Atomic Mass of Radium (Ra)
Radium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 226, and the bracketed value [226] marks that estimate.
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Radium by the numbers
One Radium atom carries 88 protons in its nucleus; add the neutrons of its commonest isotope (about 138) and you get its mass number, roughly 226.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 88 |
| Relative atomic mass | [226] (no stable isotope) |
| Protons | 88 |
| Neutrons (commonest isotope) | 138 |
| Electrons | 88 |
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 ([226] 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 Radium 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 Radium a whole number?
No — Radium has no stable isotopes, so chemists use the bracketed estimate [226] rather than a measured average.
How many neutrons does Radium have?
Neutrons = mass number − atomic number. For Radium's commonest isotope that is about 226 − 88 = 138, though the exact count differs between isotopes.