Chemistry problem - formal charges

[Sarahisme]

would any chemistry wizz's be able to help me out with this problem...?

 

i just don't see why the formal charge on the sulphur atom is zero, i thought i would be +3.

 

Thanks

 

Sarah

[Sarahisme]

oops nevermind, i just can't add up to 12. *shame*

[woelen]

would any chemistry wizz's be able to help me out with this problem...?

 

i just don't see why the formal charge on the sulphur atom is zero' date=' i thought i would be +3.

 

Thanks

 

Sarah [/quote']

What do you mean with formal charge? Is this oxidation state? The oxidation state of the sulphur in the ion you show (sulfate) is +6. All oxygens have oxidation state -2. In reality, the charge distribution is not so dramatic. All oxygens have a slight negative charge and the central sulphur might have a certain positive charge. The total charge of the sulfate ion equals -2.

In your model, two oxygens are negatively charged, but in reality, all oxygens of the ion are similar and you cannot distinguish between them.

[Mendelejev]

To obtain the formal charge of an atom in a compound : just count how many bonds there are + the number of free electrons. And compare that number with the number of valence electrons.

 

In his example S has six bonds (a double bond with oxygen counts for two.) and no free electrons. So a total of six. S, group VIa, so six valence electrons. 6 - 6 makes a formal charge of 0.

 

Oxygen had one bond with S, six free electrons, total is 7, but group VIa, so 6 - 7 is -1 => formal charge of oxygen.

 

Other oxygen had two bonds with S, four free electrons, total is 6, so formal charge of 0

 

The sum of all the formal charges is the charge of the compound. Here 0 + 0 + 0 - 1 - 1 = a total of -2. Correct, becouse SO4 is SO4 2-