selenium vs chemical element

selenium and chemical element both are nouns.

Word	Noun	Adjective	Verb	Adverb
selenium	Yes	No	No	No
chemical element	Yes	No	No	No

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

selenium: a toxic nonmetallic element related to sulfur and tellurium; occurs in several allotropic forms; a stable grey metallike allotrope conducts electricity better in the light than in the dark and is used in photocells; occurs in sulfide ores (as pyrite)
chemical element: any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter

Other hypernyms of selenium include element, antioxidant.

selenium (noun)	chemical element (noun)
a toxic nonmetallic element related to sulfur and tellurium; occurs in several allotropic forms; a stable grey metallike allotrope conducts electricity better in the light than in the dark and is used in photocells; occurs in sulfide ores (as pyrite)	any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter