Where are all my electrons?

When drawing lewis structures we may sometimes run into a problem. One such problem arrises in the drawing of SO2. SO2 has one single bond and one double bond. We could just give one Oxygen Atom a single bond and one Atom a double bond, but this would not be accurate. The reason is because the electrons that form these double bonds are constantly vibrating between the two Oxygen Atoms. A more appropriate way to draw the Lewis structure would be using it's resonance structure, which looks like so


A double headed Arrow separates the two lewis structures letting us know that if we were to take a snapshot of the atom at any given time, it would look like one of these two diagrams.

One other point brought up in class today was Ions. Ions are slightly more complex to draw in a Lewis, structure, but fortunately not too complex. Their are two main steps that are need to remember.
1. The first is that the molecule is not neutral and therefore has either more or less electrons. If the Ion has a charge of 2-, then it has two extra valence electrons.
2. The second difference between these and neutral molecules is that you must bracket in your entire Lewis structure and write the charge of the Ion outside of the brackets at the top right, as you would for a power of.

To put both lessons together, the following shows the Resonance structure of the Ion NO3(-)