AP Chemistry / Unit 2: Compound Structure and Properties
2.6 Resonance and Formal Charge
Notes
- Resonance: more than one valid Lewis structure; the real molecule is an average (e.g., $\mathrm{O_3}$, $\mathrm{NO_3^-}$).
- Formal charge = valence electrons − (lone pair electrons + bonds). The best structure has formal charges closest to 0, with negatives on more electronegative atoms.
Practice
The bonds in ozone ($\mathrm{O_3}$) are:
Resonance averages them.
Formal charge on C in CO (triple bond, one lone pair on C):
$4-(2+3)=-1$.
The preferred Lewis structure has:
Minimizing charge separation.
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