State the defects of the table proposed by Mendeleev.

Defects of table proposed by Mendeleev:

1. Anomalous pairs — The following elements did not follow Mendeleev's principles:
   1. Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1
   2. Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6
   3. Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9
2. Position of isotopes — According to Mendeleev's periodic table isotopes should be given separate place in the table since they have different atomic mass. But they were not assigned separate places.
3. Grouping of chemically dissimilar elements — Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium etc.) but they have been placed together in the first group.
4. Separation of chemically similar elements — Elements that are chemically similar, such as gold and platinum have been placed in separate groups.
5. Electron arrangement — It does not explain the electron arrangement of elements.
6. Position of hydrogen — Hydrogen was not given a fixed position. It was considered in group IA as well as in group VIIA because it forms both a positive ion, viz. in HCl and a negative ion, viz. in NaH.