Halogens

The salt-makers: a gas, a gas, a liquid and a solid, all one electron short of a full shell.

Group 17 is the only column of the periodic table where you can see a gas, a liquid and a solid side by side at room temperature. Fluorine and chlorine are gases, bromine is a dark red liquid and iodine is a grey-black crystal that puffs into purple vapour when warmed. These are the halogens, and below iodine sit two rare and radioactive relatives, astatine and tennessine.

The name halogen means salt-former, and it fits. When a halogen meets a metal, the result is a salt. Sodium chloride, the salt on your chips, is the most familiar, but every halogen makes salts of its own. The halogens are among the most reactive non-metals, and their hunger for a single extra electron drives almost everything they do.

What they have in common

Each halogen atom has seven electrons in its outer shell, one short of a full set. Gaining one more electron completes the shell, so halogens take electrons from metals to form negative ions with a charge of minus one, called halide ions. On their own, halogen atoms pair up, so the elements exist as the diatomic molecules F2, Cl2, Br2 and I2.

They are strongly coloured, and the colour deepens down the group: fluorine is a very pale yellow, chlorine a greenish yellow, bromine a deep red-brown and iodine so dark it looks almost black. All of them are poisonous and all have sharp, choking smells.

Because halogens are so reactive, none is found free in nature. Chlorine and bromine come mainly from sea water and salt deposits, fluorine from the mineral fluorite and iodine from seaweed and certain brines.

How they change down the group

Reactivity decreases down the group, the reverse of the alkali metals. A small fluorine atom pulls an incoming electron very close to its nucleus, so it grabs electrons more fiercely than any other element. Fluorine has the highest electronegativity of any element and reacts with almost everything, including glass and some noble gases. Chlorine is less aggressive, bromine less still, and iodine is reactive only in comparison with most other non-metals.

This gives rise to a neat set of displacement reactions. A more reactive halogen will push a less reactive one out of its salt. Bubble chlorine through a solution of potassium bromide and the solution turns orange as bromine is released. Add bromine to potassium iodide and brown iodine appears.

Melting and boiling points rise down the group because larger molecules attract one another more strongly. Fluorine boils at about minus 188 degrees Celsius and chlorine at about minus 34 degrees. Bromine melts at about minus 7 degrees and boils at about 59 degrees, so it is a liquid at room temperature and evaporates readily. Iodine melts at about 114 degrees Celsius. Astatine and tennessine are predicted to melt at several hundred degrees, but their values are estimates.

Where you meet them

Fluoride ions in toothpaste and in some water supplies harden tooth enamel against decay. Fluorine is also in the non-stick coating on frying pans and in many refrigerant gases and medicines. Chlorine kills bacteria in drinking water and swimming pools, and it is a building block for bleach, PVC plastic and a huge range of solvents and disinfectants.

Bromine compounds are used as flame retardants in furniture and electronics. Iodine is the halogen your body needs. The thyroid gland uses it to make hormones that control growth and metabolism, which is why iodine is added to table salt in many countries. Iodine solution is also a classic antiseptic and a test for starch, turning blue-black when starch is present.

Odd ones out

Fluorine is extreme even for a halogen. It is so reactive that for decades chemists were injured trying to isolate it, and it forms compounds with elements that otherwise refuse to react. Yet fluorine's partner bonds are so strong that many fluorine-containing compounds are among the most inert substances known, which is why non-stick coatings survive the heat of a frying pan.

Astatine breaks the pattern of behaviour. It is so radioactive that no one has ever seen a visible piece of it, and it is the rarest naturally occurring element in the Earth's crust. Its chemistry, studied with tiny numbers of atoms, shows some metallic character, and this site's dataset classes it as a metalloid rather than a halogen. Tennessine, made only a few atoms at a time, is predicted to lean even further away from typical halogen behaviour. Chlorine also has a small surprise: although fluorine is the most electronegative element, it is chlorine that releases the most energy when a single atom gains an electron, because the tiny fluorine atom is crowded with electrons that repel the newcomer.

The numbers

Melting points rise down the list, from Fluorine at -220 °C to Tennessine at 450 °C.

Density in g/cm³ (g/L for gases). Values for synthetic elements are predictions.

Did you know?

• Bromine is one of only two elements that are liquid at ordinary room temperature; the other is the metal mercury.
• Fluorine has the highest electronegativity of any element and reacts with almost every other substance.
• Halogen means salt-former, and table salt is the halogen compound sodium chloride.
• Iodine sublimes when warmed, turning straight from grey-black crystals into purple vapour.
• Astatine is so rare and radioactive that no visible sample of it has ever existed.
• A more reactive halogen displaces a less reactive one from its salt, so chlorine can free bromine from bromide solution.

Frequently asked questions

Written by Anthony. Data from Periodic-Table-JSON (CC BY-SA).