Bromine

Bromine (Br) is a reactive nonmetal with atomic number 35 and an atomic mass of 79.904. Its electron configuration is [Ar] 3d10 4s2 4p5, which gives it 7 valence electrons.

Bromine key data

Symbol	Br
Atomic number	35
Atomic mass (standard atomic weight)	79.904
Molar mass	79.904 g/mol
Protons	35
Neutrons (most abundant isotope, bromine-79)	44
Electrons (neutral atom)	35
Valence electrons	7
Category	Reactive nonmetal
Period	4
Group	17
Block	p-block
Noble gas notation	[Ar] 3d10 4s2 4p5
Full electron configuration	1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5
Electrons per shell	2, 8, 18, 7

Atomic mass: IUPAC CIAAW abridged standard atomic weights, 2024. Most abundant isotope: NIST Atomic Weights and Isotopic Compositions.

Measured properties of bromine

From PubChem, the US National Institutes of Health. Public domain.

Melting point	265.95 K (−7.2 °C), 91st of 103 by melting point
Boiling point	331.95 K (58.8 °C), 82nd of 93 by boiling point
Density	3.11 g/cm³, 69th of 96 by density
State at room temperature	Liquid
Electronegativity	2.96 (Pauling), 6th of 95 by electronegativity
Van der Waals radius	183 pm, 79th of 99 by atomic radius
First ionisation energy	11.814 eV (1139.9 kJ/mol), 11th of 102 by first ionisation energy
Electron affinity	3.365 eV (324.7 kJ/mol), 3rd of 57 by electron affinity
Common oxidation states	+5, +1, −1
Discovered	1826

Each rank counts down from the highest value, among the elements PubChem’s table gives one for, and links to that property ranked across the whole table. All of them are collected under periodic trends.

Bromine electron configuration

The full electron configuration of bromine is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5. With the noble-gas core collapsed it is [Ar] 3d10 4s2 4p5, and the outermost subshell is 4p5.

Bohr model of bromine (Br): a nucleus and four shells holding 2, 8, 18 and 7 electrons from the inside out. — Bohr model of bromine (Br). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Orbital box diagram of bromine (Br), 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p5: one box per orbital, filled by Hund’s rule, showing one unpaired electron, in 4p. — Orbital diagram of bromine (Br). Image © ScienceQuest, CC BY 4.0: free to reuse with credit to ScienceQuest and a link to this page, as the image licence explains.

Why group and block follow from this is set out in how the periodic table maps electron configuration, and the electron configuration of every element is listed on one page.

Mass of one bromine atom

A molar mass is grams per mole, so dividing by the Avogadro constant gives the mass of a single atom, or for an element with more than one natural isotope the average mass of its atoms, and the reciprocal gives how many atoms sit in a gram.

Mass of one atom

one atom = 79.904 / (6.02214076 × 10²³)
= 1.3268 × 10⁻²² g

Atoms in one gram

atoms per gram = 6.02214076 × 10²³ / 79.904
= 7.5367 × 10²¹ atoms

Bromine on the periodic table

Bromine is in period 4, group 17, in the p-block.

The other elements of group 17 are fluorine, chlorine, iodine, astatine and tennessine.