Döbereiner's triads

In the history of the periodic table, Döbereiner's triads were an early attempt to sort the elements into some logical order and sets based on their physical properties.

In 1817, a letter by Ferdinand Wurzer [de] reported Johann Wolfgang Döbereiner's observations of the alkaline earths; namely, that strontium had properties that were intermediate to those of calcium and barium.

This gave Mr. Döbereiner cause to inquire rigorously into the stoichiometric value of strontium oxide by a great series of experiments.

For a moment, Mr. Döbereiner found himself thereby caused to doubt the independent existence of strontium; however, this withstood both his analytical and synthetic experiments.

[4] As the techniques for accurately measuring atomic masses improved, the Döbereiner's triad was found to fail to remain strictly valid.