Atomic Mass of Bromine (Br)
The relative atomic mass of Bromine is 79.904 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Teachers call it "one fact, fully handled" — exactly what this page does for Bromine. The answer sits at the top in bold; scroll for the working, the shell-by-shell breakdown and the two questions students ask most. Need the whole picture instead — history, characteristics, uses, exam questions? The full Bromine page is one click away.
Bromine by the numbers
Z = 35 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (79.904 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 35 |
| Relative atomic mass | 79.904 u |
| Protons | 35 |
| Neutrons (commonest isotope) | 45 |
| Electrons | 35 |
Atomic mass vs mass number
Mass number (A) counts particles — protons + neutrons in one specific atom, always a whole number. Relative atomic mass is the experimentally measured average over all natural isotopes, so it comes out fractional (79.904). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Bromine a whole number?
No. 79.904 u is the average of Bromine's natural isotopes, which is why it lands between whole numbers. The mass number of a single isotope, though, is always whole.
How many neutrons does Bromine have?
Neutrons = mass number − atomic number. For Bromine's commonest isotope that is about 80 − 35 = 45, though the exact count differs between isotopes.