Concepts

Class 10
Chapter 5 Class 10 - Periodic Classification of Elements

• 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.

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

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

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

• He could only find 3 such triads at that time
• Other elements did not satisfy this rule

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.