Overview
This lecture covers how to calculate formal charges, use them to predict the most reasonable Lewis structures, and explains the concept of resonance in molecules.
Calculating Formal Charge
- Formal charge is the hypothetical charge an atom would have if bonding electrons were shared equally.
- Formal charge = (Valence electrons of free atom) - (Lone pair electrons) - ½(Bonding electrons).
- The sum of formal charges in a neutral molecule must be zero; in an ion, it equals the ion’s charge.
- Formal charge is a bookkeeping tool, not an actual charge on the atom.
Examples of Formal Charge Calculation
- Example: In ( \mathrm{ICl_4^-} ), I has a formal charge of -1, each Cl is 0; sum is -1.
- Example: In ( \mathrm{BrCl_3} ), Br and each Cl have formal charges of 0.
- Example: For ( \mathrm{NCl_3} ), N and all Cl atoms have formal charges of 0.
- Example: In CO, C is -1 and O is +1.
Using Formal Charge to Predict Structures
- Prefer Lewis structures where all formal charges are zero.
- If unavoidable, choose structures with the smallest nonzero formal charges.
- Place negative formal charges on more electronegative atoms.
- In CO₂, the best structure has C double bonded to two O, with all formal charges zero.
- In SCN⁻ (thiocyanate), the most likely structure minimizes atoms with nonzero formal charge.
Resonance and Resonance Hybrids
- Resonance occurs when multiple valid Lewis structures exist for one molecule by moving electrons, not atoms.
- The actual molecule is a resonance hybrid, an average of all resonance forms.
- Resonance forms are shown with double-headed arrows between them.
- In ( \mathrm{NO_2^-} ), both N–O bonds are equivalent due to resonance, despite two possible structures.
- In ( \mathrm{CO_3^{2-}} ), three equivalent resonance structures exist; all C–O bonds are identical in reality.
Key Terms & Definitions
- Formal Charge — Hypothetical charge calculated if bonding electrons are shared equally.
- Resonance — The existence of two or more valid Lewis structures for one molecule with the same atom arrangement.
- Resonance Hybrid — The actual electronic structure, representing the average of all resonance forms.
Action Items / Next Steps
- Practice calculating formal charges and drawing resonance structures using the provided online quiz.
- Review examples of formal charge calculations from the lecture.