Atomic Mass of Erbium (Er)
The relative atomic mass of Erbium is 167.26 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 Erbium. 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 Erbium page is one click away.
Erbium by the numbers
One Erbium atom carries 68 protons in its nucleus; add the neutrons of its commonest isotope (about 99) and you get its mass number, roughly 167.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 68 |
| Relative atomic mass | 167.26 u |
| Protons | 68 |
| Neutrons (commonest isotope) | 99 |
| Electrons | 68 |
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 (167.26). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Erbium a whole number?
No. 167.26 u is the average of Erbium'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 Erbium have?
Neutrons = mass number − atomic number. For Erbium's commonest isotope that is about 167 − 68 = 99, though the exact count differs between isotopes.