1. What are the 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.