- Bromine (Br) is a halogen with atomic number 35, positioned in Group 17 of the periodic table between chlorine and iodine.
- It is unique among nonmetals in being a liquid at room temperature, appearing as a dense, reddish-brown liquid with a volatile and strongly irritating vapor. Bromine has seven valence electrons—two in the 4s subshell and five in the 4p subshell—and typically exhibits oxidation states of −1, +1, +3, +5, and +7, with −1 being the most common in ionic compounds. Its atomic structure consists of thirty-five protons, typically forty-five neutrons, and thirty-five electrons arranged in four shells.
- Naturally occurring bromine is composed of two stable isotopes: bromine-79 (⁷⁹Br, 50.69%) and bromine-81 (⁸¹Br, 49.31%).
- Bromine is relatively scarce in Earth’s crust, with an abundance of about 2.5 parts per million, but it is more common in the oceans, where it occurs primarily as bromide ions (Br⁻) in seawater at about 65 milligrams per liter. It is typically obtained from brines found in underground wells, salt lakes, and the ocean. The largest producers are the United States, Israel, China, and Jordan, with significant extraction from the Dead Sea due to its exceptionally high bromide concentration.
- Bromine was discovered in 1826 independently by two chemists—Antoine-Jérôme Balard in France and Carl Löwig in Germany. Balard isolated it from brine residues and recognized it as a new element, naming it from the Greek word brōmos, meaning “stench,” in reference to its pungent, choking odor.
- Bromine’s reactivity lies between that of chlorine and iodine, and it forms compounds with many elements. Inorganic bromides, such as sodium bromide (NaBr) and potassium bromide (KBr), are common salts, while organic bromine compounds have historically been widely used as pesticides, fumigants, and flame retardants. However, many of these compounds—particularly brominated flame retardants and methyl bromide—have been restricted or banned due to their environmental persistence, toxicity, and role in ozone depletion.
- In industry, bromine compounds are still used in flame retardants, drilling fluids, water treatment chemicals, and certain pharmaceuticals. Silver bromide (AgBr) was once essential in photographic film due to its light-sensitive properties. Some bromine-based substances, such as ethylene dibromide, were historically used as gasoline additives to prevent engine knocking but have been phased out for environmental reasons.
- Chemically, bromine is highly reactive and can displace iodine from iodide salts while being displaced by chlorine from bromides. It reacts with metals to form bromides, with hydrogen to produce hydrogen bromide (HBr), and with many organic compounds to form brominated derivatives. Its vapor is highly irritating to the eyes, skin, and respiratory tract, and prolonged exposure can be dangerous.
- Biologically, bromine has only recently been recognized as an essential trace element in humans and animals, playing a role in the formation of collagen cross-links through the enzyme peroxidasin. However, the required amount is extremely small, and excess exposure to bromine or its compounds can cause bromism, a condition characterized by neurological symptoms, skin eruptions, and mucous membrane irritation.
- From an environmental perspective, bromine and its compounds must be carefully managed due to their potential toxicity and environmental persistence. Brominated organic pollutants can bioaccumulate in food chains and have been linked to endocrine disruption and developmental effects in wildlife. International regulations, such as the Montreal Protocol, have led to the reduction of certain bromine-containing chemicals that contribute to ozone layer depletion.
