Overscrupulous chemistry major here. Both Harry and Snape are wrong. By the Pauli exclusion principle an orbital can only host two electrons. But at the same time, there is no outermost orbital - valence shells are only oversimplified description of atom. Actually, so oversimplified that no one should bother writing it down. Speaking of HOMOs of carbon atom (highest [in energy] occupied molecular orbitals), each has only one electron.
The notion that (neutral) Carbon has 4 electrons to share and prefers to have 4 electrons shared with it is so oversimplified that no one should bother writing it down?
That is, umm, a surprising viewpoint to me.
Just a quick question.
Harry James Potter Evans-Verres and the Methods of Rationality: Chapter 18
HJPEV: So, Professor, can you tell me how many electrons are in the outermost orbital of a carbon atom?"
Severus's smile widened. "Four," he said. "It is a useless fact which no one should bother writing down, however."
Did Snape really know that, or did he read Harry's mind?
As to the possibility, this passage, "Harry stared into Severus's cold gaze and remembered that the Sorting Hat had warned him not to meet anyone's eyes while thinking about - Harry dropped his gaze to Severus's desk." occurs after that already quoted.
(If I do something wrong here, which merits a downvote, would you mind specifying the error in question?)