series vs helium group

series and helium group both are nouns.

Word	Noun	Adjective	Verb	Adverb
series	Yes	No	No	No
helium group	Yes	No	No	No

As nouns, helium group is a hyponym of series; that is, helium group is a word with a more specific, narrower meaning than series:

series (noun)	helium group (noun)
(mathematics) the sum of a finite or infinite sequence of expressions	the series of inert gases
a periodical that appears at scheduled times
a serialized set of programs
(sports) several contests played successively by the same teams
similar things placed in order or happening one after another
a group of postage stamps having a common theme or a group of coins or currency selected as a group for study or collection
(electronics) connection of components in such a manner that current flows first through one and then through the other