Atomic Weights

By the early nineteenth century, chemists were striving to organize their rudimentary knowledge of the chemical elements. It was known that differing weights of elements reacted to form compounds. For example, they found that 3 grams of magnesium metal reacted with precisely 2 grams of oxygen to form magnesium oxide with no residual magnesium or oxygen. The same weight of oxygen, however, required 5 grams of calcium metal to react completely to form calcium oxide. Table 1 summarizes these relative combining weights.

Chemists gradually discovered that such relative weights in chemical reactions were fundamental characteristics of the elements. The English chemist John Dalton realized that all the known combining weights were nearly whole-number multiples of the combining weight of the lightest element—hydrogen. In 1803, he proposed an atomic theory in which all other elements would be built from multiple hydrogen atoms. Consequently, he based his scale of atomic weights on hydrogen being equal to 1.

Although Dalton's theory was found to be unrealistically simple, he did compel chemists to adopt a standard scale of atomic weights. Because the combining weight of oxygen is approximately 16 times that of hydrogen, the preceding chart can be revised, as shown in Table 2 .

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