Atomic Mass of Sodium (Na)
The relative atomic mass of Sodium is 22.990 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 Sodium. 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 Sodium page is one click away.
Sodium by the numbers
Z = 11 fixes the protons; the neutron count can vary between isotopes, which is why the atomic mass is an average (22.990 u) rather than a round number.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 11 |
| Relative atomic mass | 22.990 u |
| Protons | 11 |
| Neutrons (commonest isotope) | 12 |
| Electrons | 11 |
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 (22.990). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Sodium a whole number?
No. 22.990 u is the average of Sodium'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 Sodium have?
Neutrons = mass number − atomic number. For Sodium's commonest isotope that is about 23 − 11 = 12, though the exact count differs between isotopes.