Atomic Mass of Rubidium (Rb)
The relative atomic mass of Rubidium is 85.468 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Rubidium by the numbers
Z = 37 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (85.468 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 37 |
| Relative atomic mass | 85.468 u |
| Protons | 37 |
| Neutrons (commonest isotope) | 48 |
| Electrons | 37 |
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 (85.468). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Rubidium a whole number?
No. 85.468 u is the average of Rubidium'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 Rubidium have?
Neutrons = mass number − atomic number. For Rubidium's commonest isotope that is about 85 − 37 = 48, though the exact count differs between isotopes.