State Dobereiner law of triads giving examples.

Explanation

Dobereiner made groups of there elements each, having similar chemical properties and called them triads. He arranged the three elements in a triad in an increasing order of atomic mass and showed that the atomic mass of the middle element was approximately equal to the mean of the atomic masses of the other elements.
Examples : Lithium (Li), Sodium (Na), Potassium (K) from Dobereiner's tried.