# The atomic weight of magnesium is reported as 24.3amu, yet no atom of magnesium has the mass of 24.3amu, how can this be explained?

Because the quoted atomic mass is the weighted average mass of a range of $M g$ isotopes.
For magnesium, $Z$ $=$ $12$; there are 12 positively charged particles, protons, in its nucleus. This is what defines the metal as magnesium.
Individual magnesium atoms may have 12 or more or less neutrally charged massive particles in its nucleus. About 80% of the nuclei have 12 neutrons; 10% 13 neutrons, and another 10% 14 neutrons. The weighted average of the isotopic masses give an average molecular mass. Capisce? As elements get larger ($Z >$), the isotopic spread gets correspondingly large.