Döbereiner’s Triads

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:

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:

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:

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.