Er vs metallic element

Er and metallic element both are nouns.

Word	Noun	Adjective	Verb	Adverb
Er	Yes	No	No	No
metallic element	Yes	No	No	No

As nouns, metallic element is a hypernym of Er; that is, metallic element is a word with a broader meaning than Er:

Er: a trivalent metallic element of the rare earth group; occurs with yttrium
metallic element: any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.

Other hypernyms of Er include metal.

Er (noun)	metallic element (noun)
a trivalent metallic element of the rare earth group; occurs with yttrium	any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.