Question

What were the defects and anomalies in Mendeleeff's Periodic Table and how are they resolved by Moseley? 

Answer 1

Anomalies and Defects in Meneleeff’s Periodic Table:

1. The wrong order of atomic masses of some elements could not be explained on the bases of chemical properties cobalt At. mass 58.9 (higher mass) comes first and Nickel (Ni) with lower mass (58.7) comes latter. Argon (Ar) with At. mass (39.9) precedes K (39.1).
2. Position of isotopes i.e. atoms of the same element with similar chemical properties but different At. masses should have been given different places (According to Mendeleeff) but were not placed separately.
3. The correct position could not be assigned to Hydrogen in the periodic table. Hydrogen resembles alkali metals of group I. Also Hydrogen resembles Halogen of group 17.
4. Elements that are chemically similar such as gold and platinum have been placed in separate groups.
5. It does not explain the electronic arrangement of elements.
6. Elements such as copper and silver have no resemblance with alkali metals [Na, Li] but they have been placed together in the first group.
   Moseley arranged the elements in increasing order of atomic number and most of the defects in Mendeleeff’s periodic table disappeared.