Lu vs metallic element

Lu and metallic element both are nouns.

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

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

Lu: a trivalent metallic element of the rare earth group; usually occurs in association 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 Lu include metal.

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