The German Chemist J.W. Dobereiner in 1829, grouped certain elements in the groups of three, called Dobereiner's triads or simply as triads. The three elements in a triad hd similr chemicl properties. In these triads, the tomic mass of the middle element is nearly equal to the mean of the first and third elements in each triad. But the idea of classifiction could nt be applied to all the elements..
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