What were the drawbacks of Mendeleev's Periodic Table?
Following are the drawbacks or limitations of Mendeleev's periodic table :
1. Hydrogen’s position was not justified in Mendeleev’s periodic table. He positioned hydrogen in the first column above alkali metals. He did so because hydrogen and alkali metals have similar properties. For example, hydrogen reacts with halogens, oxygen, and sulphur to form compounds whose formulae are similar to those of alkali metals.
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Hydrogen and alkali metals reacting with halogens
However, hydrogen also resembles halogens in many ways. Like halogens, hydrogen is a gas, and exists as a diatomic molecule (H2). It forms covalent compounds like halogens unlike alkali metals. Hence, it can also be placed above the halogen group.
Therefore, Mendeleev was not able to explain the position of hydrogen. In other words, the position of hydrogen in Mendeleev’s periodic table was not justified. This was the first limitation of Mendeleev’s periodic table.
2. The discovery of isotopes revealed another limitation of Mendeleev’s periodic table. Since Mendeleev’s periodic table was based on atomic masses of elements, isotopes should be placed in different columns despite the fact that they represent the same element.