Er vs atomic number 68

Er and atomic number 68 both are nouns.

Word	Noun	Adjective	Verb	Adverb
Er	Yes	No	No	No
atomic number 68	Yes	No	No	No

As nouns, Er and atomic number 68 are synonyms defined as:

Er and atomic number 68: a trivalent metallic element of the rare earth group; occurs with yttrium

Other synonyms of Er include erbium.

Er (noun)	atomic number 68 (noun)
a trivalent metallic element of the rare earth group; occurs with yttrium	a trivalent metallic element of the rare earth group; occurs with yttrium