Give Limitations of Mendeleevs Classification.

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I would like to elaborate the points, all of you have given as follow:

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. 

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.

3. Although Mendeleev arranged the elements in the increasing order of their atomic masses, there were instances where he placed an element with a slightly higher atomic mass before an element with a slightly lower atomic mass. 

For example, cobalt, whose atomic mass is 58.9, was placed before nickel whose atomic mass is 58.7. This was done to maintain consistency in the properties of the elements present in a group i.e., to group the elements with similar properties together.