Pu vs chemical element

Pu and chemical element both are nouns.

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

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

Pu: a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239
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 Pu include element.

Pu (noun)	chemical element (noun)
a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239	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