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For elements that can have an expanded octet, how can we tell when the atom can no longer take on any more valence electrons? Would the max number of valence electrons an atom with an expanded octet can have equal the 8 valence electrons in the s and p orbitals, plus the amount of electrons in a d orbital (10)? so would the max number of valence electrons an expanded octet can have just be 18 (10+8)?
For elements that can have an expanded octet, usually the max number of valence electrons the central atom can take on is the amount of its outer valence electrons. For example, in PCl5, phosphorus has 5 valence electrons, so usually, it can bond with at most 5 other atoms. Again, in XeF6, xenon has 8 valence electrons, so when xenon bonds with 6 fluorine atoms, xenon has a lone pair left over, showing a total of 8 valence electrons (two in the lone pair, 6 used in bonding).
Well, usually the most stable structure would be the one with zero formal charge on every atom, which means the central atom would have the same number of electrons closest to it as the number of valence electrons. For example, SF6 can make 6 bonds because it has 6 valence electrons. XeF2 makes 2 bonds(contributes one electron for each bond) and has 3 lone pairs, amounting to 8 electrons. Otherwise, if it were to make more or less, then there would be a formal charge not zero on a certain atom, possibly the central atom when bounded to atoms like halogens. For example, SF5+ would have Sulfur with 5 bonds, making it's formal charge +1, so usually if the central atom deviates from its number of valence electrons, some kind of atom in the molecule has a formal charge other than 0. At least, that's how I think about it.
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