chance event vs occurrence

chance event and occurrence both are nouns.

Word	Noun	Adjective	Verb	Adverb
chance event	Yes	No	No	No
occurrence	Yes	No	No	No

As nouns, occurrence is a hypernym of chance event; that is, occurrence is a word with a broader meaning than chance event:

chance event: anything that happens suddenly or by chance without an apparent cause
occurrence: an event that happens

Other hypernyms of chance event include happening, natural event, occurrent.

chance event (noun)	occurrence (noun)
anything that happens suddenly or by chance without an apparent cause	an event that happens
	an instance of something occurring