Atomic Mass of Caesium (Cs)
The relative atomic mass of Caesium is 132.91 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Caesium by the numbers
Z = 55 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (132.91 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 55 |
| Relative atomic mass | 132.91 u |
| Protons | 55 |
| Neutrons (commonest isotope) | 78 |
| Electrons | 55 |
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 (132.91). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Caesium a whole number?
No. 132.91 u is the average of Caesium'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 Caesium have?
Neutrons = mass number − atomic number. For Caesium's commonest isotope that is about 133 − 55 = 78, though the exact count differs between isotopes.