Formal Charge (College Board AP® Chemistry)

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

• 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

• V is the number of valence electrons

• B is the number of bonding electrons

• N is the number of nonbonding electrons

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