Question

about doberiner triads

Answer 1

Döbereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. In 1817, a letter reported Johann Wolfgang Döbereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium.[1] By 1829, Döbereiner had found other groups of three elements (hence "triads") whose physical properties were similarly related.[2] He also noted that some quantifiable properties of elements (e.g. atomic weight and density) in a triad followed a trend whereby the value of the middle element in the triad would be exactly or nearly predicted by taking the arithmetic mean of values for that property of the other two elements.

Answer 2

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In Doneriner's traid, the atomic mass of the middle element was almost the mean or average of the atomic mass of the first and third elements.
In Doneriner could identify only three traids of elements. These are: Li, Na, K ; Ca, Sr, Ba and Cl, Br, I.