Döbereiner's triads

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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[verification needed]
Group A element
Atomic mass
Group B element
Actual atomic mass
Mean of 1 & 3
Group C element
Atomic mass
Nitrogen
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[edit]