Question

How is the Lewis structure of an ion written?

Answer 1

The same way you draw the structure of the original molecule or atom, except with `X` more or `X` less electrons, depending on the charge.

i.e. if charge is `-2`, then add `2` electrons (because the charge of an electron is `-1.602xx10^(-19) C` and is thus the same sign as the charge), and vice versa for charges of `pmX`.

So if you use the "counting valence electrons" method, you can draw `SO_4^(2-)` by saying that:

`S: 6`
`O_4: 6*4 = 24`
`"From 2- charge: " 2`

Total: `6+24+2 = color(green)(32)`

Count the electrons in here:
http://chemwiki.ucdavis.edu/
where the above is all of the resonance structures of `SO_4^(2-)`.

With formal charge defined here as `"Actual " e^(-) - "owned " e^-`:

"Owned" electrons:

• `6` per thionyl oxygen (`S=O`), so `12` total.
• `7` per oxygen with a formal charge of `-1` (from `6 - 7`), so `14` total.
• `6` per sulfur, so `6` total.

Sure enough, it matches:
`12+14+6 = color(green)(32)`