Formal charge

Definition

Formal charge is a theoretical charge assigned to an atom in a molecule, calculated based on the number of valence electrons minus the sum of non-bonding electrons and half of the bonding electrons. It helps determine the most stable Lewis structure for a molecule.

5 Must Know Facts For Your Next Test

1. Formal charge is calculated using the formula: $$FC = V - (N + \frac{B}{2})$$ where $V$ is the number of valence electrons, $N$ is the number of non-bonding electrons, and $B$ is the number of bonding electrons.
2. The sum of formal charges in a neutral molecule must be zero; for an ion, it must equal the ion's charge.
3. Structures with formal charges closest to zero are generally more stable.
4. When multiple Lewis structures are possible, those distributing negative formal charges on more electronegative atoms are favored.
5. Formal charges help predict reactivity and preferred resonance structures in molecules.

Review Questions

• What formula is used to calculate formal charge?
• How does the sum of formal charges relate to the overall charge of a molecule or ion?
• Why are structures with formal charges closest to zero generally more stable?

Related terms

Valence Electrons: Electrons in the outermost shell of an atom that participate in chemical bonding.

Lewis Structure: A diagram showing the arrangement of valence electrons among atoms in a molecule.

Resonance: A concept where two or more valid Lewis structures can represent a single molecule. These structures are called resonance forms.