Formal Charge

The Octet Rule & Formal Charge

Atoms form chemical bonds to get the electron configuration of their closest noble gas
The octet rule explains why atoms usually want eight electrons in their valence shell in order to have more stability
In some cases, it is possible to have multiple Lewis structures while still following the octet rule
Each of these variations is a resonance structure and they contribute to the formation of the hybrid of resonance
In order to figure out the resonance structure that contributes the most, the formal charge (FC) of all the atoms in the molecule is determined

Formal charge is a charge assigned to all the atoms in a molecule by assuming that all the bonding electrons are shared equally between the atoms, regardless of differences in electronegativity
The formula of FC is

$FC = V - \frac{1}{2} B - N$

- V is the number of valence electrons
- B is the number of bonding electrons
- N is the number of nonbonding electrons

Worked example

What is the formal charge of boron in the BH₄^-ion?

Answer:

Boron is a Group 13 element, by using the periodic table information it can be deduced that it has 3 valence electrons.
Hydrogen is in Group 1, therefore it has one valence electron.
Since the BH₄^-ion has lost one electron, the total number of valence electrons in the structure is 8.
By following the steps to draw a Lewis diagram, the structure should look like this:

resonance-and-formal-charge-5

Lewis formula of BH₄^–

The number of bonding electrons in B is 8 and the number of nonbonding electrons in B is 0

$FC = V - \frac{1}{2} B - N$

$FC = 3 - \frac{1}{2} \times 8 - 0$

$FC = - 1$

Choosing the preferred resonance structure

The concept of formal charge can help select competing resonance structures
The following rules should be applied when analyzing which of the resonance structures contributes the most
- The sum of the formal in neutral molecules is zero
- The sum of the formal charges in ions is the charge of the ion
- The smaller the formal charge (or 0), the better it is. Large formal charges are not common in atoms
- Negative formal charges should be assigned to the most electronegative atom

Worked example

What is the formal charge on the two resonance structures shown?

resonance-structures-of-hydrogen-cyanide

Resonance structures of hydrogen cyanide

Deduce which is the preferred structure

Answer:

Structure 1

$FC = V - \frac{1}{2} B - N$

$FC on hydrogen = 1 - \frac{1}{2} (2) - 0 = 0$

$FC on carbon = 4 - \frac{1}{2} (8) - 0 = 0$

$FC on nitrogen = 5 - \frac{1}{2} (6) - 2 = 0$

Structure 2

$FC = V - \frac{1}{2} B - N$

$FC on hydrogen = 1 - \frac{1}{2} (2) - 0 = 0$

$FC on carbon = 4 - \frac{1}{2} (6) - 2 = - 1$

$FC on nitrogen = 5 - \frac{1}{2} (8) - 0 = + 1$

Structure 1 is the preferred structure because
- The sum of the formal charges is zero since its a neutral molecule
- All the formal charges on the atoms are 0
- It has the least electronegative atom (the carbon atom) in the central position

Structure 2 has +1 on N and -1 on C
- The negative formal charge in Structure 2 is not on the most electronegative ion, therefore, the structure is unstable

Exam Tip

The terms Lewis Diagrams, Lewis Structures or Lewis Formulas mean the same thing

Formal Charge (College Board AP Chemistry)

Revision Note

The Octet Rule & Formal Charge

Formal Charge

Worked example

Choosing the preferred resonance structure

Worked example

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Author: Martín