Well to begin with, it wouldn't collect 1 atom. The event horizon of the black holes we're talking about would be billions of times smaller than even a proton. If you added the mass of a subatomic particle to it, it would still be just as small, it would take countless numbers of amazingly rare collisions for it to grow large enough to consume even 1 atom. And by countless, I mean it would take longer than the age of the universe.