Bromine, (Chemical Element, Symbol Br, Atomic Number 35)
Bromine is an element in Group 7 of the Periodic Table. Members Group 7 are also known as Halogens.
(The other halogens - that is, members of the same group are Fluorine, Chlorine, Iodine and Astatine. These have some properties in common with bromine while there are also certain "trends" or patterns in the variation of other properties across this group of elements.)
Each bromine atom consists of 35 protons, 35 electrons plus a quantity of neutrons whose exact number* depends on the particular isotope of bromine (see below).
*The standard atomic mass of bromine is 79.904, which is often rounded to 80 in simple (e.g. school textbook) versions of the Periodic Table. Using a mass number of 80 for bromine, the number of neutrons in each atom would be 80 - 35 = 44.
Free bromine is not found in nature, but bromine atoms are present in naturally occuring colourless soluble crystalline mineral halide salts (similar in many ways to table salt).
History and Discovery of Bromine
Discovery of Bromine: Bromine was isolated independently by two scientists - the French chemist Antoine Jérôme Balard (in 1825) and the German chemist Carl Jacob Löwig (in 1826).
- In 1825 Carl Jacob Löwig isolated bromine from a mineral water spring from his hometown Bad Kreuznach. He used a solution of the mineral salt saturated with chlorine and extracted the bromine with diethyl ether. After evaporation of the ether a brown liquid remained. Using this liquid as a sample for his work Löwig applied for a job in the laboratory of Leopold Gmelin in Heidelberg. Publication of Löwig's work to isolate bromine in this way was delayed so Balard's isolation of bromine was published results first.
- In 1826 Antoine Jérôme Balard found bromides (chemicals containing bromie) in the ash of seaweed from the salt marshes of Montpellier. The seaweed was used to produce iodine, but also contained bromine. Balard distilled bromine from a solution of seaweed ash saturated with chlorine.
The properties of the substance isolated by Balard appeared to fall between those of the elements chlorine and iodine. Initially he tried to show that substance was iodine monochloride (ICl), but after failing to reach that conclusion he became convinced that he had found a new element - which he called "muride" after the Latin word muria for brine (salt water). After Balard's results had been repeated and confirmed by other leading scientists the work was presented at a lecture and published (1826). In his paper Annales de Chimie et de Physique 2nd series 32: 337–381 Balard changed the name from "muride" to "brôme", the word brôme (bromine) being derived from the Greek word for "stench". Different historical accounts mention different scientists as having suggested this change of name.
Early Use of Bromine: Bromine was not produced in large quantities until 1860. Apart from some minor early medical applications the first commercial use of bromine was for photographic processing. Potassium bromide and sodium bromide were used as anticonvulsants and sedatives in the late 19th and early 20th centuries, until superseded other drugs.
Elemental Bromine (under "standard conditions")
Elemental bromine consists of molecules, rather than atoms, of bromine.
Two atoms of bromine form each bromine molecule, hence molecules of bromine are said to be diatomic and have the chemical formula Br2.
Elemental bromine is a dense, mobile, slightly transparent fuming red-brown liquid at room temperature and readily evaporates into a red/brown vapor (a similar colour to that of nitrogen dioxide). Bromine vapour (bromine gas) has a strong odour. It smells similar to chlorine gas or bleach, and is very irritating to the mucous membranes includig those of the nose, throat and beyond. Bromine is one of the only two elements that is a liquid at room temperature, the other being mercury which is the only metal to exist in liquid form at room temperature and pressure.
Hazardous Substance: Bromine liquid and bromine vapour are both corrosive and toxic (poisonous).
Bromine and Health
Elemental bromine (Br2) is toxic and causes burns. Bromine must therefore be transported with care and attention to safety issues - hence it is often transported in steel tanks lined with lead and supported by strong metal frames.
As a pharmaceutical, the bromide ion, Br-, has inhibitory effects on the central nervous system.
Early Medical uses of bromine:
Potassium bromide (KBr) and sodium bromide (NaBr) were used as anticonvulsants and sedatives in the late 19th and early 20th centuries until they were superseded by chloral hydrate, and then the barbiturates. However, some bromine salts are still used in some antiepileptic drugs.
Compounds of Bromine (in general)
There are several categories of compounds of bromine, including:
A "bromide" is a chemical compound that contains the "bromide" Br - anion. Bromides include both ionic compounds such as caesium bromide (CsBr) and covalent compounds such as sulphur dibromide (SBr2).
Bromide ions are essential for eosinophils (white blood cells), which use them to produce antiparasitic brominating compounds by the action of eosinophil peroxidase, an enzyme which can use chloride ions, but preferentially uses bromide when available. The average concentration of bromide in human blood varies with age and gender. Bromide ions also occur in relatively high concentration in seawater and in many types of seafood, hence bromide concentrations in the blood may be heavily influenced by the relative quantity of seafood in the person's diet.
A "bromate" is a chemical compound that contains the "bromate" BrO3− anion. Examples of bromates include sodium bromate, (NaBrO3), and potassium bromate, (KBrO3). Concerns have been expressed about the possibility of bromates in drinking water because they are suspected of being a human carcinogen - that is, of causing cancer in humans.
A "perbromate" is a compound that contains the BrO4− anion or the –BrO4 functional group. The perbromate ion (BrO4−) is a strong oxidizing agent.
Compounds of bromine occur in and are required by marine animals and plants and are produced by certain forms of sealife, such as algae. Bromide salts are also found in seawater.
This section is beyond the scope of most courses in school-level chemistry, e.g. GCSE Chemistry and A-Level Chemistry (UK). Specific examples of compounds of bromine are mentioned in the list of "Uses of Bromine" at the bottom of this page.
Isotopes of Bromine
Bromine has two stable isotopes, Bromine-79 (50.69 %) and Bromine-81 (49.31%).
There are also more than 20 known radioisotopes of bromine, many of which are products of nuclear fission.
The radioactive isotopes of bromine are relatively short lived, the longest half-life being that of Bromine-77 (2.376 days).
Isotopes of bromine include:
Content of Nucleus:
35 protons, 44 neutrons
Stable isotope that accounts for approx. 50.69% of known bromine by weight.
35 protons, 46 neutrons
Stable isotope that accounts for approx. 49.31% of known bromine by weight.
Uses of Bromine (including bromine combined with other elements to form compounds of bromine)
- Fire Retardant Chemicals: Some compounds containing bromine are used as chemical flame retardants. Their purpose is to reduce the flammability of combustible materials and so reduce the risk and extent of fires if products made from such materials are set alight. Such "brominated flame retardants" are used in the manufacture of a wide range of common products such as cables, connectors, plastic covers and household items such as carpets, upholstery, paints and kitchen appliances. They are also used in some fabrics used not only for clothing but also in the production of furniture and homewares. Brominated flame retardants are also used in the computer and electronics industries to protect printed circuit boards from the risk of fire e.g. due to overheating or other causes.
An example of a bromine compound used as a flame retardant was tris(dibromopropyl)phosphate, (Br2C3H5O)3PO but this was later found to be a carcinogenic (cancer-causing) so its use has been severely restricted.
- Disinfectants: Bromine has uses as a bleaching agent and sanitizer (although not around food).
Some compounds of bromine are used against bacteria, algae and fungi to disinfect hospitals, offices, retail premises, hotels, and even domestic premises (houses).
- Drilling Wells: Calcium bromide (CaBr2), sodium bromide (NaBr), or zinc bromide (ZnBr2) are added to drilling wells to increase the efficiency of the drilling process.
- Water Purification: Some compounds of bromine are used in water purification systems. However, excess bromine renders water toxic and unsafe to drink so water purification systems must be managed carefully and use of such chemicals and the the quality of the resulting water checked thoroughly.
- Pesticides: Some compounds of bromine are used to make chemical pesticides used in agriculture to reduce the cost of damage done by pests attacking crops of e.g. potatoes, tomatoes, and other fruits. and etc.. The haloalkane methyl bromide is used as a fumigate to protect stored grains and other produces by destroying pests.
- Photographic Films: Silver bromide (AgBr) is a light sensitive chemical used in traditional photography, i.e. using films as opposed to digital photography. The microscopic particles of silver bromide undergo a chemical change when exposed to light, so it can be used to record an image formed by a pattern on light incident on the film for the brief moment the shutter of a camera is opened to take a photograph. Potassium bromide was also used in traditional film photography but for different chemical reasons to prevent one cause of "foggy images" in photographs.
- Dyes: In 1909 the German chemist Paul Friedländer found that the Tyrian purple dye contains bromine. In this form, bromine had been known to and used by humanity thousands of years. There are many other examples of chemicals that include bromine in use in the textile industry, e.g. tetrabutylammonium bromide (C16H36BrN).
- Extraction of Gold