rydberg constant vs rydberg

rydberg constant and rydberg both are nouns.

Word	Noun	Adjective	Verb	Adverb
rydberg constant	Yes	No	No	No
rydberg	Yes	No	No	No

As nouns, rydberg constant and rydberg are synonyms defined as:

rydberg constant and rydberg: a wave number characteristic of the wave spectrum of each element

Other synonyms of rydberg constant include rydberg unit.

rydberg constant (noun)	rydberg (noun)
a wave number characteristic of the wave spectrum of each element	a wave number characteristic of the wave spectrum of each element