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Dobereiner's Law of Triads

Dobereiner, in 1817 suggested a relationship between the properties of elements and their atomic weights.


Johann Wolfgang Dobereiner

According to Dobereiner, the atomic weight of the middle element is nearly the same as average of the atomic weights of other two elements.

Example : If we take lithium, sodium and potassium, the atomic weight of sodium is the mean of the atomic weights of lithium and potassium i.e., (7+ 39)/2 = 23. The elements that were grouped as triads are given in table 5.1 below.



Table 5.1 Dobereiner's Triads of Elements




Mean atomic weight

Lithium (7)



(7+ 39)/2 = 23





Calcium (40)

Strontium (87.5)

Barium (137.5)

(40 +137.5)/2 = 88.75





Phosphorus (31)

Arsenic (76)

Antimony (120)

(31 + 120)/2 = 75.5





Sulphur (32)

Selenium (79)

Tellurium (127.5)

(32+ 137.5)/2 = 79.25





Chlorine (35.5)

Bromine (80)

Iodine (127)

(35.5 + 127)/2 = 81.25





Defect of Dobereiner's triad
The defect with Dobereiner's triad was that all the known elements could not be classified as triads. For example iron, manganese, nickel, cobalt, zinc and copper are six similar elements which cannot be placed in the traid.


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