(I) They did not apply to elements with very low or very high masses.
(ii) Dobereiner\’s triads couldn\’t fit all of the elements.
(iii) As the methods to calculate atomic mass improved, Dobereiner’s triads’ validity began to decrease.
For example, in the triad of F, Cl and Br, the arithmetic mean of atomic masses of F and Br is not equal to the atomic mass of CI.