Confusing bonding question- SO4 2- ... will rep best answer!!!! Watch

Lola1244
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For the SO4 2- ion, the sulfur is bonded to four oxygens in a tetrahedral shape. I get that.

What I don't get is why the sulfur is bonded with two double bonds to to two oxygens and to the other oxygens by single bonds.
Why can it not have four double bonds:
a double bond with an oxygen- uses up two electrons
a double bond with another oxygen- four electrons have now been used up in total
a double bond with another oxygen- all its outer electrons have now been used up
a double bond with the last oxygen- in this double bond there would be two electrons from the oxygen and the two extra electrons the molecule has due to the 2- charge

I am so confused!! Any help is appreciated and will rep best answer
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RMNDK
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(Original post by Lola1244)
For the SO4 2- ion, the sulfur is bonded to four oxygens in a tetrahedral shape. I get that.

What I don't get is why the sulfur is bonded with two double bonds to to two oxygens and to the other oxygens by single bonds.
Why can it not have four double bonds:
a double bond with an oxygen- uses up two electrons
a double bond with another oxygen- four electrons have now been used up in total
a double bond with another oxygen- all its outer electrons have now been used up
a double bond with the last oxygen- in this double bond there would be two electrons from the oxygen and the two extra electrons the molecule has due to the 2- charge

I am so confused!! Any help is appreciated and will rep best answer
So you're saying the last oxygen forms a double coordinate bond with the sulfur atom? As in, to donate two lone pairs of electrons to form a double bond? I'm not sure that's possible.

Plus you should consider the formal charge on the sulfur atom.

Think of it like this. If each oxygen atom made a single sigma bond with the sulfur atom, the formal charge would be +2. This is because, sulfur had 6 electrons to give away, once it does, it will now have 4 electrons and what you do to calculate formal charge is the number of valence electrons, minus the electrons it has now. (Not specifically that, there's an actual formula. It might help to draw a Lewis Diagram)

The structure of a molecule needs to have the least amount of formal charge. We can reduce +2 to 0. If two oxygen molecules form a pi bonds, then the sulfur will now have 6 electrons instead of 4 because it's getting some from the oxygen. Now the formal charge on the sulfur atom is 0. Great.

As for that -2 charge, if you calculate the formal charge on the oxygen atoms, they're all -1. Once they shared the electrons with sulfur, that went up to 0 because they brought 6 electrons, it forms two bonds, that's 2 electrons and it has 4 spare electrons. That's 6-2-4 = 0.
There are only two oxygen molecules with a -1 charge. Hence SO42-

This might not make sense
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Pigster
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https://www.youtube.com/watch?v=8AOw_EWmkGk
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Lola1244
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(Original post by RMNDK)
So you're saying the last oxygen forms a double coordinate bond with the sulfur atom? As in, to donate two lone pairs of electrons to form a double bond? I'm not sure that's possible.

Plus you should consider the formal charge on the sulfur atom.

Think of it like this. If each oxygen atom made a single sigma bond with the sulfur atom, the formal charge would be +2. This is because, sulfur had 6 electrons to give away, once it does, it will now have 4 electrons and what you do to calculate formal charge is the number of valence electrons, minus the electrons it has now. (Not specifically that, there's an actual formula. It might help to draw a Lewis Diagram)

The structure of a molecule needs to have the least amount of formal charge. We can reduce +2 to 0. If two oxygen molecules form a pi bonds, then the sulfur will now have 6 electrons instead of 4 because it's getting some from the oxygen. Now the formal charge on the sulfur atom is 0. Great.

As for that -2 charge, if you calculate the formal charge on the oxygen atoms, they're all -1. Once they shared the electrons with sulfur, that went up to 0 because they brought 6 electrons, it forms two bonds, that's 2 electrons and it has 4 spare electrons. That's 6-2-4 = 0.
There are only two oxygen molecules with a -1 charge. Hence SO42-

This might not make sense
Thanks for your reply but we have not done formal charges in chem a level and I don't think it's on the spec ... So how would you be able to do this??? Is it just a case of remembering??
Many thanks
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charco
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(Original post by Lola1244)
Thanks for your reply but we have not done formal charges in chem a level and I don't think it's on the spec ... So how would you be able to do this??? Is it just a case of remembering??
Many thanks
The reply always depends on the level studied.

You should know that electrons have to be located in regions of space (orbitals) around the central atom and that there is a limit to the number of orbitals that can 'fit' around the atom.

In the case of sulfur there are up to six orbitals available, making room for 12 electrons, i.e. 6 bonding pairs. (two single bonds and two double bonds).

However there is also Occam's razor to consider, in that it is possible to describe the bonding in terms of two shared pairs and two dative coordinate pairs from the sulfur, making four single bonds to four oxygen atoms with the two 'extra' charges filling up the oxygen octets.

Evidence for the 'actual' arrangement is provided by bond strength, ionic shape and bond length, derived from spectroscopic and crystallographic studies.

The ion is tetrahedral with all four bonds identical.

The bond strength and lengths are all the same, being intermediate between single and double bonds.

The 'best' answer therefore (at this level) is that all four bonds are single, but with contributions from a delocalised pi system derived from partially overlapping d (sulfur) and p(oxygen) orbitals.
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Shraddha123
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Sulphur has 6 valence e- in its outermost shell So42- HAS 2- charge so two oxygen will contain single bond while other two have double bond
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alow
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(Original post by Shraddha123)
Sulphur has 6 valence e- in its outermost shell So42- HAS 2- charge so two oxygen will contain single bond while other two have double bond
This thread was posted in 2015.
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Nabila77
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btw what hybridisation state for so42-? it is sp3 or sp3d??? i need your help
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