Question

Why is modern periodic table considered to be an improvement over mendeleev's table?

Answer 1

Because Mendeleev's Periodic Table was based on atomic mass and not on atomic number so it was not possible to display isotopes in his periodic table.

He grouped the elements on the basis of the compounds they formed with oxygen and water. But hydrogen formed both bonds as those of alkali metals as well as akin those of halogens. Therefore the position of hydrogen was questionable.