Magnesium has three stable isotopes. One has 12 neutrons the second has 13 and the third has 14. Which one is the most abundant? How do you know?

12

Explanation:

If you look at the periodic table, you can see that magnesium has an atomic mass of 24.305.  Subtract the number of protons from this to get neutrons.

24.305 - 12 = 12.305

12.305 is the average amount of neutrons a magnesium atom can have.  A magnesium atom can have 12, 13, or 14 neutrons.  The average, based on the natural abundance, is 12.305.  To find which isotope is more abundant, you need to pick the number that is closest to the average.  This would make the most abundant isotope the one with 12 neutrons.

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