bromine

bromine (Br), chemical element, a deep red noxious liquid, and a member of the halogen elements, or Group 17 (Group VIIa) of the periodic table.

History

Bromine was discovered in 1826 by the French chemist Antoine-Jérôme Balard in the residues (bitterns) from the manufacture of sea salt at Montpellier. He liberated the element by passing chlorine through an aqueous solution of the residues, which contained magnesium bromide. Distillation of the material with manganese dioxide and sulfuric acid produced red vapors, which condensed to a dark liquid. The similarity of this procedure to that for making chlorine suggested to Balard that he had obtained a new element similar to chlorine. (The German chemist Justus von Liebig appears to have obtained the element before Balard, but he wrongly considered it to be iodine chloride.) Because of the bad odor of the element, the French Academy of Sciences suggested the name bromine, from the Greek word bromos, meaning “bad smell” or “stench.”

Occurrence and distribution

A rare element, bromine is found in nature dispersed throughout Earth’s crust only in compounds as soluble and insoluble bromides. Some enrichment occurs in ocean water (65 parts per million by weight), in the Dead Sea (approximately 5 grams per liter [0.7 ounce per gallon]), in some thermal springs, and in rare insoluble silver bromide minerals (such as bromyrite, found in Mexico and Chile). Natural salt deposits and brines are the main sources of bromine and its compounds. Jordan, Israel, China, and the United States led the world in bromine production in the early 21st century; other important bromine-producing countries during that period include Japan, Ukraine, and India.

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Natural bromine is a mixture of two stable isotopes: bromine-79 (50.54 percent) and bromine-81 (49.46 percent). Of the 17 known radioactive isotopes of the element, bromine-77 has the longest half-life (57 hours).

Physical and chemical properties

Free bromine is a reddish brown liquid with an appreciable vapor pressure at room temperature. Bromine vapor is amber in color. Bromine has a pungent odor and is irritating to the skin, eyes, and respiratory system. Exposure to concentrated bromine vapor, even for a short time, may be fatal. Like the other halogens, bromine exists as diatomic molecules in all aggregation states.

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About 3.41 grams (0.12 ounce) of bromine dissolve in 100 milliliters (0.1 quart) of water at room temperature. The solution is known as bromine water. Like chlorine water, it is a good oxidizing agent, and it is more useful because it does not decompose so readily. It liberates free iodine from iodide-containing solutions and sulfur from hydrogen sulfide. Sulfurous acid is oxidized by bromine water to sulfuric acid. In sunlight bromine water decomposes, with release of oxygen, as in the following equation:

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From bromine water a hydrate (a clathrate) can be isolated that contains 172 water molecules and 20 cavities capable of accommodating the bromine molecules. Bromine dissolves in aqueous alkali hydroxide solutions, giving bromides, hypobromites, or bromates, depending on the temperature. Bromine is readily extracted from water by organic solvents such as carbon tetrachloride, chloroform, or carbon disulfide, in which it is very soluble. In the organic solvents it gives an orange solution.

The electron affinity of bromine is high and is similar to that of chlorine. It is, however, a less powerful oxidizing agent, chiefly because of the weaker hydration of the bromide ion as compared with the chloride ion. Similarly, a metal-bromine bond is weaker than the corresponding metal-chlorine bond, and this difference is reflected in the chemical reactivity of bromine, which lies between that of chlorine and that of iodine. An organic bromo compound resembles the corresponding chloro derivative but is usually more dense, less volatile, less combustible, and less stable.

Bromine combines violently with the alkali metals and with phosphorus, arsenic, aluminum, and antimony but less violently with certain other metals. Bromine displaces hydrogen from saturated hydrocarbons and adds to unsaturated hydrocarbons, though not as readily as chlorine does.

The most stable oxidation state of the element is −1, in which bromine occurs naturally. But oxidation states of 0 (elemental bromine, Br₂), +1 (hypobromite, BrO⁻), +3 (bromite, BrO⁻₂), +5 (bromate, BrO⁻₃), and +7 (perbromate, BrO⁻₄) are also known. The first ionization energy of bromine is high, and compounds containing bromine in positive oxidation numbers are stabilized by appropriate ligands, mainly oxygen and fluorine. Compounds with the oxidation numbers +1, +3, +4, +5, and +7 all contain covalent bonds.

Production and use

The chief commercial source of bromine is ocean water, from which the element is extracted by means of chemical displacement (oxidation) by chlorine in the presence of sulfuric acid through the reaction

The product of the reaction is a dilute solution of bromine, from which the element is removed by blowing air through it. The free bromine is then mixed with sulfur dioxide, and the mixed gases are passed up a tower down which water is trickling. The following reaction takes place in the tower:

resulting in a mixture of acids that is much richer in bromide ion than seawater. A second treatment with chlorine liberates bromine, which is freed from chlorine and purified by passage over moist iron filings.

Commercial bromine generally contains up to 0.3 percent chlorine. It is usually stored in glass bottles or in barrels coated with lead or Monel metal.

The industrial usage of bromine had been dominated by the compound ethylene bromide (C₂H₄Br₂), which once was added to gasoline with tetraethyl lead to prevent deposition of lead in the engine. Since the renunciation of leaded gasoline, bromine compounds have mainly been used in flame retardants, but ethylene bromide is still an important compound because of its use to destroy nematodes and other pests in soils. Bromine is also used in the production of catalysts, such as aluminum bromide.

Bromine has other uses, as in making various dyes and the compounds tetrabromoethane (C₂H₂Br₄) and bromoform (CHBr₃), which are used as liquids in gauges because of their high specific gravity. Until the development of barbiturates in the early 20th century, bromides of potassium, sodium, calcium, strontium, lithium, and ammonium were used widely in medicine because of their sedative action. Silver bromide (AgBr), an important component of photographic film, is, like silver chloride and iodide, light sensitive. Traces of potassium bromate (KBrO₃) are added to wheat flour to improve baking. Other bromine compounds of significance include hydrogen bromide (HBr), a colorless gas used as a reducing agent and a catalyst in organic reactions. A solution of the gas in water is called hydrobromic acid, a strong acid that resembles hydrochloric acid in its activity toward metals and their oxides and hydroxides.