Question

Q. Why did Mendeleev leave gaps in his periodic table?

Answer 1

The two criteria used by Mendeleev to classify the elements in his periodic table are :-

★ increasing atomic masses

★ Grouping of elements which exihibited similar properties, placed under the same vertical column ( group ).

Mendeleev left some gaps in his periodic table for the elements which were not known at that time, to ascertain that elements having the same properties fell in the same vertical column or group , without disturbing the arrangements​ of previous elements.

also there was no mention of Noble gases like helium, neon, argon in Mendeleev's preodic table as those gases were not known that time.

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Answer 2

Mendeleev had left some gaps in his periodic table because he had believed that more elements would be discovered later.

Hope it helps!!