lithium vs atomic number 3

lithium and atomic number 3 both are nouns.

Word	Noun	Adjective	Verb	Adverb
lithium	Yes	No	No	No
atomic number 3	Yes	No	No	No

As nouns, lithium and atomic number 3 are synonyms defined as:

lithium and atomic number 3: a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several minerals

Other synonyms of lithium include Li.

lithium (noun)	atomic number 3 (noun)
a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several minerals	a soft silver-white univalent element of the alkali metal group; the lightest metal known; occurs in several minerals