State Dobereiner's law of traids giving one example .

Explanation

Dobereiner made groups of three elements each, having similar chemical properties and called them traids. He arranged the three elements in a traid 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 two elements .
Examples: Lithium (Li), Sodium (Na) , Potassium (K) form Dobereiner's traid.