What are the merits and demerits …

Merits are as follows:-
 i. There was a regular gradation in physical and chemical properties of elements.
ii. The group number of an element indicates highest oxidation state that it can attain.
iii. There were many vacant spaces in Mendeleev's periodic table for the elements to be discovered. He  named them Eka-Boron, Eka-Aluminium and Eka-silicone He also predicted the properties of these  undiscovered elements including atomic masses. These elements were discovered as Sc ,Ga and Ge  with same features as he predicted.
iv. Mendeleev’s arrangement helped to correct atomic masses of a number of elements.

Demerits of Mendeleev’s periodic table
i. Mendeleev could not assign a correct position to hydrogen in the periodic table as hydrogen resembles alkali metals as well as halogens.
ii. Isotopes had to be given the same position in the periodic table though isotopes of the same elements have different atomic masses.
iii. It was observed that at certain places, an element of higher atomic mass Cobalt (Co=58.93) has been placed before an element of lower atomic mass Nickel (Ni = 58.71).
iv. Some elements placed in the same sub - group varies in their properties. Manganese is placed with halogens which totally differ in the properties.