Br · Element 35 · Halogen
Br is the symbol for bromine, element 35. It is a group 17 halogen in period 4. At ordinary room temperature, elemental bromine is a reddish-brown liquid made of Br₂ molecules. [1]
Answers
Br stands for bromine. Its atomic number, 35, counts the protons in every bromine nucleus. A neutral bromine atom also has 35 electrons. Bromine is a non-metal halogen, not a transition metal. [1]
Br identifies the element or one atom; Br₂ is the two-atom molecule in elemental bromine. Br⁻ is a bromide ion, with one more electron than the neutral atom. These symbols do not describe the same particle. [1] [2]
A neutral bromine atom has four occupied principal shells. Its ground-state configuration is [Ar] 3d¹⁰ 4s² 4p⁵, giving shell populations of 2, 8, 18 and 7. The fourth shell contains seven electrons. [2]
The number 35 is bromine’s proton count, not its neutron count. Its two natural stable isotopes are Br-79 and Br-81. They have 44 and 46 neutrons respectively, calculated as 79 minus 35 and 81 minus 35. [3]
NIST lists representative natural abundances of 50.69% for Br-79 and 49.31% for Br-81. The decimal atomic weight, about 79.904, is an average across isotopes; it cannot be used to assign a fractional neutron count to one atom. These NIST representative values differ slightly from the IAEA values used in the isotope table below; abundance values depend on the sample and evaluation. [3] [5] [4]
CIAAW gives bromine a standard atomic-weight interval of [79.901, 79.907] because isotope proportions vary naturally. The familiar 79.904 is a useful abridged value. [4] [5]
Bromine’s electronegativity is 2.96 on the Pauling scale, a relative measure of how strongly it attracts electrons in a bond. At 20 °C it is liquid, although it also releases vapor. Its liquid state and its electronegativity describe different properties. [1]
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where bromine sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All bromine atoms have the same number of protons, 35, and differ only in their number of neutrons: those variants are its isotopes. The most common, Br-79, has 44 neutrons and makes up 50.65% of natural bromine.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Br-79 most common | 44 | 50.65% | Stable | — |
| Br-81 | 46 | 49.35% | Stable | — |
The classroom shortcut, rounded atomic mass minus atomic number, gives 45 for bromine, but the most common bromine atom has 44 neutrons: the atomic mass, 79.904, is an average over all natural isotopes.
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 120 pm — that is 0.12 nm, so about 4.17 million of them side by side would span a millimetre.
Thermal range
Bromine is liquid over a 66 K window, from 266 K to 332 K.
Where it sits
Bromine sits in period 4, group 17. Everything in group 17 shares the same outer-electron count, which is why they behave so similarly.
Other halogens
All halogensThe story
Bromine (Br) is element 35, a halogen in group 17, between chlorine and iodine. It is one of only two elements that are liquid at room temperature, the other being mercury. Bromine is a dense, dark red-brown liquid that gives off a choking orange-brown vapor. It freezes at −7.35 °C (265.8 K), boils at 58.85 °C (332 K) and has a density of 3.10 g/cm³.
Bromine has 35 electrons in four shells, arranged 2, 8, 18, 7, with the configuration [Ar] 3d¹⁰ 4s² 4p⁵. With seven outer electrons it readily gains one more to form the bromide ion, Br⁻, as in sodium bromide. The element itself is made of Br₂ molecules.
Bromine has two stable isotopes, bromine-79 and bromine-81, in almost equal amounts (about 50.7% and 49.3%). This gives bromine compounds a recognizable pair of peaks in mass spectra.
In 1825 the German student Carl Jacob Löwig obtained a red-brown liquid from the water of a mineral spring at Bad Kreuznach, but his publication was delayed. In 1826 Antoine-Jérôme Balard, a young chemist in Montpellier, found the same substance in the brine of salt marshes after treating it with chlorine, and published first, so he is credited with the discovery.
Balard first proposed the name muride, from the Latin muria, brine; it was changed to brome, from the Greek bromos, meaning stench, because of its strong smell.
Bromides were soon used in photography and, from the 1850s, as the first effective sedatives and anticonvulsants.
Applications
The largest use of bromine is in brominated flame retardants for plastics in electronics, cables, insulation and furniture. Some of these, such as polybrominated diphenyl ethers (PBDEs), persist in the environment and are now banned or restricted under the Stockholm Convention.
Bromine is a building block for pharmaceuticals, dyes and agricultural chemicals, and silver bromide was the light-sensitive compound of photographic film. Methyl bromide, once a widely used soil fumigant, has been phased out under the Montreal Protocol, apart from quarantine and pre-shipment uses, because it damages the ozone layer. Ethylene dibromide, once added to leaded gasoline, largely disappeared with leaded fuel.
Where it comes from
2.4 mg/kg of Earth's crust · more abundant than 59% of elements
Bromine is too reactive to occur free. It is found as bromide salts, mostly dissolved in seawater, which holds about 65 parts per million, and in salt lakes and underground brines. Earth's crust contains only about 2.4 parts per million.
Bromine is extracted from brines by oxidizing the bromide with chlorine and blowing the bromine out with air or steam. The richest sources are the Dead Sea, worked by Israel and Jordan, and underground brines in Arkansas in the United States; China is also a major producer (USGS).
Handling
Liquid bromine is highly toxic and corrosive. It causes deep, painful burns on skin, and its vapor severely irritates the eyes, nose and lungs; high concentrations can cause fluid in the lungs that may appear hours after exposure. In the United States the OSHA limit for bromine vapor in workplace air is 0.1 parts per million.
Bromine is a strong oxidizer. It reacts violently with many metals, such as aluminum and potassium, and with reducing agents and many organic compounds.
Sources checked .
Symbol, atomic number, group and period, molecular liquid at 20 °C and Pauling electronegativity.
Neutral Br configuration; the four shell populations are counted from occupied subshells. Ion electron counts follow from charge.
Br-79 and Br-81, representative abundances and atomic masses. Neutron counts are derived from mass number minus 35.
Standard interval [79.901, 79.907], natural isotopic variation and abridged atomic-weight convention.
Abridged bromine atomic weight 79.904 ± 0.003.