Answer:
1) Mendeleev's periodic table grouped elements on the basis of their atomic masses, while the modern periodic table groups elements on the basis of atomic number.
2) As we know, the isotopes of an element will have the same atomic number. This is because there number of protons will be same. Hence, in the modern periodic table the isotopes of an element share the same place.
3) Cobalt is places on the 9th position and nickel on the 10th position. There were arguments before as the atomic mass of nickel was more than cobalt.
4) A position for the hydrogen atom was finally allocated in the modern periodic table which had been a issue in Mendeleev's periodic table. Hydrogen is placed on top of the alkali metals i.e the first group.
