Bromine

Bromine

Bromine is the third member of the halogen family, the elements that make up Group 17 in the periodic table. Its atomic number is 35, its atomic mass is 79.904, and its chemical symbol is Br.

Properties

Bromine is one of only two elements that is liquid at room temperature, the other being mercury. It is a beautiful deep reddish brown liquid that evaporates easily, giving off strong fumes that irritate the throat and lungs. Bromine boils at a temperature of 137.8°F (58.8°C), and its density is 3.1023 grams per cubic centimeter. Bromine's freezing point is 18.9°F (-7.3°C).

Like all halogens, bromine is a very reactive element, less reactive than fluorine or chlorine, but more reactive than iodine. It reacts vigorously with many metals, sometimes explosively. Bromine even reacts with relatively inert elements such as platinum and palladium.

Occurrence and Extraction

Bromine is a moderately abundant element with an estimated abundance of about 1.6-2.4 parts per million in the Earth's crust. It is far more abundant in seawater, where its abundance is estimated to be about 65 parts per million.

The element is far too reactive to occur as an element in the Earth's crust, and is found primarily in the form of sodium bromide (NaBr) and potassium bromide (KBr). Since both compounds are highly soluble in water, they tend to dissolve when water washes over the Earth's surface. They are then carried into the oceans. Very large underground deposits of sodium and potassium bromide were formed when ancient seas dried up and were buried by earth movements. Those deposits, known as salt domes are a major dry-land source of bromine today.

Discovery and Naming

Compounds of bromine have been known for many hundreds of years. One of the most famous of these compounds was a dye known as Tyrian purple, or royal purple. The dye was obtained from a particular mollusk found on the shores of the Mediterranean Sea. The cost of extracting the dye from the mollusks was so great that only very rich people could afford it, thus accounting for its alternative name.

Bromine was discovered at almost the same time in 1826 by the German chemist Carl Löwig (1803-1890) and the French chemist Antoine-Jérôme Balard (1802-1876). Löwig, a student at the University of Heidelberg at the time, probably made his discovery first. However, Balard was the first to publish the results of his research. As a consequence, credit for the discovery of bromine is usually given to both men. Bromine was named after the Greek word bromos, which means "stench."

Uses

The most important use of bromine today is in making flame retardant materials. Many of the materials used in making clothing, carpets, curtains, and drapes are highly flammable. To reduce the risk that such materials will catch fire, they may be soaked in a bromine compound, or the bromine compound may be applied after the product has been manufactured. In either case, the bromine compound prevents the material from catching fire, although the material may still smolder or char.

A highly controversial use of one bromine compound, methyl bromide (CH₃Br), is as a pesticide. Methyl bromide is sprayed on the surface or injected directly into the ground on a farm. Some of the methyl bromide escapes into the air where it damages the ozone layer. Concern about this problem has led world leaders to recommend an end to the use of methyl bromide as a pesticide shortly after the beginning of the new millennium.

Bromine and its compounds are becoming increasingly popular in the purification of public water supplies and swimming pools. Bromine kills bacteria in the same way that chlorine does, but it appears to be even more effective than chlorine.