serial vs in series

serial is a noun but in series is not a noun.

Word	Noun	Adjective	Verb	Adverb
serial	Yes	Yes	No	No
in series	No	Yes	No	No

As adjectives, serial and in series are synonyms defined as:

serial and in series: of or relating to the sequential performance of multiple operations

Other synonyms of serial include nonparallel.

serial (noun)	in series (noun)
a serialized set of programs
a periodical that appears at scheduled times

serial (adjective)	in series (adjective)
pertaining to or occurring in or producing a series	of or relating to the sequential performance of multiple operations
pertaining to or composed in serial technique
of or relating to the sequential performance of multiple operations
in regular succession without gaps