In the end, the mass values on the periodic table are not atomic mass, nor mass number. Instead, the values on the periodic table are average atomic masses.
The averages are calculated as weighted averages, so that the value represents a “typical” atom.
As a simple example of weighted average would could think about people.
Let’s imagine that you are at a small party at a friend’s house. Looking around, you realize that 9 of the 10 people there weigh 100 lbs while the other person weighs 200 lbs. If you were asked the average weight of the people at the party you would not answer 150 lbs. You would probably say 110 lbs (because (9*100 + 1*200)/10 = 110). Note that this answer is a little odd, since there is no one attending the party who actually weighs that amount (they are all either more or less).
If we make the math a little more difficult (but not much more) we could say that you go to much larger party where 90% of the people weigh 100 lbs and 10% of the people weigh 200 lbs. Asked for the average you should still say 110 lbs (because (90*100+10*200)/100 = 110).
This is the way that the average mass of atoms is calculated. For instance, we know (from using a mass spectrometer) that 98.9% of the world’s carbon weighs 12.000 amu and that 1.1% of the world’s carbon weighs 13.000 amu. So, if we do the math (98.9*12.000+1.1*13.000)/100, we get 12.011 which is the mass on the periodic table for carbon.
What that means is that the average mass of a carbon atom is 12.011 amu, even though there are NO atoms of carbon that weigh that much.
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