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 by their physical properties. In 1829, the German chemist Johann Wolfgang Döbereiner noted that there were groups of three elements (hence "triads") which had similar physical properties. 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 near exactly predicted by taking the arithmetic mean of values for that property of the other two elements.

Predicted vs actual atomic mass of the central atom of each triad

Group A element Atomic mass	Group B element Actual atomic mass Mean of 1 & 3	Group C element Atomic mass
Lithium 14.0	Phosphorus 31.0 31.0	Arsenic 74.9
Calcium 40.1	Strontium 87.6 87.6	Barium 137.3
Chlorine 35.5	Bromine 79.9 79.9	Iodine 126.9
Sulfur 32.1	Selenium 79.0 79.9	Tellurium 127.6
Carbon 12.0	Nitrogen 14.0 14.0	Oxygen 16.0
Iron 55.8	Cobalt 58.9 57.3	Nickel 58.7

References

• "Johann Wolfgang Dobereiner". Retrieved 2013-01-15.
• "A Historic Overview: Mendeleev and the Periodic Table" (PDF). Retrieved 2013-01-15.