What are the defects of Mendeleev’s periodic table?

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Detailed Solution

The defects of Mendeleev’s periodic table include irregular atomic mass order, uncertain position of hydrogen, isotopes not explained, and incorrect groupings of some elements. Although Mendeleev’s periodic table was a great achievement, it had several limitations. With further discoveries in chemistry, these defects became clear.

Main Defects

Position of Hydrogen: Hydrogen showed properties of both Group 1 (alkali metals) and Group 17 (halogens), so its position was uncertain.
Isotopes: Isotopes of the same element have different atomic masses, but they could not be placed properly in his table.
Order of Atomic Mass: Some elements had to be placed against increasing atomic mass order to match properties (e.g., cobalt and nickel).
Grouping Issues: Elements with different properties were placed in the same group in some cases.
Atomic Number Not Considered: The table was based on atomic mass, not atomic number, which was later found to be more accurate.

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