Shapes - Chemistry Watch

Sloshy
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Hi, I am struggling to understand how to deduce the shape and angle of a molecule that does not follow the standard bonding pairs and lone pairs rule for example Tetrahedral is 109.5, 4 bonding 0 lone pairs.

When faced with a molecule like S02 I would expect it to have a bond angle of 104.5 since it has 2 bonding pairs and 2 lone pairs around the central atom sulfur . However this is not the case it has a bond angle of 120 but I’m unsure this is determined.
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splitter2017
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OK so a sulfur atom has 6 electrons in its outer shell. When bonding in SO2, a dative covalent bond forms between S and one O. The other O is bonded to S by a double covalent bond. This leaves one lone pair.

If you add up the areas of electron density it equates to 3. These repel and spread out as far apart as possible to give 120 degrees bond angles (in an exam).
In reality, the lone pair will repel more strongly than bonding pairs and the angle will be ~ 119 degrees.

e.g. http://study.com/cimages/multimages/...e_molecule.png
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mansnothot
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Per lone pair subtract 2.5 degrees from 109.5 degrees.
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Sloshy
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(Original post by splitter2017)
OK so a sulfur atom has 6 electrons in its outer shell. When bonding in SO2, a dative covalent bond forms between S and one O. The other O is bonded to S by a double covalent bond. This leaves one lone pair.

If you add up the areas of electron density it equates to 3. These repel and spread out as far apart as possible to give 120 degrees bond angles (in an exam).
In reality, the lone pair will repel more strongly than bonding pairs and the angle will be ~ 119 degrees.

e.g. http://study.com/cimages/multimages/...e_molecule.png
Thanks for your help but how did you know it would form a dative covalent bond and a covalent bond? Couldn’t it have formed two covalent bonds and had two lone pairs left?
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splitter2017
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It always helps to draw a dot cross diagram. If you were to have two lone pairs on sulfur and two bonding pairs on sulfur. This would mean that each oxygen would have an unpaired electron which is an unstable arrangement. We also know that SO2 is uncharged so the oxygen's can't simply have an extra electron from another species.

Hope this helps
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Sloshy
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(Original post by splitter2017)
It always helps to draw a dot cross diagram. If you were to have two lone pairs on sulfur and two bonding pairs on sulfur. This would mean that each oxygen would have an unpaired electron which is an unstable arrangement. We also know that SO2 is uncharged so the oxygen's can't simply have an extra electron from another species.

Hope this helps
Hi thanks again for your help, the last thing I’m unsure how to do is determing it’s shape, since the sulfur has two bonding pairs and a lone pair how would I predict the shape it will make?
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splitter2017
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(Original post by Sloshy)
Hi thanks again for your help, the last thing I’m unsure how to do is determing it’s shape, since the sulfur has two bonding pairs and a lone pair how would I predict the shape it will make?
The shape is not to do with the pairs of electrons but rather the GROUPS or AREAS of electron density.

There are actually 3 bonding pairs and 1 lone pair it is just 2 of those bonding pairs are in 1 area of electron density (the double bond).
We count the areas of electron density to determine shapes (in this case 3) I have made this table which should illustrate the scenarios. The best way is to learn these rules off-by heart and with practice it will be easy.

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