What is doberneir triad

According to Dobereiner, all elements occurred in groups of three, when arranged in increasing order of atomic masses. He referred to these groups as triads. In a traid the elements had similar chemical properties.  Traids of the Dobereiners classification: Traid1:

Traid2:

Traid3:

Dobereiner’s law of triads states that, the atomic mass of the middle element of a triad is the arithmetic mean of the atomic masses of the other two elements.   Example: In the triad of lithium, sodium and potassium. The atomic mass of lithium is 7 and  the atomic mass of potassium is 39. The average of masses of lithium and potassium gives atomic mass of sodium 23.  Drawbacks: All the known elements could not be arranged in the form of triads. This law did not hold good for elements with very low or very high atomic mass.  Example: The arithmetic mean of the atomic masses of fluorine 19 and bromine 80, which comes to 49.5, varies significantly from the atomic mass of chlorine, which is 35.5.