Atomic Mass of Barium (Ba)
The relative atomic mass of Barium is 137.33 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 Barium. 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 Barium page is one click away.
Barium by the numbers
One Barium atom carries 56 protons in its nucleus; add the neutrons of its commonest isotope (about 81) and you get its mass number, roughly 137.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 56 |
| Relative atomic mass | 137.33 u |
| Protons | 56 |
| Neutrons (commonest isotope) | 81 |
| Electrons | 56 |
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 (137.33). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Barium a whole number?
No. 137.33 u is the average of Barium'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 Barium have?
Neutrons = mass number − atomic number. For Barium's commonest isotope that is about 137 − 56 = 81, though the exact count differs between isotopes.