The specific usage I'm thinking of is something like:
"He exerted a single ____ of effort to..." (blah blah blah).
The word I'm thinking of is "modicum" but after looking up the definition, that apparently just means "small quantity." I'm looking for a single word describing the smallest amount - like modicum - but also implies "indivisible" or something like that. Preferably a noun, so it fits the usage above without rewriting.
Am I crazy, or does a word like this exist? I swear a word like this exists.