Nb vs metallic element

Nb and metallic element both are nouns.

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

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

Nb: a soft grey ductile metallic element used in alloys; occurs in niobite; formerly called columbium
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 Nb include metal.

Nb (noun)	metallic element (noun)
a soft grey ductile metallic element used in alloys; occurs in niobite; formerly called columbium	any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.