According to Dobereiner’s law of triads, when elements in the periodic table are arranged in an increasing order of their atomic masses, a group of elements share similar chemical properties. This is known as triads (3 elements together). 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 have atomic masses 40, 88 and 137 and form 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