Atomic Mass of Actinium (Ac)
Actinium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 227, and the bracketed value [227] marks that estimate.
Teachers call it "one fact, fully handled" — exactly what this page does for Actinium. The answer sits at the top in bold; scroll for the working, the shell-by-shell breakdown and the two questions students ask most. Need the whole picture instead — history, characteristics, uses, exam questions? The full Actinium page is one click away.
Actinium by the numbers
Z = 89 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average ([227] u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 89 |
| Relative atomic mass | [227] (no stable isotope) |
| Protons | 89 |
| Neutrons (commonest isotope) | 138 |
| Electrons | 89 |
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 ([227] 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 Actinium 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 Actinium a whole number?
No — Actinium has no stable isotopes, so chemists use the bracketed estimate [227] rather than a measured average.
How many neutrons does Actinium have?
Neutrons = mass number − atomic number. For Actinium's commonest isotope that is about 227 − 89 = 138, though the exact count differs between isotopes.