# Use the periodic table to find the percentage abundance of neon-20, assuming that neon has only one other, neon-22.  How do I find the atomic mass of neon-22? Is neon 20's atomic mass 20.18? I...

Use the periodic table to find the percentage abundance of neon-20, assuming that neon has only one other, neon-22.

How do I find the atomic mass of neon-22? Is neon 20's atomic mass 20.18? I don't get any of this at all! How do I find the percentage abundance?

ncchemist | eNotes Employee

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The atomic mass listed on the periodic table for each element is the weighted average of all of the isotopes for the element with their relative abundences taken into account.  Neon's average molecular weight is listed as 20.18.  If we assume that there are only 2 isotopes of neon found (neon-20 and neon-22), then this simplifies the calculations.  The exact weight of an atom of neon-20 is 20 and the exact weight of neon-22 is 22.  If we call the percentage of neon-22 as "x" then the percentage of neon-20 is "1=x", since both percentages must add up to 100% (or 1.0).  We can set up the following equation:

`22x + (1-x)20 = 20.18`

`22x + 20-20x = 20.18`

```2x + 20=20.18`

`2x=0.18`

`x=0.09`

So we know that the percent abundance of neon-22 is 9% and neon-20 is 91%.

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