Atomic Mass of Radon (Rn)
Radon has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 222, and the bracketed value [222] marks that estimate.
Teachers call it "one fact, fully handled" — exactly what this page does for Radon. 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 Radon page is one click away.
Radon by the numbers
One Radon atom carries 86 protons in its nucleus; add the neutrons of its commonest isotope (about 136) and you get its mass number, roughly 222.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 86 |
| Relative atomic mass | [222] (no stable isotope) |
| Protons | 86 |
| Neutrons (commonest isotope) | 136 |
| Electrons | 86 |
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 ([222] 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 Radon 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 Radon a whole number?
No — Radon has no stable isotopes, so chemists use the bracketed estimate [222] rather than a measured average.
How many neutrons does Radon have?
Neutrons = mass number − atomic number. For Radon's commonest isotope that is about 222 − 86 = 136, though the exact count differs between isotopes.