`underset("x")(H-N=C=O)harrunderset("y")(H-overset(o+)N-=C-overset(Theta)O)harrunderset("z")(H-overset(Theta)N-C-=overset(o+)O)`
which resonating structure is least stable

Options

A. x
B. y
C. z
D. All are equivalent

Answer

To determine which resonating structure among x, y, and z is the least stable, we will analyze each structure based on the principles of resonance stability.

### Step 1: Identify the Resonating Structures
- **Structure X**: H-N=C=O (neutral molecule, no charges)
- **Structure Y**: H-N^+(C)=O^-(negative charge on oxygen, positive charge on nitrogen)
- **Structure Z**: H^+(N)-C=O^-(positive charge on nitrogen, negative charge on oxygen)

### Step 2: Analyze the Charges
- **Structure X**: This structure is neutral and does not have any charges, which generally contributes to its stability.
- **Structure Y**: The positive charge is on nitrogen (less electronegative) and the negative charge is on oxygen (more electronegative). This arrangement is stable as it follows the rule of having negative charges on more electronegative atoms.
- **Structure Z**: The positive charge is on oxygen (more electronegative) and the negative charge is on nitrogen (less electronegative). This arrangement is unstable, as it violates the principle that negative charges should be on more electronegative atoms.

### Step 3: Consider the Number of Bonds
- All three structures have similar bonding patterns, but the charge distribution significantly affects their stability.

### Step 4: Conclusion
Based on the analysis:
- Structure X is neutral and stable.
- Structure Y is stable due to the correct placement of charges.
- Structure Z is least stable because it places a positive charge on the more electronegative oxygen atom.

Thus, the least stable resonating structure is **Structure Z**.