How is the atomic mass of magnesium determined?

The atomic mass of magnesium is determined by taking into account the masses of its isotopes and their relative abundances. Magnesium has three stable isotopes: Mg-24, Mg-25, and Mg-26. The atomic mass is a weighted average of these isotopes' masses. For example, if Mg-24 has an abundance of about 79%, Mg-25 about 10%, and Mg-26 about 11%, the atomic mass is calculated as:

Atomic mass of Mg = (0.79 * 24) + (0.10 * 25) + (0.11 * 26) = 24.305 u