Atomic weight refers to the average weight of an atom of an element, as compared to the isotope of carbon which is designated to have a weight of twelve. The great historian of chemistry Aaron Ihde has referred to atomic weights as a problem "so formidable that it taxed the ingenuity of the best chemists of Europe for half a century." The problem originated as a result of John Dalton's atomic theory, first presented between 1803 and 1805. Dalton emphasized the importance of knowing the weights of atoms and outlined an experimental method for determining those weights.

The one problem with Dalton's suggestion was that chemists had to know the formulas of chemical compounds before they could determine the weights of atoms. But they had no way of knowing chemical formulas without a dependable table of atomic weights. Dalton himself had assumed that compounds...