atomic number 18 vs inert gas

atomic number 18 and inert gas both are nouns.

Word	Noun	Adjective	Verb	Adverb
atomic number 18	Yes	No	No	No
inert gas	Yes	No	No	No

As nouns, inert gas is a hypernym of atomic number 18; that is, inert gas is a word with a broader meaning than atomic number 18:

atomic number 18: a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere
inert gas: any of the chemically inert gaseous elements of the helium group in the periodic table

Other hypernyms of atomic number 18 include argonon, noble gas.

atomic number 18 (noun)	inert gas (noun)
a colorless and odorless inert gas; one of the six inert gases; comprises approximately 1% of the earth's atmosphere	any of the chemically inert gaseous elements of the helium group in the periodic table