Valence electron count in compounds

[DrMattOglesby]

hi guys,
i just ran across a problem asking me to identify the valence electrons on a sulfur atom in a sulfate ion.

my understanding is that sulfate ion = 4 oxygen + 1 sulfur atom and has a 2- charge.

now, in the answer to this question, it reads:

sulfur in a sulfate ion has 12 valence electrons, its original six...
*which i was able to find; 1s2,2s2,2p6,3s2,3p4

plus 6 more from the oxygens to which it is bonded.
*i followed this only after looking at the molecular structure here
**so now my electron distribution looks like this:
1s2,2s2,2p6,3s2,3p6 .... but then it goes on to say:

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.

**so my question is why did they 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.

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[SketchLazy]

hi guys,
i just ran across a problem asking me to identify the valence electrons on a sulfur atom in a sulfate ion.

my understanding is that sulfate ion = 4 oxygen + 1 sulfur atom and has a 2- charge.

now, in the answer to this question, it reads:

sulfur in a sulfate ion has 12 valence electrons, its original six...
*which i was able to find; 1s2,2s2,2p6,3s2,3p4

plus 6 more from the oxygens to which it is bonded.
*i followed this only after looking at the molecular structure here
**so now my electron distribution looks like this:
1s2,2s2,2p6,3s2,3p6 .... but then it goes on to say:

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.

**so my question is why did they 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.

Ok using filling order for the sulfur atom you get 6 valence electrons:

3s2 and 3p4

With the 4 oxygen atoms, that makes 12 valence electrons altogether.

Since you have sulfur connected to four oxygen atoms, you have sp3 orbital hybridization making four sigma bonds taking 8 electrons:

|| || || ||
sp3 sp3 sp3 sp3

Now the two other bonds have to be ∏ bonds. S orbitals can not make pi bonds so even though 4s orbitals are lower in energy, they can't be used to make the double bonds between the oxygen and sulfur because the 4 sp3 hybrids are in the way. This is why the D orbitals are used, they have a spatial configuration similar to P orbitals so they can be used to overlap with the p orbitals from the oxygen atoms to make a ∏ bond. So you have two ∏ bonds using up the last 4 electrons.

I don't think you will be expected to know anything this complicated for the MCAT.

 

[DrMattOglesby]

Ok using filling order for the sulfur atom you get 6 valence electrons:

3s2 and 3p4

With the 4 oxygen atoms, that makes 12 valence electrons altogether.

Since you have sulfur connected to four oxygen atoms, you have sp3 orbital hybridization making four sigma bonds taking 8 electrons:

|| || || ||
sp3 sp3 sp3 sp3

Now the two other bonds have to be ∏ bonds. S orbitals can not make pi bonds so even though 4s orbitals are lower in energy, they can't be used to make the double bonds between the oxygen and sulfur because the 4 sp3 hybrids are in the way. This is why the D orbitals are used, they have a spatial configuration similar to P orbitals so they can be used to overlap with the p orbitals from the oxygen atoms to make a ∏ bond. So you have two ∏ bonds using up the last 4 electrons.

I don't think you will be expected to know anything this complicated for the MCAT.

humph!
yeah, that did take some special train of thought!
good job on kicking that question's butt though~AND you made perfect sense too, im proud to say i followed all of your steps and saw the logic to each of them.
it wouldnt seem like the MCAT should ask that question, but as premeds, we definitely do have that knowledge =]
thanks for your help!!!