Atomic Mass of Plutonium (Pu)
Plutonium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 244, and the bracketed value [244] marks that estimate.
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Plutonium by the numbers
One Plutonium atom carries 94 protons in its nucleus; add the neutrons of its commonest isotope (about 150) and you get its mass number, roughly 244.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 94 |
| Relative atomic mass | [244] (no stable isotope) |
| Protons | 94 |
| Neutrons (commonest isotope) | 150 |
| Electrons | 94 |
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 ([244] is shown in brackets because it is only an estimate). Exams love this one-mark distinction.
Why is the mass in brackets?
Every isotope of Plutonium is radioactive and vanishes too quickly to build up a natural mixture — so the IUPAC convention prints the mass number of the longest-lived isotope in square brackets instead of a standard atomic weight.
Students also ask
Is the atomic mass of Plutonium a whole number?
No — Plutonium has no stable isotopes, so chemists use the bracketed estimate [244] rather than a measured average.
How many neutrons does Plutonium have?
Neutrons = mass number − atomic number. For Plutonium's commonest isotope that is about 244 − 94 = 150, though the exact count differs between isotopes.