Question

why are nobel gases grouped together on the periodic table?

Answer 1

This is because, for the main reason that these elements that are mostly monatomic, are unreactive, they do not react, or in other words, exchange, and or share valence electrons through chemical bonds, as they already posses the maximum number of valence electrons in their valence shell, which is 8.

Answer 2

Answer:  

Nobel gases also referred as inert gases, they are grouped together because they are the most stable elements. This explains why they are nonreactive. They do not form chemical bonds because they only have a little tendency to either gain or lose an electron, since they have 8 electrons in the outer shell (except for helium as it has 2 electrons in the outer shell). As a result, they do not react with any other element in the periodic table.