10th Class Science Periodic Classification of Elements Dobereiners Classification

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       Dobereiner’s Classification

In the year 1829, Johann Wolfgang Dobereiner, a German scientist, was the first to classify elements into groups based on John Dalton's assertions. He grouped the elements with similar chemical properties into clusters of three, called 'Triads'. The distinctive feature of a triad was the atomic mass of the middle element. When elements were arranged in order of their increasing atomic mass, the atomic mass of the middle element was approximately the arithmetic mean of the other two elements of the triad.

Elements	Atomic mass	Average Atomic Mass
Lithium	7
Berellium	9	9
Potassium	11

               

Elements	Atomic mass	Average Atomic Mass
Carbon	12
Nitrogen	14	14
Oxygen	16

               

Elements	Atomic mass	Average Atomic Mass
Calcium	40
Strontium	87.5	88.1
Barium	11

Elements	Atomic mass	Average Atomic Mass
Chlorine	35
Bromine	80	80.6
Iodine	127

           Drawback of Triad Classification

• A large number of similar elements could not be grouped into triads e.g., iron, manganese, nickel, cobalt, zinc and copper are similar elements but could not be placed in the triads.
• It was possible that quite dissimilar elements could be grouped into triads.
• Dobereiner could only classify 3 triads successfully. Since he failed to arrange the then known elements in the form of triads his attempt at classification was not very successful.