(a) Why did Mendeleev leave some gaps in his periodic table? (b) In Mendeleev’s periodic table, why was there no mention of noble gases like helium, neon and argon?

(a) In order to make sure that the elements having similar properties lie in the same vertical column (group), Mendeleev left some gaps in his periodic table.

(b) The noble gases were not discovered by that time. So there was no mention of noble gases like helium, neon and argon in Mendeleev’s periodic table.