How far can an atom expand?

[Isaac Wen]

So I know that elements from period 3 and beyond can have expanded octets, but how expanded can they be? Does it have to do with how many electrons are already in their d-orbital?

[Yuelai Feng 3E]

Hi! Theoretically speaking the max. # of valence electrons is the max. "capacity" of the outermost electron shell. For example, Period 3 elements can have a maximum of 18 valence electrons. Because the third electron shell can hold 18 e- (2s+6p+10d). For Period 4 elements it should be 2s+6p+10d+14f=32 e-.
So it doesn't matter how many electrons there are in their d-orbital already, it only depends on the period it's in.
But it doesn't mean that the more expanded the atom, the more stable it is. The stability of structures still depends on their formal charge.
Hope it helps!!