Drawback of Dobereiner\’s triads:\tDobereiner could identify only three triads. He was not able to prepare triads of all the known elements\tAll the known elements could not be arranged in the form of triads.\tFor very low mass or for very high mass elements, the law was not holding good. Take the example of F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.
Law of Triads:\xa0When elements are arranged in the order of their increasing atomic masses, the atomic mass of the middle element was approximately the mean of the atomic masses of the other two elements.For example: Consider the triad of lithium, sodium and potassium. The atomic mass of sodium is the mean of the atomic masses of lithium and potassium.