Why does magnesium have an atomic mass of 24.3 instead of 24 or 25?

Question

Owen Ambrose · Accepted Answer

Because the quoted atomic mass is the weighted average of the individual isotopes.

For magnesium, #Z#, the atomic number, #=# #12#. That is there are 12 protons, 12 fundamental, massive, positively charged particles in the nucleus. #Z# of course determines the nuclear identity. However, within the nucleus, there may be 12, or 13, or 14, NEUTRONS; massive, neutrally charged, nuclear particles, that also do contribute to nuclear stability, but not to chemistry (#Z# has already determined that!). The weighted average of the individual isotopes is the atomic mass quoted on the Periodic Table. Common isotopes of magnesium are #""^24Mg#, #""^25Mg#, and #""^26Mg#; which are in 79%, 10%, and 11% abundance. The weighted average of these isotopes sum to 24.3 amu. There are a few other, less abundant isotopes, but these don't concern is very much due to their rarity. Is that right? The weighted average is just the SUM of the individual isotopes WEIGHTED according to their isotopic abundance. For the figures above, we could do the sum: Some exams ask you to quote an atomic mass after providing details about a few common isotopes. #"Average mass"# #=# #0.78xx24+0.10xx25+0.11xx26# #=# #24.3# as required. Most elements have a number of common isotopes.

Natalie Anton · Answer

undefined The atomic mass of magnesium is 24.3 because it is an average of the masses of its isotopes, which include magnesium-24 and magnesium-25. Isotopes are variants of an element with the same number of protons but different numbers of neutrons. The presence of different isotopes and their relative abundances contribute to the overall atomic mass of magnesium.