Unpopular take: I don’t think microfiction illuminates atomic structure of long stories. They’re fun but more closely related to jokes. The essence of long story is compounding accumulation across the atoms.
Realized from the answers that unconsciously I am thinking of serialized atoms. There’s no reason the atoms have to be continuous segments of text. Though I can’t think of a candidate counterexample. Maybe a thread?
I’ve always liked this quote from EM Forster:
“The king died and then the queen died” is a story. But “the king died and then the queen died of grief” is a plot.
(Note in each case you need two events, not one, the atomic unit is the link, not the node)
Character/actor, even if implied. "It was" is probably the shortest story ever, though we see the addition of another factor here, temporality. Robbe-Grillet mucked about with this