Atomic Mass of Fermium (Fm)
Fermium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 257, and the bracketed value [257] marks that estimate.
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Fermium by the numbers
One Fermium atom carries 100 protons in its nucleus; add the neutrons of its commonest isotope (about 157) and you get its mass number, roughly 257.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 100 |
| Relative atomic mass | [257] (no stable isotope) |
| Protons | 100 |
| Neutrons (commonest isotope) | 157 |
| Electrons | 100 |
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 ([257] 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 Fermium 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 Fermium a whole number?
No — Fermium has no stable isotopes, so chemists use the bracketed estimate [257] rather than a measured average.
How many neutrons does Fermium have?
Neutrons = mass number − atomic number. For Fermium's commonest isotope that is about 257 − 100 = 157, though the exact count differs between isotopes.