What is the formal charge on each atom in$$ CO_2 $$?
In order to determine formal charges for the atoms in the carbon dioxide molecule you need to take into account the fact that CO2 has three resonance structure that look like this:
SIDE NOTE: the actual structure of the carbon dioxide molecule is a hybrid between these three structures, but I'll just show you each of them separate because I don't want the answer to become too long.
The carbon dioxide molecule has a total of 16 valence electrons - 4 from the carbon atom and 6 from each of the two oxygen atoms, all of which being accounted for in the three Lewis structures above.
The easiest way to assign a formal charge on an atom is to compare the number of valence electrons that atom has with how many electrons it "gets" in a molecule - assuming bond electrons are shared equally at all times regardless of electronegitivity
Let's start with the first Lewis stricture Carbon forms 4 bonds, which means it gets 4 electrons - 1 from each bond. Since carbon has 4 valence electrons, its formal charge will be zero.
The same is true for both oxygen atoms. Both of them form 2 bonds, which means they get 2 electrons. In addition to these electrons, they both have 2 lone pairs; this brings the total number of electrons an oxygen atom gets to 6 (2 + 4). Since oxygen has 6 valence electrons, it will have a zero formal charge.
Moving on to the second Lewis structure. Carbon is in the same position it was earlier - it forms 4 bonds $$ \rightarrow $$zero formal charge. However, things have changed for the oxygen atoms. Notice the oxygen on the left now forms 3 bonds with the carbon and has 1 lone pair instead of 2.
This means that it will get 5 electrons - 3 from the bonds and 2 from the lone pair; now it has one less electron than it "needs", i.e. one less than its valence electrons. This will result in a (+1) formal charge.
The oxygen on the right forms 1 bond with the carbon and has 3 lone pairs, for a total of 7 electrons; since it has one more electron than it needs, it will automatically have a (-1) formal charge.
The third structure is identical to the second with respect to formal charges, but this time the oxygen on the left will get a (-1) formal charge and the one on the right a (+1) formal charge.
