Question

Write two defects of mendeleevs periodic table how these defects were removed in modern periodic table?

Answer 1

1)POSITION OF ISOTOPES
Isotopes of an element have similar chemical properties but different atomic masses. As the isotopes have similar chemical properties and hence occupy same group. But on the basis of atomic masses they should occupy different groups. So, Mendeleef could not explain the positions of isotopes.
2) POSITION OF LANTHANIDES & ACTINIDES
Both these families have fourteen elements each, but they could not find proper positions in the main set up of the Mendeleef’s periodic table. Instead, these elements have been placed in two separate rows at the bottom of periodic table without assigning a proper reason.