Question

why did dobereiner suggest that calcium strontium and barium were also triads

Answer 1

Answer:

In 1817, a letter reported Johann Wolfgang Dobereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium. By 1829, Dobereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related.