The main features of Mendeleev’s periodic table are:
- Elements were arranged in order of their increasing atomic weights in horizontal rows called periods.
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- Elements were arranged in vertical columns called groups according to their resemblance in properties.
- The periodic table had 8 groups and 7 periods.
- Since only 63 elements were known at that time, he had left gaps for undiscovered elements.
Some discrepancies of Mendeleev’s periodic table are as follows:
- Position of hydrogen: Hydrogen is placed in group 1A. It resembles the elements of group IA as well as elements of group VII A. Thus, position of hydrogen in the periodic table is not clear.
- Position of isotopes: On the basis of atomic mass, various isotopes of the same element should be assigned different places in the periodic table. However, no separate place was assigned to the isotopes.
- Similar elements placed separately: certain chemically similar elements have been placed in different groups, e.g. similar elements such as Cu and Hg have been placed in different groups.
- Dissimilar elements placed together: curtain elements having very few similarities with other elements were grouped together, e. g. Mn has been placed in group VII.