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Give the reasons for dividing Mendeleev’s group into two sub groups.

Answer
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Hint:We know that Mendeleev’s periodic law states that the elements properties are the periodic function of the atomic masses of elements. Mendeleev grouped sixty three elements based on the increasing order of their relative atomic mass.

Complete step by step answer:
-As we know that the Mendeleev grouped sixty three elements based on the increasing order of their relative atomic mass. We call this as Mendeleev’s periodic table. He separated the table into eight columns and seven rows. The columns were called as groups and the rows were known as periods.
Explanation:
-The normal elements were located in Groups I to VII, transition elements were present in Group VIII.
Groups I to VII have been classified to two sub-groups, whereas three elements were meant to be Group VIII.
-Periods from 4th to 7th were divided into first series and second series.
-Elements showing similar properties were located in the same group. Example: Lithium, rubidium, potassium were present in the first group.
-The reason why groups in the Mendeleev periodic table were parted into two subgroups having elements possessing the same physical property and chemical properties and they are given the labels as A and B. -The element’s properties of subgroups look alike to each other more markedly than the properties of those elements of the two-sub group groups.

Note:
We have to remember that the merits of the Mendeleev periodic table is that some gaps were left for the elements that are to be discovered. So, if a certain new element is discovered, it can be placed in a new group without disturbing any present group. Some of the demerits are that the position of hydrogen was missing. The increase in atomic mass was not systematic when moving from one element to another. Isotopes violated the Mendeleev’s periodic law.