Yes, bond length absolutely depends on resonance. Resonance significantly influences the actual bond lengths within molecules by creating an averaged electron distribution among contributing structures.
Understanding Resonance and Its Effect on Bond Length
Resonance is a concept used in chemistry to describe delocalized electrons within molecules or polyatomic ions where no single Lewis structure can fully represent the bonding. Instead, the true structure, known as a resonance hybrid, is an average of all valid contributing Lewis structures. This averaging directly impacts bond lengths.
The bond length in the hybrid resonance structure is always an average of the bond length of all surrounding atoms and central atoms. Essentially, the resonance structure affects the bond length because it reflects a shared, or delocalized, electron density, rather than discrete single or double bonds.
How Resonance Modifies Bond Lengths
- Delocalization of Electrons: In molecules exhibiting resonance, electrons (particularly pi electrons) are not confined to a single bond between two atoms. Instead, they are delocalized over three or more atoms.
- Partial Bond Character: This delocalization means that bonds that appear as single or double in individual resonance forms take on partial double bond character (or partial single bond character) in the actual resonance hybrid. For instance, a bond might have 1.5, 1.33, or 1.67 bond order, rather than a whole number.
- Intermediate Bond Lengths: Since single bonds are typically longer than double bonds, and double bonds are longer than triple bonds, a bond with partial double bond character will have a length intermediate between a pure single bond and a pure double bond. The greater the double bond character, the shorter the bond.
Examples Illustrating Resonance and Bond Length
Let's explore some classic examples where resonance directly dictates observed bond lengths.
1. Nitrate Ion (NO₃⁻)
The nitrate ion (NO₃⁻) is a perfect example where resonance structures explain the identical bond lengths observed experimentally.
- Resonance Structures: There are three equivalent resonance structures for the nitrate ion. In each structure, one nitrogen-oxygen bond is a double bond, and the other two are single bonds.
- N=O (double) and two N-O (single)
- N-O (single), N=O (double), N-O (single)
- N-O (single), N-O (single), N=O (double)
- Resonance Hybrid: The actual nitrate ion is a hybrid of these three structures. This means the pi electrons are delocalized over all three oxygen atoms.
- Observed Bond Lengths: As a result, all three N-O bonds in the nitrate ion are found to be identical in length. This length is shorter than a typical N-O single bond but longer than a typical N=O double bond. Specifically, each N-O bond has an average bond order of approximately 1.33 (one double bond character spread over three bonds).
| Bond Type | Typical Bond Length (picometers, pm) | Nitrate Ion N-O Bond Length (pm) |
|---|---|---|
| N-O Single Bond | ~140 | |
| N=O Double Bond | ~120 | |
| N-O in NO₃⁻ | ~122 |
This shows that the N-O bonds in nitrate are intermediate, reflecting their partial double bond character.
2. Benzene (C₆H₆)
Benzene is another iconic molecule demonstrating resonance.
- Resonance Structures: Benzene has two primary Kekulé resonance structures, where single and double carbon-carbon bonds alternate around the ring.
- Resonance Hybrid: The true structure of benzene is a resonance hybrid where the pi electrons are delocalized over the entire ring.
- Observed Bond Lengths: All six C-C bonds in benzene are experimentally found to be identical, with a length of approximately 139 pm. This length is intermediate between a typical C-C single bond (154 pm) and a C=C double bond (134 pm), indicating an average bond order of 1.5 for each carbon-carbon bond.
3. Carboxylate Ions (RCOO⁻)
Carboxylate ions, such as the acetate ion (CH₃COO⁻), also exhibit resonance.
- Resonance Structures: There are two equivalent resonance structures for the carboxylate group, with the negative charge and double bond alternating between the two oxygen atoms.
- Resonance Hybrid: The true structure is a hybrid where the negative charge and pi electrons are delocalized over both oxygen atoms and the carbon atom.
- Observed Bond Lengths: Both C-O bonds in a carboxylate ion are found to be identical in length, intermediate between a C-O single bond and a C=O double bond.
In summary, resonance is a crucial concept for understanding the actual bond lengths in molecules and ions where electron delocalization occurs. It explains why certain bonds are neither purely single nor purely double, but possess an intermediate character.
