Question

why was the basis of classification of elements changed from the atomic mass to the atomic number

Answer 1

see this could be because there were many isotopes i.e., elements having same atomic number but different atomic mass for eg is attached


so these could not be accomodated in the same periodic table

Answer 2

Position of isotopes according to at mass it should have different places but according to at no. No separate places are needed because they have same at no.