Resonance and Formal Charge
Introduced| English | Meaning |
|---|---|
| resonance/ˈrezənəns/ | Describing a molecule by two or more structures because no single structure shows its real bonding. |
| delocalized/dɪˈlɒkəlaɪzd/ | Describing electrons distributed over more than two atoms rather than confined to one atom or bond. |
| formal charge/ˈfɔːml tʃɑːdʒ/ | A *bookkeeping* tool, not a real charge on the atom. |
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.
Ozone has two resonance structures; the real molecule is their hybrid
A molecule with resonance is best described as...
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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?
Difficulty 2/5 · 140 coins + a small bonus on first completion today
$FC = 6 - 4 - \tfrac{1}{2}(4) = 6 - 4 - 2 = 0$.
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...
Difficulty 1/5 · 100 coins + a small bonus on first completion today
Minimizing formal charge gives the most stable, most likely structure.
The formal charges in a neutral molecule must add up to +1.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
False. Formal charges add up to the overall charge: zero for a neutral molecule, and the ion's charge for an ion.
A negative formal charge is best placed on the...
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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.