formal+charges

A //formal charge// is a value assigned to an atom in a Lewis diagram that indicates its stability (i.e. "happiness"). A formal charge of zero is ideal, meaning the atom has the best balance of bonds to lone pairs according to its valence.

The formal charge is exactly that - a formality. There is no real charge, per se, on the atoms with formal charges other than zero; this method is just a neat way of analyzing Lewis diagrams.

Here's how formal charge is calculated:

formal charge = atom's group # - (number of bonds + number of "dots")

where the number of "dots" is the number of non-bonding electrons drawn around an atom.

Here is an example:



The left-hand oxygen has a formal charge of -1 because 6 - (1+ 6) = -1.

The carbon in the center has a formal charge of 4 - (4 + 0) = 0.

The right-hand oxygen has a formal charge of 6 - (3 + 2) = +1.


 * Resonance**

Formal charges help to understand resonance structures, as well.

Any time electrons are shifted in a molecule or ion, but the backbone of the molecule (how it is put together) remains unchanged, a new resonance structure is created.

As you can see with boron trifluoride below, you can draw two resonance structures:



because all the atoms in the left-hand molecule have formal charges of zero, it is the "best" Lewis diagram. As you can see, by shifting a pair of non-bonding electrons off of F and creating a double bond with B in the right-hand molecule, you create a less favorable Lewis structure. This shows that halogens like F are not happy with double bonds, and Group 3 elements like B prefer the "hextet" rule (6 electrons, not eight).

Here is one more example:



What would the formal charges be on each atom in each resonance structure?

Here is more practice: