Atomic Mass of Californium (Cf)
Californium has no stable isotope, so no standard atomic weight exists; its most stable isotope has mass number ≈ 251, and the bracketed value [251] marks that estimate.
Teachers call it "one fact, fully handled" — exactly what this page does for Californium. 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 Californium page is one click away.
Californium by the numbers
One Californium atom carries 98 protons in its nucleus; add the neutrons of its commonest isotope (about 153) and you get its mass number, roughly 251.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 98 |
| Relative atomic mass | [251] (no stable isotope) |
| Protons | 98 |
| Neutrons (commonest isotope) | 153 |
| Electrons | 98 |
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 ([251] 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 Californium 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 Californium a whole number?
No — Californium has no stable isotopes, so chemists use the bracketed estimate [251] rather than a measured average.
How many neutrons does Californium have?
Neutrons = mass number − atomic number. For Californium's commonest isotope that is about 251 − 98 = 153, though the exact count differs between isotopes.