Bromine at a glance
- Atomic number
- 35
- Symbol
- Br
- Atomic mass
- 79.904 u
- Category
- Halogen
- Group / period
- 17 / 4
- Electron configuration
- [Ar] 3d 10 4s 2 4p 5
- State at 25 °C
- Liquid
- Melting point
- -7.35 °C
- Boiling point
- 58.85 °C
- Density
- 3.122 g/cm³
- Electronegativity
- 2.96
- 1st ionization energy
- 1140 kJ/mol
- Oxidation states
- −1, +1, +5
- Discovered
- 1826
Atomic structure
A neutral bromine atom has 35 protons and 35 electrons, and its most common isotope carries about 44 neutrons. One atom weighs about 1.33 × 10⁻²⁵ kg, so a mole of bromine (6.022 × 10²³ atoms) weighs 79.904 g. That nucleus holds 1.26 neutrons for every proton. The electrons fill 4 shells (K 2, L 8, M 18, N 7), which gives the configuration [Ar] 3d 10 4s 2 4p 5. With 7 electrons in its outer shell, it tends to gain or share electrons to complete its outer shell. It has 2 stable isotopes: Br-79 and Br-81. No other element has a stable isotope with mass number 79, so Br-79 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) bromine is a liquid. It melts at -7.35 °C and boils at 58.85 °C, so it stays liquid across a 66.2-degree range. To freeze it has to be cooled below -7.35 °C, colder than the freezing point of water. Its density is 3.122 g/cm³, 3.12 times as dense as water. Its atoms have a radius of about 115 pm (Selenium next door is 115 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 2.96 on the Pauling scale. Pulling off its outermost electron takes 1140 kJ/mol, less than Chlorine above it (1251 kJ/mol). A bond between bromine and chlorine would be almost non-polar covalent, with an electronegativity difference of 0.2. It releases 325 kJ/mol when it gains an extra electron, one of the largest electron affinities known. Its common oxidation states are −1, +1, +5, so in compounds you will usually meet it as Br⁻, Br⁺ and Br⁵⁺. Group 17 also holds Fluorine, Chlorine, Iodine, Astatine and Tennessine, which share its outer-shell pattern. Its Lewis symbol is Br surrounded by 7 dots. Handle with care: as a pure element it is toxic and corrosive.
Where it comes from
It makes up 2.4 mg/kg of Earth's crust, the 50th most abundant element there. In the human body it accounts for about 0.00029 % of body mass. It was first identified in 1826, making it the 53rd element to be discovered (counting the 11 known since antiquity together).