What is Dobereiner's law of triads? Explain with the help of one example of a Dobereiner's triad.

According to Dobereiner’s Law of triads, when elements are arranged in increasing order of their atomic masses, a group of elements with similar chemical properties is obtained. This is called triads. The atomic mass of the middle element is equal to the arithmetic mean of the atomic masses of the other two elements.

For example: Calcium, strontium and barium has atomic masses 40,088 and 137 forms a triad and have similar chemical properties. The atomic mass of strontium is approximately equal to the average of the atomic masses of calcium and barium.

Arithmetic mean of atomic masses of calcium and barium = 40+137/2 = 88.5