ElementsWhat are the halogens?
The halogens fill column 17. The name means 'salt former', because they combine with metals to make salts, such as sodium chloride. Each atom needs one more electron to fill its outer shell, so the halogens are very reactive, most of all fluorine at the top. At room temperature the family includes gases (fluorine, chlorine), a liquid (bromine) and a solid (iodine). They are used to disinfect water, and all are harmful in pure form.
The family in numbers
Melting points in this family run from -219.6 °C (fluorine) to 301.9 °C (astatine). Among its solid members, the least dense is iodine (4.93 g/cm³) and the densest is astatine (7 g/cm³). The most recently discovered member is tennessine (2010).
How the family changes down the column
The halogens are the mirror image of the alkali metals: here reactivity is greatest at the top and falls going down. Fluorine is the most reactive of all the elements and attacks almost everything, including glass. Chlorine is a little milder, bromine milder again and iodine the least reactive of the four common ones. A more reactive halogen pushes a less reactive one out of its compounds: bubble chlorine through a solution of potassium bromide and the liquid turns orange as bromine is set free.
Colours and states
The family shows all three states of matter at room temperature, and the colours deepen going down: fluorine is a pale yellow gas, chlorine a yellow-green gas, bromine a red-brown liquid and iodine a dark grey solid that gives a violet vapour when warmed. All exist as molecules of two atoms.
Where you meet them
Chlorine makes drinking water and swimming pools safe and is part of PVC plastic. Common salt is sodium chloride. A small amount of fluoride in toothpaste hardens the surface of teeth. Iodine is needed by the thyroid gland, which is why table salt is iodised, and its brown solution is an antiseptic for cuts. Astatine and tennessine, at the foot of the column, exist only in tiny amounts and are radioactive.
All halogens compared
| No. | Symbol | Name | Atomic mass | State | Melts at | Boils at | Density g/cm³ | Discovered |
|---|---|---|---|---|---|---|---|---|
| 9 | F | Fluorine | 18.99840316 | Gas | -219.6 °C | -188.1 °C | 0.001696 | 1886 |
| 17 | Cl | Chlorine | 35.45 | Gas | -101.5 °C | -34 °C | 0.003214 | 1774 |
| 35 | Br | Bromine | 79.90 | Liquid | -7.2 °C | 58.8 °C | 3.11 | 1826 |
| 53 | I | Iodine | 126.9045 | Solid | 113.7 °C | 184.4 °C | 4.93 | 1811 |
| 85 | At | Astatine | 209.98715 | Solid | 301.9 °C | no value | 7 | 1940 |
| 117 | Ts | Tennessine | 294.211 | Solid | no value | no value | no value | 2010 |
Other families: alkali metals, alkaline earth metals, transition metals, post-transition metals, metalloids, nonmetals, noble gases, lanthanides, actinides.