Atomic Mass of Beryllium (Be)
The relative atomic mass of Beryllium is 9.0122 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Beryllium by the numbers
One Beryllium atom carries 4 protons in its nucleus; add the neutrons of its commonest isotope (about 5) and you get its mass number, roughly 9.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 4 |
| Relative atomic mass | 9.0122 u |
| Protons | 4 |
| Neutrons (commonest isotope) | 5 |
| Electrons | 4 |
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 (9.0122). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Beryllium a whole number?
No. 9.0122 u is the average of Beryllium'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 Beryllium have?
Neutrons = mass number − atomic number. For Beryllium's commonest isotope that is about 9 − 4 = 5, though the exact count differs between isotopes.