Döbereiner's triads

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In the history of the periodic table, Döbereiner's triads were an early attempt at organizing the chemical elements by certain physical properties. In 1829, the German chemist Johann Wolfgang Döbereiner noted that there were triads of elements that showed similar chemical properties. He also noted that other properties of the elements followed trends in which the value of the middle element of the triad was in between those of the end members. For example, the atomic weight[clarification needed] and density of the central element of the triad was at or nearly the mean of the atomic weights of the other two members. The modern periodic table has these triads of elements in adjacent spaces (either consecutive periods (rows) in a group (column) or consecutive groups in a period).

Predicted vs actual atomic mass of the central atom of each triad[verification needed]
Element 1
Atomic mass
Element 2
Mean of 1 & 3
Actual atomic mass
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]