Döbereiner’s Triads

  • This classification was done by scientist Johann Wolfgang Döbereiner in 1817.
  • Döbereiner identified groups of 3 elements each (called triad )
  • He showed that if 3 elements were written in increasing order of atomic masses , the atomic mass of the middle element was roughly the arithmetic average of atomic masses of first and third elements.
  • He also found similarities in physical and chemical properties in these triads.
    Example: Lithium, Sodium and Potassium (Triad 1) react vigorously with Chlorine, Bromine and Iodine (Triad 3).
  • He also predicted that the law could be applied to other quantifiable properties such as density.

Triad 1

Lithium, Sodium and Potassium

The atomic masses are:

Element

Symbol

Atomic Mass

Lithium

Li

6.9

Sodium

Na

23

Potassium

K

39.1

Sum of atomic masses of first and third element in the triad = (6.9 + 39.1)/2 = 23

                                                                                               = Atomic mass of second element

Triad 2

Calcium, Strontium and Barium

The atomic masses are:

Element

Symbol

Atomic Mass

Calcium

Ca

40.1

Strontium

Sr

87.6

Barium

Ba

137.3

Sum of atomic masses of first and third element in the triad = (40.1 + 137.3)/2 = 88.7

                                                                                                Atomic mass of second element

Triad 3

Chlorine, Bromine and Iodine

The atomic masses are:

Element

Symbol

Atomic Mass

Chlorine

Cl

35.5

Bromine

Br

79.9

Iodine

I

126.9

Sum of atomic masses of first and third element in the triad = (35.5 + 126.9)/2 = 81.2

                                                                                                Atomic mass of second element

Limitations of Döbereiner’s triads

  • He could only find 3 such triads at that time
  • Other elements did not satisfy this rule
  • Thus the system was not found to be useful

 

Here, Average of Atomic Mass of first and third element

 =Average of 14 and 74.9

 =(14+74.9)/2

 =88.9/2

 =44.45

  Atomic Mass of Phosphorus

Hence they do not form a triad.

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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo