chromium vs metallic element

chromium and metallic element both are nouns.

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

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

chromium: a hard brittle multivalent metallic element; resistant to corrosion and tarnishing
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 chromium include metal.

chromium (noun)	metallic element (noun)
a hard brittle multivalent metallic element; resistant to corrosion and tarnishing	any of several chemical elements that are usually shiny solids that conduct heat or electricity and can be formed into sheets etc.