Formal charge
Formal charge is a bookkeeping number for an atom in a Lewis structure: valence electrons minus lone-pair electrons minus number of bonds. The best structure has formal charges as close to zero as possible.
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Analogy
Like splitting a restaurant bill evenly: you check if each person ends up paying their fair share.
Example
Valence electrons = 5Lone-pair electrons = 0Bonds = 4Formal charge = 5 − 0 − 4 = +1
In practice
Formal charge helps you pick the most likely Lewis structure when more than one can be drawn.
Practice
Flashcards
- Formula for formal charge?Valence electrons − lone-pair electrons − number of bonds.
Interview questions
- Draw the Lewis structure of the nitrate ion and explain resonance and formal charge.
- Total valence electrons: 24
- Three equivalent resonance forms
- Formal charges: N +1, one O −1 in each form
- Real bond order 4/3
Questions people ask
Is formal charge the real charge on an atom?
No. Formal charge assumes every bond is shared perfectly equally, which is rarely true. Real charge distribution depends on electronegativity. Formal charge is a tool for comparing possible Lewis structures, and the sum of all formal charges must equal the overall charge of the molecule or ion.
How do I choose between structures using formal charge?
Prefer the structure where formal charges are smallest, and where any negative charge sits on the more electronegative atom. Total formal charge must stay the same for each option. A structure with all atoms at zero is usually the best choice.