Question

give mentally periodic law
why he left some gaps in periodic table

Answer 1

Mendeleev's periodic law states that "the physical and chemical properties of all elements the periodic function of their atomic masses".
In order to make sure that the elements having similar properties fell in the same vertical column or group, he left some vacant places for elements which were yet to be discovered though he could predict their properties.

Answer 2

Mendeleev leave some gaps as he was assure that there are some elements which are not discover at that time. when the was discovered they are easily fitted into the table and this is also a merits of Mendeleev periodic table.

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