6. List three drawbacks of Mendeleev’s Periodic table.
Answers
Answer:
Position of hydrogen: Hydrogen is placed in group I. ...
Anomalous pairs: In certain pairs of elements, the increasing order of atomic masses was not obeyed. ...
Position of isotopes: Isotopes are the atoms of the same element having different atomic masses but same atomic number.
Answer:
1. Position of hydrogen:- hydrogen resembles both, alkali metals (IA) and the halogens (VIIA) in properties so Mendeleev could not decide where to place it.
2. Position of Isotopes:- As atomic weight of Isotopes differ, they should have been placed in different position in Mendeleev's periodic table. But there was no such places for Isotopes in his table.
3. Cause of periodicity:- Why physical and chemical properties repeated in a group.
4. Anamolous pairs of elements:- There were some pair of elements which did not follow increasing order of atomic weights.
Example: Ar&K, Te&I, Co&Ni, Th&Pa
5. Gaps in his period table:- Mendeleev left several gaps in his periodic table, proposing that some of the elements were still undiscovered
Example: Eka Aluminium - Gallium(Ga), Eka Silicon - Germanium(Ge), Eka Boron - Scandium(Sc), Eka Mangenese - Technetium(Tc).
6. Like elements were placed in different groups.
Example: Platinum (Pt) and Gold(Au) have similar properties but were placed in different groups Pt(VIII) and Au(IB).
7. Unlike elements were placed in same groups.
Example: Cu, Ag and Au were placed in Ist group with Na, K etc while they differ in their properties.
8. It was not clear that lanthanides and Actinides are related with IIIA group or IIIB group.