|MadSci Network: Chemistry|
Why was Carbon-12 chosen as the standard for AMU units?
The answer is an accident of history.
Initially, the easiest measurement available was to determine the relative molecular masses of substances which are gases at room temperature. This is because of Avogadro's Principle, which states that "pressure and temperature being equal, an amount of any gas which occupies a given volume will have the same number of molecules." That is, a gallon of hydrogen and a gallon of oxygen will have the same number of molecules, pressure and temperature being equal. This allows one to establish, for example, that oxygen masses sixteen times as much as hydrogen.
Since oxygen forms compounds with more elements than practically any other, oxygen was a natural standard for an atomic mass scale. Then, chemical analysis would allow one to determine the atomic mass of any other element which formed a compound with oxygen. And since hydrogen, the lightest element, weighed 1/16 as much as oxygen, the mass of oxygen was defined as exactly 16.
In the early 20th Century, while chemists were still determining atomic masses as they always had, by chemical analysis, physicists seized on the technique of mass spectrometry, by which one could measure the mass of a single atom or molecule. And therein lay the problem.
In a chemical analysis, one is never determining the mass of an isolated atom or molecule, always the average mass of a huge ensemble of atoms or molecules. (My basic principle: "Chemistry never happens to isolated atoms or molecules. Only physics happens to isolated atoms or molecules.") Therefore, chemists assigned natural oxygen -- which is a mixture of 16O, 17O and 18O -- a mass of exactly 16.
On the other hand, physicists, who were actually measuring the masses of individual atoms or molecules, found it much simpler to assign a mass of exactly 16 to 16O itself. This led to discrepencies (in the third or fourth significant figure, which can be important), and eventually the conflict was resolved by adopting a compromise in 1961: 12C would be assigned a mass of exactly 12. This placed the new scale approximately midway between the two old, oxygen-based scales.
To give credit where credit is due, much of this can be found at David Dice's site on material relating to introductory chemistry.
Try the links in the MadSci Library for more information on Chemistry.