Question

How do you tell the preferred structure of SO4(2-) based on the formal charges? I know how to find the formal charges but I dont understand how to convey which is the preferred stucture based on the different formal charges.

Answer 1

In sulphate ion S atom is bonded to four oxygen. Assume four oxygen bonded by single bond

If you calculate formal charge on S atom, it comes out to be +2 and on oxygen atom -1.

There are four oxygen, that means -4 charge. That does not make any sense.

Fromal charges should be near to zero.

There are three non-bonding electron pair on each oxygen. We assume two of the oxygen give one non-bonding pair of electron to form double bond with sulphur.

Now if you calculate formal charge on S = 6 - 0 - 12/2 = 0

and on two doubly-bonded oxygen, O = 6 - 4 - 4/2 = 0

Therefore now the structure of sulphate ion, contains two doubly bonded oxygen and two singly bonded oxygen carrying uninegative charge.