Atomic Mass of Nobelium (No)
Nobelium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 259, and the bracketed value [259] marks that estimate.
Looking for Nobelium's atomic mass without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Nobelium page is one click away.
Nobelium by the numbers
One Nobelium atom carries 102 protons in its nucleus; add the neutrons of its commonest isotope (about 157) and you get its mass number, roughly 259.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 102 |
| Relative atomic mass | [259] (no stable isotope) |
| Protons | 102 |
| Neutrons (commonest isotope) | 157 |
| Electrons | 102 |
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 ([259] 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 Nobelium 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 Nobelium a whole number?
No — Nobelium has no stable isotopes, so chemists use the bracketed estimate [259] rather than a measured average.
How many neutrons does Nobelium have?
Neutrons = mass number − atomic number. For Nobelium's commonest isotope that is about 259 − 102 = 157, though the exact count differs between isotopes.