Resonance and Formal Charge
Introduced| English |
|---|
| resonance/ˈrezənəns/ |
| delocalized/dɪˈlɒkəlaɪzd/ |
| formal charge/ˈfɔːml tʃɑːdʒ/ |
When one drawing isn't enough
- Some molecules refuse to fit a single Lewis picture.
- You can draw two or three that look equally right.
- The real molecule is a blend of them all.
- A quick bookkeeping trick tells you which drawing to trust.
Resonance: an average of structures
- Resonance 共振 means a molecule needs several Lewis structures.
- The true structure is an average, or hybrid, of them.
- The electrons are spread out, or delocalized 离域, across the molecule.
A molecule with resonance is best described as...
The real molecule is one steady average, not a flipping between forms.
In a resonance hybrid, the electrons are spread out, or ____, over the molecule.
Resonance means the electrons are delocalized, not fixed to one bond.
Formal charge
- Formal charge 形式电荷 checks how fairly electrons are shared:
- It compares an atom's electrons in the structure to its neutral count.
An oxygen atom (6 valence) has 4 lone electrons and 4 bonding electrons. Its formal charge?
$FC = 6 - 4 - \tfrac{1}{2}(4) = 6 - 4 - 2 = 0$.
Choosing the best structure
- The best Lewis structure keeps formal charges closest to zero.
- Any negative formal charge should sit on the most electronegative atom.
- The formal charges across the molecule must sum to its total charge.
The best Lewis structure has formal charges that are...
Minimizing formal charge gives the most stable, most likely structure.
The formal charges of all atoms must add up to the molecule's overall charge.
For a neutral molecule they sum to zero; for an ion, to its charge.
A negative formal charge is best placed on the...
The most electronegative atom holds negative charge most comfortably.
Ozone $\text{O}_3$ has two resonance structures.
- In each one, one O--O bond is single and the other is double.
- The real molecule averages them: two equal bonds, each of order 1.5.
Assign the formal charges
Formal charge = valence electrons - lone electrons - bonds. Assign each atom in the nitrate ion.
Resonance structures are not real states the molecule flips between -- it is one steady average of them, all the time. Formal charge is a bookkeeping tool, not a real charge on the atom. And the best structure minimizes formal charges and places any negative one on the most electronegative atom.
Resonance means a molecule is the average of several Lewis structures, with electrons delocalized across it. Formal charge ($=$ valence $-$ lone $- \tfrac{1}{2}$ bonding electrons) is a bookkeeping check: the best structure keeps formal charges near zero, with any negative charge on the most electronegative atom.