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Atomic Weights

Since he had no means of isolating and weighing individual atoms, Dalton set the hydrogen atom as the standard and assigned it a mass of 1. Dalton assumed that water, the only hydrogen-oxygen compound known at the time, had a 1:1 ratio of hydrogen to oxygen atoms. From this and the best available value of the combining ratio by mass, he assigned oxygen the atomic weight of 7. Of course the place Dalton went wrong was the assumption that the hydrogen-oxygen in water was 1:1.

Once scientists understood that water contains two hydrogen atoms for each oxygen they concluded that an oxygen atom must weigh 16 times as much as a hydrogen atom.

Today we can measure the masses of individual atoms very accurately with a device called the mass spectrometer. Measurements of mass on this instrument are made by comparing the mass of an atom to the mass of a reference atom. The carbon-12 atom has been chosen as the standard and has been arbitrarily assigned a mass of exactly 12 atomic mass units (amu). One atomic mass unit is one-twelfth the mass of a carbon-12 atom.

Average Atomic Weights

Most elements occur in nature as mixtures of isotopes. We can determine the average atomic mass of an element by using the masses of the various isotopes and their relative abundances. For example chlorine exists naturally as two isotopes: chlorine-35, with a mass of 34.96885 amu and a fractional abundance of 0.75771, and chlorine-37, with a mass of 36.96590 amu and a fractional abundance of 0.24229. We calculate the average atomic mass (also called atomic weight) of chlorine as follows: (0.75771)(34.96885 amu) + (0.24229)(36.96590 amu) = 35.453 amu.

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