Atomic Mass of Promethium (Pm)
Promethium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 145, and the bracketed value [145] marks that estimate.
This is the fast lane: one page, one fact, zero digressions. Read the quick answer, check the table, and if your exam needs the full derivation, the sections below walk through it step by step — in Bangla and English, like every page here. Need the whole picture instead — history, characteristics, uses, exam questions? The full Promethium page is one click away.
Promethium by the numbers
Z = 61 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average ([145] u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 61 |
| Relative atomic mass | [145] (no stable isotope) |
| Protons | 61 |
| Neutrons (commonest isotope) | 84 |
| Electrons | 61 |
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 ([145] 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 Promethium 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 Promethium a whole number?
No — Promethium has no stable isotopes, so chemists use the bracketed estimate [145] rather than a measured average.
How many neutrons does Promethium have?
Neutrons = mass number − atomic number. For Promethium's commonest isotope that is about 145 − 61 = 84, though the exact count differs between isotopes.