Atomic Mass of Magnesium (Mg)
The relative atomic mass of Magnesium is 24.305 u — the weighted average of its naturally occurring isotopes, on the carbon-12 scale.
Looking for Magnesium's atomic mass without wading through a whole chapter? Here is the direct answer first, then the tables and derivations examiners want to see. Every number on this page comes from the same engine that powers the interactive table. Need the whole picture instead — history, characteristics, uses, exam questions? The full Magnesium page is one click away.
Magnesium by the numbers
One Magnesium atom carries 12 protons in its nucleus; add the neutrons of its commonest isotope (about 12) and you get its mass number, roughly 24.
| Quantity | Value |
|---|---|
| Atomic number (Z) | 12 |
| Relative atomic mass | 24.305 u |
| Protons | 12 |
| Neutrons (commonest isotope) | 12 |
| Electrons | 12 |
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 (24.305). Exams love this one-mark distinction.
Students also ask
Is the atomic mass of Magnesium a whole number?
No. 24.305 u is the average of Magnesium'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 Magnesium have?
Neutrons = mass number − atomic number. For Magnesium's commonest isotope that is about 24 − 12 = 12, though the exact count differs between isotopes.