MadSci Network: Chemistry
Query:

Re: Why is the relative atomic mass of an element never a whole number

Date: Wed Nov 1 08:18:54 2006
Posted By: Adil Bukhari, Undergraduate, Computer Science, University of Maryland, College Park
Area of science: Chemistry
ID: 1161796665.Ch
Message:

Kenneth, Your were partially correct, but let me fill you in on the rest of the answer...

The relative atomic mass of any element on the periodic table is just a sum of how often each isotope occurs in nature times its atomic mass. If that's a bit confusing, here's an example...

The element chlorine (Cl) is present largely as two different isotopes, Cl-35 and Cl-37. Cl-35, with a mass of 35, is much more common, representing about 75% of all naturally existing Chlorine. Cl-37, with mass of 37, on the other hand, makes up roughly 25% of naturally existing chlorine. To figure out the relative atomic mass, we just use this formula:

Relative Atomic Mass = (Isotope 1 occurrence)(Isotope 1 Atomic Mass) + (Isotope 2 occurrence)(Isotope 2 Atomic Mass) +...

For chlorine, this means:

Relative Atomic Mass = ((75/100)*35) + ((25/100)*37), which equals 35.5.

Of course, there are many more less common isotopes, which cause the relative atomic mass to actually be around 35.453.
Relative Atomic Mass has no units, since they are all cancelled out through the calculation!
Check this website for more information.
Well, hope that helps,
Adil B.


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