Bromine (Br)
Bromine (Br) is a halogen with atomic number 35 and a standard atomic weight of 79.904. At room temperature it is a liquid that melts at -7.2 °C and boils at 59 °C.
Bromine key facts
Bromine at a glance
Identity
| Symbol | Br |
|---|---|
| Atomic number (Z) | 35 |
| Atomic weight | 79.904 ± 0.003 (CIAAW 2024) |
| Category | Halogen |
| Group | 17 — halogens |
| Period | 4 |
| Block | p-block |
| CAS number | 7726-95-6 |
Atomic properties
| Electron configuration | [Ar] 3d10 4s2 4p5 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5 |
|---|---|
| Electrons per shell | 2, 8, 18, 7 |
| Oxidation states | -1, +1, +3, +5 Less common: +2, +4, +7 |
| Electronegativity (Pauling) | 2.96 |
| First ionisation energy | 11.814 eV (1,139.9 kJ/mol) 2nd: 21.591 eV · 3rd: 34.871 eV |
| Electron affinity | 3.364 eV |
| Atomic radius (empirical) | 115 pm |
| Covalent radius | 114 pm |
| Van der Waals radius | 185 pm |
Physical properties
| State at 25 °C | Liquid |
|---|---|
| Melting point | -7.2 °C · 19 °F · 265.95 K |
| Boiling point | 58.8 °C · 137.8 °F · 331.95 K |
| Density | 3.1028 g/cm³ |
| Specific heat capacity | 0.474 J/(g·K) |
| Thermal conductivity | 0.005 W/(m·K) |
| Crystal structure | Orthorhombic |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 79Br 50.65%, 81Br 49.35% |
| Stable isotopes | 6 |
| Known isotopes | 34 |
| Abundance in Earth's crust | 2.4 mg/kg |
| Abundance in seawater | 67.3 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Bromine electron configuration
Bromine has 35 electrons. Written in short form, its ground-state configuration is [Ar] 3d10 4s2 4p5 — the electrons of the noble gas argon, plus 3d10 4s2 4p5.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5.
As a member of group 17 it has 7 valence electrons in its outer shell (shell 4), which largely decide its chemistry.
Discovery and name
1825–1826 — Antoine Jérôme Balard; independently Carl Jacob Löwig · France / Germany.
Greek brōmos, stench.
Known for
One of only two elements liquid at room temperature (with mercury) — a red-brown, fuming liquid. Used in flame retardants and water treatment.
Isotopes of bromine
Naturally occurring bromine is a mix of 2 isotopes. Each has 35 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 79Br | 35 | 44 | 50.65% | stable |
| 81Br | 35 | 46 | 49.35% | stable |
34 isotopes of bromine are known in our data set (half-lives as published in nuclear data tables).
Bromine compared with group 17
The other elements in group 17 (halogens), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Fluorine | 9 | 18.998 | -220 °C | -188 °C | 1.696 g/L | 3.98 |
| Chlorine | 17 | 35.45 | -101 °C | -34 °C | 3.2 g/L | 3.16 |
| Bromine | 35 | 79.904 | -7.2 °C | 59 °C | 3.103 g/cm³ | 2.96 |
| Iodine | 53 | 126.90 | 114 °C | 184 °C | 4.933 g/cm³ | 2.66 |
| Astatine | 85 | [210] | 302 °C (est.) | — | 7 g/cm³ (pred.) | 2.2 |
| Tennessine | 117 | [294] | — | — | 7.2 g/cm³ (pred.) | — |