Answer
Resonance Structures Explained
Resonance structures are multiple valid Lewis structures that represent the same molecule. These structures:
- Have the same molecular formula
- Show the same atom connectivity
- Contain the same total number of electrons
- Differ only in the arrangement of electrons (especially π-electrons and lone pairs)
They help represent the delocalization of electrons within molecules where a single Lewis structure is insufficient.
Example 1: Resonance in the Allyl Ion
The allyl ion CH2=CH–CH2– has two major resonance structures:
- Structure 1: Negative charge on the terminal CH2 group
- Structure 2: Double bond shifts, and the negative charge moves to the opposite end
Here, the π electrons from the C=C bond move toward the adjacent C–C bond, creating delocalization across the three-carbon chain.
Example 2: Resonance in the Amidate Ion
The amidate ion, typically derived from deprotonated amides, can resonate between two forms:
- R–C(=O)–NH–: Lone pair is on nitrogen, C=O double bond intact
- R–C(–O–)=NH: Double bond shifts from C=O to C=N, and the negative charge resides on oxygen
Thus, the correct resonance form contains one C=N bond and one C–O bond, sharing electron density across the atoms.
Important Tip: Resonance structures must obey all valence rules and represent real delocalization. They are not in equilibrium with each other; instead, the actual molecule is a hybrid of all valid resonance structures.