atomic number 18 vs noble gas

atomic number 18 and noble gas both are nouns.

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

As nouns, noble gas is a hypernym of atomic number 18; that is, noble 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
noble gas: any of the chemically inert gaseous elements of the helium group in the periodic table

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

atomic number 18 (noun)	noble 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