Pu vs element

Pu and element both are nouns.

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

As nouns, element is a hypernym of Pu; that is, 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
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 chemical element.

Pu (noun)	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	an artifact that is one of the individual parts of which a composite entity is made up; especially a part that can be separated from or attached to a system
	an abstract part of something
	the most favorable environment for a plant or animal
	a straight line that generates a cylinder or cone
	the situation in which you are happiest and most effective
	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
	one of four substances thought in ancient and medieval cosmology to constitute the physical universe