[JohnTan]

Hi everyone, I'm just getting back into bonding and am getting puzzled by a few things. For instance, the sulfate ion SO4^-2. I understand that the structure has the sulfur atom as the center, with two oxygen atoms double bonded to this and the other two oxygen ions single bonded. However, I am also lead to believe that a double bond consists of TWO shared pairs of electrons. Taking this into account I cannot figure out what the molecule will look like in terms of lewis structure. The way I have drawn it shows only single bonds, with sulfur contributing two electrons in two of the bonds, and one electron from each being shared in the other two. I hope someone can help me with this, it will be much appreciated.

[Schrödinger]

Can you please post the structure that you have drawn?

[JohnTan]

i'll try!
                  xx
               ^x_x O ^x_x
      -   xx     oo    xx
        ^x_x O ^x_o  S  ^x_o O ^x_x -
           xx    oo     xx
                ^x_x O ^x_x
                   xx

where x = oxygen electron and o = sulfur electron

[JohnTan]

No, that's completely wrong..i actually have no idea now

[Schrödinger]

i'll try!
                  xx
               ^x_x O ^x_x
      -   xx     oo    xx
        ^x_x O ^x_o  S  ^x_o O ^x_x -
           xx    oo     xx
                ^x_x O ^x_x
                   xx

where x = oxygen electron and o = sulfur electron

What we have between the red oxygens and sulphur is a co-ordinate bond. Both the electrons are being donated by S. If you want to draw a double bond between 2 atoms, then 4 electrons need to be shared between 2 atoms.

[JohnTan]

but if this is done, won't sulfur have too many electrons?

[Schrödinger]

Octet rule is not followed for all atoms.

See http://en.wikipedia.org/wiki/Expanded_octet

[JohnTan]

that certainly throws a spanner in the works. thank you for your help

[Schrödinger]

No problem.