Draw the Lewis structure for OCN
(the atoms are bonded in the order given). Include all formal charges and resonance structures. Circle which structure would be the MAJOR contributor.
The Correct Answer and Explanation is:
To draw the Lewis structure for the OCN⁻ ion (also called the cyanate ion), with atoms bonded in the order O–C–N, follow these steps:
Step 1: Count total valence electrons
- Oxygen (O) = 6 valence electrons
- Carbon (C) = 4 valence electrons
- Nitrogen (N) = 5 valence electrons
- Extra electron due to the negative charge = 1 electron
Total = 6 + 4 + 5 + 1 = 16 valence electrons
Step 2: Draw possible resonance structures
Let’s construct 3 possible resonance structures while maintaining the O–C–N bonding order.
Structure A:
mathematicaCopyEditO=C=N⁻
- Oxygen: double bonded to carbon
- Carbon: double bonds with both O and N
- Nitrogen: lone pair and negative formal charge
Formal charges:
- O: 6 – (4 non-bonding + 2 bonding) = 0
- C: 4 – (0 non-bonding + 4 bonding) = 0
- N: 5 – (6 non-bonding + 2 bonding) = –1
Structure B:
mathematicaCopyEditO–C≡N⁻
- Oxygen: single bond to carbon
- Carbon: triple bond to nitrogen
- Nitrogen: one lone pair, negative charge
Formal charges:
- O: 6 – (6 non-bonding + 1 bonding) = –1
- C: 4 – (0 non-bonding + 4 bonding) = 0
- N: 5 – (2 non-bonding + 3 bonding) = 0
Structure C:
mathematicaCopyEdit⁻O≡C–N
- Oxygen: triple bond to carbon, negative charge
- Carbon: single bond to nitrogen
- Nitrogen: full octet
Formal charges:
- O: 6 – (2 non-bonding + 3 bonding) = +1
- C: 4 – (0 non-bonding + 4 bonding) = 0
- N: 5 – (6 non-bonding + 1 bonding) = –2
Step 3: Identify the MAJOR contributor
Structure B (O–C≡N⁻) is the major resonance contributor because:
- All atoms follow the octet rule
- The negative charge is on oxygen, the most electronegative atom
- Formal charges are minimized and more stable
Final Answer:
✅ Major resonance structure: O–C≡N⁻
Other valid structures: O=C=N⁻ and ⁻O≡C–N
This explains electron delocalization and shows the cyanate ion is resonance stabilized.
