2.17 Relative Masses

Chemists came up with atomic weights for the elements long before they could isolate individual atoms. But how did they do that? How did they weigh individual atoms?

Simple: they didn’t! Instead, they carefully weighed samples of those elements and reacted them with samples of other elements -- also carefully weighed out. That enabled them to come up with mass combining ratios and from these, they were able to determine what they believed were relative masses of the various elements.

Three important concepts to keep in mind:
* The lightest element was arbitrarily assigned a mass of 1.00. (That was of course, the element H, hydrogen.)
* All the other elements were then assigned masses relative to this 1.0 based on their mass combining ratios. So for example hydrogen samples reacted with fluorine samples that weighed precisely 19 times as much. Thus relative to a hydrogen atom weighing 1.0, a fluorine atom would weigh 19.0.
* The assumption was made that the compounds formed were always in a 1:1 atom ratio, thus their sample combining mass ratio would be equivalent to their atomic mass ratio. These two short jing videos should help you if you get stuck!
http://www.screencast.com/users/KRBecker/folders/relative%20masses

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