Bromine

Category: halogen Atomic number: 35 Colour: deep red Melting point: −7°C (19°F) Boiling point: 59°C (138°F) First identified: 1826

Bromine is one of only a handful of elements that are liquids at normal atmospheric conditions. It is deep red, oily and toxic, with a nasty smell (the Greek word bromos means stench). It was discovered in 1826 by Antoine-Jérôme Balard; he took some brine, evaporated most of the liquid, then passed chlorine gas through it. When you do this, the bromine evaporates and can be collected, as an orange-red liquid, which Balard correctly guessed was an undiscovered element. This happens because salty water, and especially that found in the Dead Sea, contains bromides (a bromine atom that has acquired a negative charge).

Bromine was used far more widely a few decades ago – photography used the light-sensitive nature of silver bromide, potassium bromide was used as a tranquillizer, leaded petrol contained dibromomethane, and bromomethane (also known as methyl bromide) was used to fumigate soil. Some of these uses are now redundant, as better alternatives have been discovered, while others have been banned – the Montreal Protocol banning CFCs also called for a reduction in the use of many of these compounds, because bromine atoms damage the atmosphere too. It has, however, proved difficult to find replacements for some uses of bromomethane: in many places it is still used to kill pests in the soil, and to treat wood that is being transported. Bromine compounds continue also to be widely used as a fire retardant in plastic casings, such as those on laptop computers, and in flame-retardant substances in fire extinguishers.

Donning the Purple

Tyrian purple, a dye produced from the mucus excreted by the sea snail Bolinus brandaris (also known as the spiny dye-murex), was once a symbol of great wealth and power – the hardy, vivid dye was expensive to produce as it required thousands of snails to produce a tiny quantity. The magnificent purple togas worn by Roman emperors were dyed using it, and this is the origin of the phrase ‘donning the purple’, meaning ‘taking power’.