Atomic Mass of Protactinium (Pa)
The relative atomic mass of Protactinium is 231.04 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Protactinium by the numbers
Z = 91 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (231.04 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 91 |
| Relative atomic mass | 231.04 u |
| Protons | 91 |
| Neutrons (commonest isotope) | 140 |
| Electrons | 91 |
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 (231.04). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Protactinium a whole number?
No. 231.04 u is the average of Protactinium'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 Protactinium have?
Neutrons = mass number − atomic number. For Protactinium's commonest isotope that is about 231 − 91 = 140, though the exact count differs between isotopes.