Atomic Mass of Terbium (Tb)
The relative atomic mass of Terbium is 158.93 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Teachers call it "one fact, fully handled" — exactly what this page does for Terbium. 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 Terbium page is one click away.
Terbium by the numbers
Z = 65 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (158.93 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 65 |
| Relative atomic mass | 158.93 u |
| Protons | 65 |
| Neutrons (commonest isotope) | 94 |
| Electrons | 65 |
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 (158.93). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Terbium a whole number?
No. 158.93 u is the average of Terbium's natural isotopes, which is why it lands between whole numbers. The mass number of a single isotope, though, is always whole.
How many neutrons does Terbium have?
Neutrons = mass number − atomic number. For Terbium's commonest isotope that is about 159 − 65 = 94, though the exact count differs between isotopes.