Question

1. why was it necessary to change the basis of classification from atomic mass to atomic number?
2. find the atomic mass of first element if three element in dobereiner triad in which the atomic mass of the middle element is 23 and last element is 29.

Answer 1

it is because if the representation would have been on the atomic masses then the isotopes would have given a separate place and thus the means of classification was just a joke ..
so , atomic number was chosen as the basis