Döbereiner’s triads: the elements that came in threes

Who was Johann Döbereiner?

Johann Wolfgang Döbereiner (1780–1849) was a German chemist who trained as a pharmacist before becoming a chemistry professor at the University of Jena. He is also remembered for Döbereiner’s lamp of 1823, an early lighter in which a stream of hydrogen gas caught fire on contact with platinum, one of the first practical uses of a catalyst.

A clue from strontium

In 1817 Döbereiner noticed something curious about strontium. Its atomic weight fell almost exactly halfway between those of calcium and barium, and its chemistry was in between as well: the three metals form similar compounds, and strontium’s behavior sits between that of its two relatives. With only one example, though, it could easily have been a coincidence.

The triads of 1829

In 1829 Döbereiner published a broader version of the idea. He described several groups of three, or triads, in which the elements are chemically alike and the middle atomic weight is close to the mean of the outer two. With modern atomic weights the pattern is easy to check. Calcium (40.1), strontium (87.6) and barium (137.3): the average of calcium and barium is 88.7, close to strontium. Chlorine (35.5), bromine (79.9) and iodine (126.9): the average of the outer two is 81.2. Lithium (6.9), sodium (23.0) and potassium (39.1): the average is 23.0, almost exactly the weight of sodium. Sulfur, selenium and tellurium formed a fourth triad.

Physical properties followed the same order. Chlorine is a gas, iodine is a solid, and bromine, discovered only three years earlier, is a liquid in between; the color of the three halogens deepens and their reactivity drops in the same sequence.

What the triads showed

The triads were among the first evidence that atomic weight is connected to how an element behaves. They also showed that elements belong to natural families. Each of Döbereiner’s triads still sits together in a single column of the modern table: the alkali metals in group 1, the alkaline earth metals in group 2, the chalcogens in group 16 and the halogens in group 17.

Today we can see why the averages work. The three members of each triad are evenly spaced in atomic number: lithium, sodium and potassium are 8 places apart, and the members of the other triads are 18 places apart, so their atomic weights rise in roughly equal steps.

The limits of the triads

Triads could not organize most of the elements. Of the more than 50 elements known in 1829, only about a dozen fit neatly into groups of three, and Döbereiner offered no way to link one triad to another or to place the elements left over. Viewed with modern data, the rule also fails inside families: fluorine, chlorine and bromine are all halogens, yet chlorine’s atomic weight (35.5) is nowhere near the average of fluorine and bromine (49.5).

Later chemists, among them Leopold Gmelin and Jean-Baptiste Dumas, found more families and numerical patterns, but a complete system had to wait until reliable atomic weights became available in the 1860s.