Atomic Mass of Oxygen (O)
The relative atomic mass of Oxygen is 15.999 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 Oxygen. 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 Oxygen page is one click away.
Oxygen by the numbers
One Oxygen atom carries 8 protons in its nucleus; add the neutrons of its commonest isotope (about 8) and you get its mass number, roughly 16.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 8 |
| Relative atomic mass | 15.999 u |
| Protons | 8 |
| Neutrons (commonest isotope) | 8 |
| Electrons | 8 |
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 (15.999). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Oxygen a whole number?
No. 15.999 u is the average of Oxygen'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 Oxygen have?
Neutrons = mass number − atomic number. For Oxygen's commonest isotope that is about 16 − 8 = 8, though the exact count differs between isotopes.