What was the contribution of Dobereiner towards classification of elements?

that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements.

In 1829, Johann Döbereiner recognised triads of elements with chemically similar properties, such as lithium, sodium and potassium, and showed that the properties of the middle element could be predicted from the properties of the other two.

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