Atomic Mass of Osmium (Os)
The relative atomic mass of Osmium is 190.23 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
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Osmium by the numbers
One Osmium atom carries 76 protons in its nucleus; add the neutrons of its commonest isotope (about 114) and you get its mass number, roughly 190.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 76 |
| Relative atomic mass | 190.23 u |
| Protons | 76 |
| Neutrons (commonest isotope) | 114 |
| Electrons | 76 |
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 (190.23). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Osmium a whole number?
No. 190.23 u is the average of Osmium'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 Osmium have?
Neutrons = mass number − atomic number. For Osmium's commonest isotope that is about 190 − 76 = 114, though the exact count differs between isotopes.