Question

In Mendeleev's periodc table,why was there no mention of noble gases like helium,neon and argon?

Answer 1

Mendeleev would have no clear reason to suspect there were missing elements when they can be separated by multiple units. But he did notice trends, and was able to predict some holes based on those trends. The entire column of noble gases, though, could be missed because it was a periodic gap, not a gap in the period.

Answer 2

Gases like He, Ne, Ar etc., are inert gases and don't react with other atmospheric gases and at that time no such technology was invented which could detect such inert and reactive gases and were simply left unknown. That's why, these gases weren't there in Mendeleev's periodic table.