Johann Dobereiner gave the idea of trends among physical and chemical properties of several groups of three elements, the relationship referred to as

Dobereiner noticed that the middle element of each of the triads had an atomic weight about half way between the atomic weights of the other two. Also the properties of the middle element were in between those of the other two members. Dobereiner's relationship referred to as the law of triads, seemed to work only for a few elements.