Question about sulfur's valence electrons in sulfate ion!!!

[pup329]

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My book (Kaplan MCAT Gen Chem pg 27) says that for the sulfur atom in the sulfate ion, sulfur's 3s and 3p subshells can only contain 8 of these 12 electrons; the other 4 electrons have entered the sulfur atom's 3d subshell, which in elemental sulfur is empty.

Why did "the other 4 electrons" enter into the 3d subshell INSTEAD of going into the 4s subshell first? it is my understanding that the 4s subshell fills before 3d based on its lower energy level found by adding the principal quantum numbers to their respective azimuthal quantum numbers???

 

[etp123]

There's always exceptions to everything. Quantum numbers are a theoretical model and not a guarantee an element will follow those rules. Sometimes using the d orbital is actually lower energy than an s orbital like you see happen with some transition metals (I was told to think of it like the d orbital having way more space for the electron to roam around in, hence actually making it lower energy). Usually the answer to weird funky exceptions is that somehow that configuration affords a lower energy state, for whatever reason. Sorry, not sure if I was very helpful lol

 

[medium yus]

Valence electron count in compounds