What are the demerits of Mendeleev's periodic table?

Explanation

Demerits of Mendeleev's periodic table:
i. The whole number atomic mass of the elements cobalt (Co) and nickel (Ni) is the same, that is, 59. Therefore, there was an ambiguity regarding the sequence of these elements in the Mendeleev's periodic table.
ii. Isotopes were discovered long time after Mendeleev developed the periodic table. Isotopes have same chemical properties but different atomic masses. Therefore, isotopes could not be given a proper place in Mendeleev's periodic table.
iii. In Mendeleev's periodic table, elements are arranged in an increasing order of atomic masses. Howere, the rise 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.
iv. No fixed position was given to hydrogen in Mendeleev's periodic table because it resembled alkali metals (group I) as well as halogens (group VII).