Döbereiner's triads

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Johann Wolfgang Döbereiner, who attempted to sort the elements in an order which consisted of 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 published[1] a report of his previous observations 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 nearly 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[verification needed]
Element 1
Atomic mass
Element 2
Actual atomic mass
Mean of 1 and 3
Element 3
Atomic mass
Lithium
6.9
Sodium
23.0
23.0
Potassium
39.1
Calcium
40.1
Strontium
87.6
88.7
Barium
137.3
Chlorine
35.5
Bromine
79.9
81.2
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[edit]

  1. ^ Döbereiner, Johann Wolfgang (1829). "An Attempt to Group Elementary Substances according to Their Analogies". Annalen der Physik und Chemie. 15: 301–307. an attempt which I made twelve years ago to group substances by their analogies.