Shape of a sulphate ion

Hi,

Please could someone help me workout how to correctly work out and draw the shape of a sulphate ion including bond angles.

I started off thinking that the group is 6, the bonds formed are 4 because there are 4 oxygen atoms and then I added a 2 for the charge. The total of this gives me 12 which I halved to give me 6 including a total of 4 bonding pairs and 2 lone pairs although I’m not sure I’ve done this right so any help would be much appreciated.

Thanks :smile:

Reply 1

EierVonSatan

21

Original post by Redandblue678

Hi,

Please could someone help me workout how to correctly work out and draw the shape of a sulphate ion including bond angles.

I started off thinking that the group is 6, the bonds formed are 4 because there are 4 oxygen atoms and then I added a 2 for the charge. The total of this gives me 12 which I halved to give me 6 including a total of 4 bonding pairs and 2 lone pairs although I’m not sure I’ve done this right so any help would be much appreciated.

Thanks :smile:

The rule that you are using there is pretty limited in scope, to be honest. I don't reccommend you use it :no:

Neutral S can form a maximum of 6 bonds (6 outer electrons to pair up), neutral O can form a maximum of 2 bonds. Then there are two electrons left over. Therefore 4 oxygens want to gain 8 electrons => 6 from sulfur and 2 from the 'extra' 2 electrons. Does that help?

Reply 2

Redandblue678

OP

12

Original post by EierVonSatan

The rule that you are using there is pretty limited in scope, to be honest. I don't reccommend you use it :no:

Neutral S can form a maximum of 6 bonds (6 outer electrons to pair up), neutral O can form a maximum of 2 bonds. Then there are two electrons left over. Therefore 4 oxygens want to gain 8 electrons => 6 from sulfur and 2 from the 'extra' 2 electrons. Does that help?

Hi,

This is the only rule I’ve actually ever been introduced to.

I’m also really struggling to follow. I get the bit about how the neutral S can form a maximum of 6 bonds but then you lose me when you start talking about the oxygen. Where are you getting the 2 from?

Reply 3

EierVonSatan

21

Original post by Redandblue678

Hi,

This is the only rule I’ve actually ever been introduced to.

I’m also really struggling to follow. I get the bit about how the neutral S can form a maximum of 6 bonds but then you lose me when you start talking about the oxygen. Where are you getting the 2 from?

Oxygen wants 8 electrons in its outer shell, it has 6 at the moment, so wants to accept two more - so two bonds :smile:

Reply 4

Redandblue678

OP

12

I’m still lost. I can’t seem to visualise it. Am I wrong to think that it forms a square planar shape with a 90 degree bond angle?

Reply 5

EierVonSatan

21

Original post by Redandblue678

I’m still lost. I can’t seem to visualise it. Am I wrong to think that it forms a square planar shape with a 90 degree bond angle?

No.

Try this - if sulfur is connected to 4 oxygen atoms, how many double bonds must there be?

Reply 6

Redandblue678

OP

12

4 pairs of double bonds?

Reply 7

EierVonSatan

21

Original post by Redandblue678

4 pairs of double bonds?

S has 6 electrons; 4 double bonds would need 8 electrons.

It has 2 double bonds and 2 single bonds using up all 6 of the electrons - this means there is no lone pairs on the sulfur. Happy with this?

That means you've got 4 areas of bonding pairs around the sulfur. Do you know what shape this makes?

Original post by Redandblue678

Hi,

Please could someone help me workout how to correctly work out and draw the shape of a sulphate ion including bond angles.

I started off thinking that the group is 6, the bonds formed are 4 because there are 4 oxygen atoms and then I added a 2 for the charge. The total of this gives me 12 which I halved to give me 6 including a total of 4 bonding pairs and 2 lone pairs although I’m not sure I’ve done this right so any help would be much appreciated.

Thanks :smile:

Shapes of molecules and ions is an empirical area of chemistry where the theories are shoe-horned to fit in with the experimental data.

The shapes and bond angles of the molecules and ions of period 2 elements fit very well with the hybridisation model.
The shapes of the molecules and ions in period 3 elements demonstrate that the valence shell can be expanded or extended when highly electronegative elements are involved. This allows for coordination numbers higher than 4.

The best approach when trying to decide on a structural shape is to draw out possible Lewis (electron dot-cross) arrangements. With compounds or ions containing oxygen there is also the possibility of dative coordinate bonding to fill the outer shell.

If we have a look at SO₂ there are two possible arrangements. One in which both oxygen atoms share two pairs of electrons and the sulfur atom has an expanded octet.
There is also a structure in which one sulfur-oxygen bond is dative and the other is a double bond. This would present resonance (delocalisation) and both bonds would have a bond order of 1.5.

So how do we decide on the "correct" structure?
We look at the empirical evidence for bond length and strength and the bond angle.
The bond length for both S-O bonds is shorter than single but longer than double. It seems that our model "fits".

If we extend this argument to the sulfate ion, we can see that there are two possible structures that can be drawn.
One has two double bonds between S=O and two single bonds between S-O. This requires octet expansion of the sulfur valence shell to 12 electrons.
The other structure has four dative coordinate bonds between sulfur and oxygen. This does not require octet expansion and it has the advantage of simplicity (Occam's razor)

The empirical evidence shows identical bond lengths between double and single S-O bond lengths. This sways our opinion towards the first structure, however, the identical nature of all of the bonds shows that delocalisation (resonance) would produce a bond order of 1.5.

Both "answers" will lead to an identical result in that we have four identical bonds and hence four electron domains around the central sulfur atom. These will arrange in the form of a perfect tetrahedral arrangement.

Reply 9

Pigster

20

Original post by Redandblue678

Thanks

http://www.kentchemistry.com/links/bonding/lewisdotstruct.htm

You probably won't need to know about formal charge, but it is easy to get your head around.

Shape of a sulphate ion

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