Draw three possible Lewis structures (resonance structures) for N2O and determine the formal charges in each structure. (6 pts) 3. The N-N bond length in N2O is 112 pm, slightly longer than a typical N≡N bond; and the N-O bond length is 119 pm, slightly shorter than a typical N=O bond. Based on these data, which resonance structure best represents N2O in question 2 ? Explain your answer. (4 pts)
The Correct Answer and Explanation is:
To address this problem about N2O (nitrous oxide) and its Lewis structures, we need to draw the possible resonance structures and determine the formal charges in each. Let’s break it down step by step.
Step 1: Draw Three Possible Lewis Structures
- Structure 1:
- N≡N−O (N≡N with a single bond to O)
- The triple bond (N≡N) between the two nitrogen atoms is common in N2O. Oxygen forms a single bond with the second nitrogen atom and carries a lone pair.
- Formal charges: Nitrogen on the left has a formal charge of 0. Nitrogen in the center has a formal charge of +1. Oxygen has a formal charge of −1.
- Structure 2:
- N=N=O (N=N double bond with a double bond to O)
- This structure features a double bond between the nitrogen atoms and a double bond to oxygen.
- Formal charges: The formal charge on the left nitrogen is 0, the center nitrogen has a formal charge of 0, and oxygen has a formal charge of 0.
- Structure 3:
- N−N≡O (N−N with a triple bond to O)
- In this structure, the first nitrogen has a single bond to the second nitrogen, and the second nitrogen forms a triple bond with oxygen.
- Formal charges: The left nitrogen has a formal charge of +1, the center nitrogen is neutral (formal charge 0), and oxygen has a formal charge of −1.
Step 2: Formal Charges Calculation
For each structure, formal charge can be calculated using the formula:Formal Charge=Valence electrons−(Lone pair electrons+12×Bonding electrons)\text{Formal Charge} = \text{Valence electrons} – \left( \text{Lone pair electrons} + \frac{1}{2} \times \text{Bonding electrons} \right)Formal Charge=Valence electrons−(Lone pair electrons+21×Bonding electrons)
- Structure 1 (N≡N−O):
- Left N: 5 (valence) – (2 + 3) = 0 (formal charge)
- Center N: 5 (valence) – (0 + 6) = +1 (formal charge)
- O: 6 (valence) – (6 + 2) = −1 (formal charge)
- Structure 2 (N=N=O):
- Left N: 5 (valence) – (2 + 4) = 0 (formal charge)
- Center N: 5 (valence) – (2 + 4) = 0 (formal charge)
- O: 6 (valence) – (4 + 4) = 0 (formal charge)
- Structure 3 (N−N≡O):
- Left N: 5 (valence) – (4 + 2) = +1 (formal charge)
- Center N: 5 (valence) – (2 + 4) = 0 (formal charge)
- O: 6 (valence) – (2 + 6) = −1 (formal charge)
Step 3: N-N and N-O Bond Lengths
Given that:
- The N-N bond length in N2O is 112 pm, slightly longer than the typical N≡N bond, which suggests some delocalization or partial bonding.
- The N-O bond length is 119 pm, slightly shorter than the typical N=O bond, suggesting some partial double bond character.
Step 4: Which Resonance Structure is Most Accurate?
Given the bond length data:
- Structure 2 (N=N=O) best represents N2O, as it has partial double bond character between N-N and N-O. The bond lengths observed (N-N being longer and N-O being shorter) are most consistent with this structure.
- The other structures (N≡N−O and N−N≡O) would suggest more discrete bonding, which doesn’t match the observed bond length data.
Final Answer:
The N=N=O resonance structure is the best representation of N2O, as it accounts for the bond lengths being intermediate between typical N≡N and N=O bonds. This structure also minimizes formal charges (0 for both nitrogen atoms and oxygen), making it the most stable and likely structure.
