Answer 1

Demerits of Mendeleev’s periodic table:

• Hydrogen resembles alkali metals and halogens. So, a correct position could not be assigned to hydrogen in the periodic table.
• The position of isotopes could not be explained. Isotopes are atoms of the same element with similar chemical properties but different atomic masses. If the elements are arranged according to atomic masses, the isotopes should be placed in different groups of the periodic table.
• At certain places, an element of higher atomic mass has been placed before an element of lower atomic mass. Example: Cobalt (Co = 58.93) is placed before nickel (Ni = 58.71).