Formal charge can be calculated for any atom in a given Lewis Dot Structure. The sum of all formal charges in a structure must equal the total charge of the structure. Formal charges are a bookkeeping tool, used to evaluate Lewis Dot Structures. Formal charges do not represent a physical quantity, but they can be useful in predicting the behavior of a molecule.
CALCULATION
The formula for formal charge is a simple subtraction using valence electrons (VE):
Formal Charge = (VE in the neutral atom*) – (net VE on the atom in the Lewis structure*)
For a simple example, look at the Lewis Dot Structure for carbon dioxide (CO2).
::O=C=O::
Formal charge can be calculated on any of the three atoms. The two oxygen atoms are equivalent, so only two calculations are needed, one for carbon, one for oxygen.
Carbon:
Four valence electrons on the neutral atom
Four valence electrons on the bonded atom (8 bonded electrons X 1/2)
Formal charge = 4 – 4 = 0
Oxygen:
Six valence electrons on the neutral atom
Six valence electrons on the bonded atom (4 bonded electrons X 1/2, plus 4 lone electrons)
Formal charge = 6 – 6 = 0
Carbon dioxide is a neutral molecule, and the sum of all formal charges is zero (neutral), so this checks out.
EVALUATION
Sometimes more than one possible Lewis Dot Structure can be drawn for a molecule. A comparison of the formal charges in the structures can often suggest which one is the better structure. While not an absolute rule, typically the structure with the charges closest to zero is best, and negative formal charges should on the more electronegative atoms.
Consider sulfur dioxide (SO2). It could be drawn in either of the following two ways.
1. ::O=S(:)=O:: (normally “(:)” is drawn as a lone pair drawn above the S)
2. ::O=S(:)-O:::
Calculating the formal charges (from left to right):
1. O = 0, S = 0, O = 0
2. O = 0, S = +1. O = -1
The first structure has all formal charges of zero, while the second structure has two charges. (Notice that the -1 is on oxygen, which is more electronegative than sulfur.) The first structure is considered the better Lewis structure.
REALITY and FORMAL CHARGE
In the sulfur dioxide example, two Lewis structures were possible. These represent resonance structures of sulfur dioxide, so the true structure is something of an average of all valid structures. (There is also a third structure that is the mirror-image of the second.) Because the first structure is the most favorable, the expectation is that the average would be weighted in favor of that structure. An actual measurement of bond length is required to find the actual weight. Based on the average, the expectation is that the S-O bonds will be slightly polar. (A comparison of electronegativity gives the same prediction.)
REVIEW NOTES for VALENCE
*Valence electrons are the electrons in the outermost energy of an atom. For a neutral atom, this is the sum of “s” and “p” orbital electrons in the highest energy level, with a maximum number of eight. Excluding the “d” and “f” block metals, simply count the number of columns in the periodic table from left to right until you reach the atom of interest. Arsenic, for example is in the fifth column (because we ignore all of the d-block transition metals), and has five valence electrons.
*In Lewis Dot structures, only valence electrons are shown. To determine the valence electrons on a given atom in a Lewis structure, count all lone electrons on the atom as one VE and count all covalently bonded electrons around the atom as one half of a VE.
