Atomic Mass Units

I mentioned that these are relative atomic weights because they were compared to hydrogen (then oxygen and then carbon-12). One way of dealing with atomic weights that gets around the "relative" aspect would be to find out how much each atom actually weighs. That was difficult and describing how it was done will have to wait for another lesson. Another way is to define a new mass unit and say that it is equal to the mass of one hydrogen atom. That unit is called the atomic mass unit (amu) and is defined, for the moment, as the mass of one hydrogen atom. Then if oxygen atoms weigh 16.00 times as much as hydrogen atoms, they weigh 16.00 amu. Similarly for any element, whatever the relative atomic weight is, that is how many amu's each atom of that element weighs. Using the current standard, the actual definition of an atomic mass unit is that it is 1/12th of the mass of one atom of the carbon-12 isotope.

Because the relative weights of hydrogen, carbon and nitrogen are 1.01, 12.01 and 14.01. We can also say that the weight of hydrogen is 1.01 amu per atom, the weight of carbon is 12.01 amu per atom and the weight of nitrogen is 14.01 amu per atom. (Actually we should say "the average weight per atom" because the weight of an atom depends not only on which element it is, but also on which isotope it is.)

Top of Page

Back to Course Homepage

E-mail instructor:

Clackamas Community College
©1998, 2002 Clackamas Community College, Hal Bender