Atomic Mass of Astatine (At)
Astatine has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 210, and the bracketed value [210] marks that estimate.
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Astatine by the numbers
Z = 85 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average ([210] u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 85 |
| Relative atomic mass | [210] (no stable isotope) |
| Protons | 85 |
| Neutrons (commonest isotope) | 125 |
| Electrons | 85 |
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 ([210] 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 Astatine 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 Astatine a whole number?
No — Astatine has no stable isotopes, so chemists use the bracketed estimate [210] rather than a measured average.
How many neutrons does Astatine have?
Neutrons = mass number − atomic number. For Astatine's commonest isotope that is about 210 − 85 = 125, though the exact count differs between isotopes.