Question

compare the arrangements of mendellev periodic table and modern periodic table

Answer 1

mendellev :
1) elements are arranged based on their atomic mass
2) position of hydrogen was not fixed.
3) there was some anomalous pairs.
4) position for isotopes was not clear.

Modern:
1) elements are arranged based on their atomic numbers.
2) position of hydrogen was in group 1.
3) no anomalous pairs
4) problem of isotopes didn't arise.