[johntdlemon]

Hi there. I've having a problem concerning the ways to draw Lewis structures.
I'll use SO₄^2- as the example here.
I learnt to draw it this way in this video I found on YouTube:http://youtu.be/8AOw_EWmkGk


However, there seems to be another way to draw the sulfate ion:

Which the uploader of the previously mentioned video said is wrong in the comments

The Wikipedia page on sulfate ion has a section on these different structures:
http://en.wikipedia.org/wiki/Sulfate_ion#Structure_and_bonding
It seems to me that according to Wikipedia, the latter structure is more preferable.

But I also found this forum topic on a similar question:
http://sgforums.com/forums/2297/topics/449090#post_10519630
The one who answered the question said it's simply that some chemist preferred one form, while others preferred another.

I'm also seeing these differences in ClO₄^- and PO₄^3-
I'm hoping if anyone can explain this for me. It's been troubling me for a while.
And I would also be grateful if you can tell me how the second structure is drawn.
Thanks a lot!

[formaldehyde23]

The reason that SO4(2-) has two different structures is the discrepancies between formal charge. One of them is preferable, but that doesn't mean that either structure couldn't exist.
First of all, formal charge is defined by the individual charge of an atom. The most favorable structure is defined by the one that has each individual atom to have the lowest formal charge possible.
Formal charge is defined by the value of the number of valence electrons of the atom in a neutral state minus the number of unbonded electrons - the number of covalent bonds that the atom is attached to.
So, to decide which structure is preferable, look at the formal charges of each atom in each diagram
First diagram: Sulfur has F.C. of 6-(6) = 0. Oxygen with two lone pairs has an F.C. of 6-(2+4) = 0. 
Oxygen with three lone pairs has a formal charge of 6-(6+1) = -1.
For your second diagram, look at the formal charges of each individual atom again.
Sulfur: 6-4 = 2.
Oxygen (all have three lone pairs of electrons): 6-(1+6) = -1.
You can see that the formal charges of the atoms in the first diagram are preferential because they are closest to zero. That means that the ion with the first Lewis Dot Structure is most stable. (The negative charge of the sulfate ion is most spread out).

[Stovn0611]

I think that the second way would be incorrect because it has extra electrons

Sulfur and Oxygen should each have 6 valence electrons then you have 2 extra electrons for the -2 charge so there should be 26 electrons total but in that picture there's 32 electrons

[johntdlemon]

Thank you both for replying! (Sorry, somehow I didn't get a notification in the email.)

Most of the stuff I found online draw sulfate the first way
But I still would like to know if anyone can explain for me this section of Wikipedia's page on sulfate. Does this mean that the 2 different diagrams are drawn based on different theories on the structure of sulfate?:

Linus Pauling used valence bond theory to propose that the most significant resonance canonicals had two π bonds involving d orbitals. His reasoning was that the charge on sulfur was thus reduced, in accordance with his principle of electroneutrality. The double bonding was taken by Pauling to account for the shortness of the S-O bond (149 pm). Pauling's use of d orbitals provoked a debate on the relative importance of π bonding and bond polarity (electrostatic attraction) in causing the shortening of the S-O bond. The outcome was a broad consensus that d orbitals play a role, but are not as significant as Pauling had believed.
Double bonds in the Pauling structure imply a molecular orbital formed from 3d orbitals on sulfur and 2p orbitals on oxygen. A widely accepted description involving pπ - dπ bonding was initially proposed by D.W.J. Cruickshank. In this model, fully occupied p orbitals on oxygen overlap with empty sulfur d orbitals (principally the dz2 and dx2-y2). However, in this description, despite there is some π character to the S-O bonds, the bond has significant ionic character. For sulfuric acid, computational analysis (with natural bond orbitals) confirms a clear positive charge on sulfur (theoretically +2.45) and a low 3d occupancy. Therefore, the representation with four single bonds is the optimal Lewis structure rather than the one with two double bonds (thus the Lewis model, not the Pauling model). In this model, the structure obeys the octet rule and the charge distribution is in agreement with the electronegativity of the atoms. The shorter S-O bonds have a different explanation]. However, the bonding representation of Pauling for sulfate and other main group compounds with oxygen is still a common way of representing the bonding in many textbooks.

Stovn0611:
I don't think there's a problem with the number of electrons in the second diagram
There're 1 sulfur and 4 oxygen -> (1+4)*6 = 30
Then plus 2 extra electrons -> 30+2 = 32

Again, thanks a lot!!

[Stovn0611]

Oh my bad, I meant to say that the first* diagram is wrong because it doesn't have the right number of electrons - although I think they can fill in electrons to get the full octets for each atom by using up the electrons they don't have yet(something more like this - http://media.web.britannica.com/eb-media/45/16145-004-7D531508.gif)

To address the question about the 2 different structures of sulfate, there can be lots of structures for the same molecule, known as resonance structures, and you can determine which one is more favorable by checking formal charges on each one. Like formaldehyde23 said earlier, the molecule with formal charges closest to zero are the most preferable form

EDIT: I just noticed in the first diagram they also put 5 electron pairs around the top oxygen in the second resonance structure so the first diagram they didn't really draw it correctly

[johntdlemon]

  Sorry, the faulty diagram would be my bad (I drew it. Forgot to copy the lone pairs before moving them)


I thought resonance is something like this:(diagram from Wikipedia)

Caption:Six resonances

Even though the structure is a little bit different, the numbers of different types of bonds don't change.



But this is the structural difference I'm asking about:(diagram again from Wikipedia)

Caption:Two models of the sulfate ion.1 with polar covalent bonds only; 2 with an ionic bond

There is simply no double bonds in them, and after reading the Wikipedia article, I'm wondering if it's a result of a different theory on how it's structured.

Thanks