Ans.
1. Hydrogen resembles alkali metals (group I) as well as halogens (group VII). Therefore, the correct position of hydrogen atom could not be decided.
2. In Mendeleev's periodic table, elements were arranged in an increasing order of atomic masses. But the increase in atomic mass is not uniform. Hence, it was not possible to predict the number of elements that could be discovered between two heavy elements.
3. The whole number of atomic mass of the elements cobalt (Co) and nickel (Ni) is the same. Therefore, it was difficult to interpret the position of these two elements in Mendeleev's periodic table.
4. Isotopes have same chemical properties but different atomic masses. Therefore, it would be difficult to place them in Mendeleev's periodic table.
