Question

why did mendeléev leave some gaps in his periodic table​

Answer 1

Mendeleev leave some gaps in his period table for some undiscovered elements which were not discovered then...

Answer 2

To make sure that elements with similar properties lie in the same group, Mendeleev left some gaps in the periodic table. These gaps were left for subsequent inclusion of the elements not known at that time. The elements were discovered later.

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