Question

What is the formal charge of SO2?

Answer 1

The formal charge on the `"SO"_2` molecule is zero, but the formal charge on each atom depends on the Lewis structure that you draw.

Explanation:

You can draw three Lewis structures for `"SO"_2`.

The actual structure is therefore a resonance hybrid of all three structures.

In calculating the formal charge, each atom "gets" all of its lone pair electrons and half of its bonding electrons.

The formal charge is then the difference between the calculated number and the number of valence electrons in the isolated atom.

`"FC " = " VE - LP - ½BP"`

Let's look at each of the contributors.

The left hand structure

`bb"S"`: `"FC" = "VE - LP - ½BP" = 6 - 2 - 8/2 = 6 – 2 – 4 = 0`

`bb"O"`: `"FC" = "VE - LP - ½BP"= 6 - 4 - 4/2 = 6 – 4 - 2 = 0`

Here, the formal charge on each atom is zero.

The other two structures

`bb"S"`: `"FC" = "VE - LP - ½BP"= 6 – 2 – 6/2 = 6 – 2 – 3 = +1`

`"The single-bonded " bb"O"`:
`"FC" = "VE - LP - ½BP"= 6 – 6 – 2/2 = 6 – 6 – 1 = -1`

`"The double-bonded " bb"O"`:
`"FC" = "VE - LP - ½BP"= 6 – 4 – 4/2 = 6 – 4 - 2 = 0`

In every structure, the formal charge on `"SO"_2` is zero.