Solomon Islands vs Bougainville Island

Solomon Islands and Bougainville Island both are nouns.

Word	Noun	Adjective	Verb	Adverb
Solomon Islands	Yes	No	No	No
Bougainville Island	Yes	No	No	No

Solomon Islands (noun)	Bougainville Island (noun)
the southern Solomon Islands that since 1978 form an independent state in the British Commonwealth	the largest of the Solomon Islands; a province of Papua New Guinea
the northernmost islands are part of Papua New Guinea; the remainder form an independent state within the British Commonwealth