A sample of magnesium contains 79% of magnesium-24 atoms ,10% of magnesium-25 atoms ,11% of magnesium-26 atoms. In most calculations the relative atomic mass of magnesium used is 24.Use this information to explain why, in this sample, magnesium has a relative atomic mass of 24.3 – 9001

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1. The relative atomic mass of magnesium is 24.3 because magnesium has more than one isotope. Most of the atoms are magnesium-24, but some are magnesium-25 and magnesium-26, which are heavier. The average (relative atomic mass) is calculated using a weighted mean, based on how much of each isotope there is.

   In a sample of 100 atoms:

   79 atoms have a mass of 24

   10 atoms have a mass of 25

   11 atoms have a mass of 26

   So the calculation is:

   Relative atomic mass =
   (79 × 24) + (10 × 25) + (11 × 26)
   = 1896 + 250 + 286
   = 2432 ÷ 100
   = 24.3

   This shows that magnesium-24 is the most common, which is why the average is closer to 24 than to 25 or 26.

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