Snape's knowledge of valence shells

You might want to post this on the hpmor subredit page instead - or in the latest open thread. I don't, however, think that a top-level discussion post is necessary for this.

In any case, Snape saying that the number of valence electrons of carbon is a meaningless fact is weak evidence that he didn't read it in Harry's mind.

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.

It's at least plausible that Snape, as a potions expert who grew up with muggles, thought there might be some connection between potions and chemistry and learned the basics of chemistry.

It's possible that he read it from Harry's mind. Snape is a powerful legilimens, does not believe that anything is cheating when he has to win, Harry had no protection or even deep knowledge of the technique, had to have been thinking of the answer, and hadn't thought to avert his gaze until later.

That said, Severus is more in tune with the Muggle world than most others in the setting, and in Chapter 61 we see Dumbledore treat him as an expert source on muggle technology :

"Severus?" the old wizard said. "What was it actually?" "A rocket," said the half-blood Potions Master, who had grown up in the Muggle town of Spinner's End. "One of the most impressive Muggle technologies."

Later in Chapter 18, we also see the phrase "Common sense is often mistaken for legilimency." Especially as a potions master and someone who retained an interest in muggle chemistry into adulthood, Snape does have more reason than most wizards to know this information. Harry also tends to overestimate both his knowledge, and both he and Quirrel tend to assume the victories of others come from innate differences rather than simple changes in planning or knowledge.

On the other hand, had Snape known that information, it would also mean he could have ended the world accidentally. Dunno what to make of that.