Re: VRD--What element was used before carbon12 as the standard for atomic mass?

Dear Jessica,
The simple answer is: the current standard based on carbon=12 was adopted in 1961. Before that it was oxygen=16. Hydrogen=1 was used until 1905.

This topic was touched on in a MadSci question from 1998: "Why was Carbon-12 chosen as the standard for AMU units?"(see 885340342.Ch).

If you need more information on this fascinating historical topic, then read on...

The decision to base atomic masses on carbon-12 was made in 1961 by the International Union of Pure and Applied Chemistry (IUPAC). Before that oxygen was the basis for specifying both relative atomic masses AND weights. Note the subtle difference!

Chemists were, and still are, in the habit of using atomic weights, that is an "average mass" based on all naturally occuring isotopes (at least here on earth). So even today, carbon has an atomic weight of 12.0107, not 12.0000 since that is the mass of the carbon-12 isotope only. Chemists were using oxygen = 16 (as determined by F.W Clarke in 1882 and formely adopted in 1905 according to the suggestion by J.S. Stas), even though we now know that oxygen has other isotopes. Physicists claimed to have had it right when they insisted that 0=16 for the O16 isotope only (as discovered by F.W. Aston in 1929)!! The difference between weight and mass had only became apparent in 1913 with the discovery of isotopes by a fellow named Soddy.

Even earlier systems were based on assuming hydrogen to have an atomic weight of 1. A natural choice since it had long been known that there was no lighter element. As early as 1803 Dalton published his "atomic theory" and postulated that atoms of different elements have different weights. In 1860 Stanislao Cannizzaro suggested that the weight of a hydrogen atom should be used as the standard to which all other weights were compared. A sytem proposed by Berzelius in 1815 wherein the weight of oxygen was taken to be 100 was not popular, since hydrogen would have a weight of 6.25 (ugly, if not actually that inconvenient)!!

Thanks for your question!

Online References:

http://www.britannica.com/bcom/eb/article/printable/2/0,5722,10262,00.html
http://www.iupac.org/r eports/1998/7001coplen/
www.bartleby.com/65/at/atomicwe .html
http://www.chemistry.ga tech.edu/stms/amu.txt
http://www.carlton.paschools.pa.sk.ca/chemical/Molemass/cannizar.htm