Valence

2-8-18-32-32-18-2

Atoms can't just connect in any imaginable way. There are rules. You can break the rules for a little while, but not forever. That's why carbon monoxide is so dangerous. It wants so badly to be in compliance—but to do so, it needs to steal one more oxygen atom. And certain combinations are just impossible. You can't make a molecule with neon in it, because its valence shell is satisfied.

But some things that seem impossible aren't, and some things that seem possible are impossible. The plausiverse is the realm of these plausibilities, the domain where possibility can't be known.