Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Matias Ekstrand on July 27, 2011, 12:41:52 PM
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I have been trying to figure out how to correctly describe atomic masses. When I look at IUPAC publications, I find that atomic mass should only be used to describe a specific isotope of an element in the unit u.
Whenever we want to describe an "average" (maybe not the correct word in the context) mass depending on natural occurrence of isotopes, we use relative atomic mass (atomic weight), which has no unit associated with it.
The problem is that I can't find any use of this outside of IUPAC publications (except for some obscure mentioning of it on Wikipedia). All the chemistry literature I can get my hands on systematically use the term atomic mass when they describe relative atomic masses, and use the atomic mass unit with it as well.
My question: Are the IUPAC publications simply ignored in this area?
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Yes. Sort of. They all mean the same thing essentially; but you have to have a very specific way of defining things for them to work in science.
What most people use, is what the IUPAC defines as the unified atomic mass unit.
Unless you are doing some particle physics or some serious isotope stuff, it is all the same within significance.
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I just find strange is that IUPAC says there should be no unit used with the relative atomic mass, yet everyone uses the unified atomic mass unit. This should matter a lot in making correct unit conversions.
If I wanted to teach chemistry, would you recommend that I included the difference between atomic mass and relative atomic mass to comply with IUPAC guidelines?
enahs: +mole snack
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No. It is irrelevant and confusing to early chemistry students.
There are units in the relative atomic mass; just when you perform the division "mathematically" there are no-longer any units.
You can not do proper chemistry/conversions without the units.
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Guess I will not go into relative atomic masses with the students then.
Thank you very much for your help in this matter. I really appreciate it.
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If I wanted to teach chemistry, would you recommend that I included the difference between atomic mass and relative atomic mass to comply with IUPAC guidelines?
Nah. If you want to confuse them just bring up the subject of deuterium exchange. Its a lot more readily accessible. For example, you can order some D2O from united nuclear and make a sinking ice cube, or show a sample of water on a scale in a bell jar "gain weight" as another sample of heavy water next to it loses weight, etc.. after you're sure they know what deuterium is, ask some pH questions... :)