I think Dr Lavelle said in class that the extra 4 valence electrons of sulfur come from the atom's subshell 3d, but the configuration of sulfur atom is 1s2 2s2 2p6 3s2 3p4.
So I wonder where those electrons come from, and how can we know the number of valence electrons of sulfur is 10 not 6 in the case of SO4 2-.
Additionally, if such a problem appears in the test, how can we know that in some cases, the number of valence e- of the atom is not the regular one?
Where do the extra valence e- come from in S of SO4 2-
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Re: Where do the extra valence e- come from in S of SO4 2-
The 4 extra valence atoms are not specifically from the sulfur atom's 3d orbital - rather, the 3d orbital is able to accommodate extra electrons being placed into them. The extra electrons in the case of the sulfate ion would come from the oxygens bonded to the sulfur atom.
The way you would determine whether an atom has more valence electrons than it naturally does would be to draw out the Lewis structure and redistribute the valence electrons of all atoms until a structure that is satisfactory is achieved.
The way you would determine whether an atom has more valence electrons than it naturally does would be to draw out the Lewis structure and redistribute the valence electrons of all atoms until a structure that is satisfactory is achieved.
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Re: Where do the extra valence e- come from in S of SO4 2-
To look out for exceptions to the octet rule in the form of expanded valence shells, remember that nonmetals in periods 3 and above can hold more than 8 electrons using their d orbitals; to remember this, it helps to recognize that the first d orbital is in the n=3 shell. Thus, elements in periods 1 and 2 (n=1 and n=2) do not have the capacity to accept more than an octet due to not utilizing a d orbital, while elements in periods 3 and above (n=3 or greater) do. Additionally, I find that when drawing Lewis structures, it helps to pay attention to the formal charges of the individual atoms in order to figure out how many double/triple bonds there are. After all bound atoms aside from the central atom have been given an octet, then assign the leftover electrons to the central atom, and if it's in period 3 or above, don't be afraid to give it more than 8!
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