each readily gains one electron to become a singly charged negative ion. The further down the group an element is, the more electron shells it has and the larger its atoms are.
Each halogen as an element is made of molecules in which two atoms are bonded together: \mathrm{F_2}, \mathrm{Cl_2}, \mathrm{Br_2} and \mathrm{I_2}.
At room temperature, fluorine is a pale yellow gas, chlorine a yellow-green gas, bromine a red-brown liquid and iodine a purple-black solid. The larger the molecules, the stronger the forces between them, so the melting and boiling points rise down the group.
Iodine sublimes easily, and when it is heated it turns into a violet gas.
The halogens act as oxidizing agents, taking electrons from other substances. Their oxidizing power decreases in the order \mathrm{F_2} > \mathrm{Cl_2} > \mathrm{Br_2} > \mathrm{I_2}: the higher up the group, the more strongly the element pulls electrons away.
When chlorine water is added to potassium bromide solution, chlorine takes electrons from the bromide ions and bromine is formed, so the solution turns orange-brown.
2\mathrm{KBr} + \mathrm{Cl_2} \longrightarrow 2\mathrm{KCl} + \mathrm{Br_2}
When chlorine water is added to potassium iodide solution, iodine is formed in the same way, and the solution turns brown. On the other hand, adding bromine or iodine to potassium chloride solution causes no reaction.
Compounds of the halogens with hydrogen are called hydrogen halides, and all of them dissolve readily in water to give acidic solutions. Hydrochloric acid, hydrobromic acid and hydroiodic acid are strong acids, but hydrofluoric acid, the solution of hydrogen fluoride in water, is a weak acid.
Hydrofluoric acid dissolves silicon dioxide, the main component of glass.
\mathrm{SiO_2} + 6\mathrm{HF} \longrightarrow \mathrm{H_2SiF_6} + 2\mathrm{H_2O}
For this reason, hydrofluoric acid is stored not in glass bottles but in polyethylene containers. This property is also used to etch scales and patterns onto glass.
Iodine hardly dissolves in water, but it dissolves well in potassium iodide solution. The brown solution made in this way is called iodine-potassium iodide solution.
When iodine-potassium iodide solution is added to a solution of starch, it turns blue-black. This is called the iodine-starch reaction, and it is used to test for iodine or for starch.
When the blue-black solution is heated, the colour disappears, and when it is cooled, the blue-black colour returns.