Draw the Lewis sructure for NCo, the oyanate anion, and calcle charges. Which statement is true:
A. In the best Lewis structure the formal charge of the onygrn is
B. In the best Lewis structure the formal charge of the osy gen is
C. In the best Lewis structure the formal charge of the carbon is
D. In the best Lewis structure the formal charge of the nitrogen is -1
E. In the best Lewis structure the formal charge of the nitrogen is -2
The Correct Answer and Explanation is:
To determine the correct Lewis structure of the cyanate ion (NCO⁻) and the associated formal charges, we must follow a systematic approach:
Step 1: Count the Total Valence Electrons
- Nitrogen (N): 5 valence electrons
- Carbon (C): 4 valence electrons
- Oxygen (O): 6 valence electrons
- Cyanate ion has a -1 charge, so add 1 more electron
Total = 5 (N) + 4 (C) + 6 (O) + 1 = 16 valence electrons
Step 2: Construct Possible Lewis Structures
We try different bonding arrangements with C in the center (as is typical), and we consider:
- N–C≡O
- N≡C–O
- N=C=O
After testing resonance and evaluating formal charges, the most stable Lewis structure is:
N⁻–C≡O⁺
:N⁻:—C≡O⁺
- Nitrogen: 1 lone pair, 1 single bond → 6 valence electrons
- Carbon: 1 single bond, 1 triple bond → 4 valence electrons
- Oxygen: 2 lone pairs, 1 triple bond → 6 valence electrons
Step 3: Calculate Formal Charges
Formal charge = [Valence electrons] – [Lone electrons + ½ Bonding electrons]
- Nitrogen:
5 – (2 lone + 2 from single bond) = 5 – 4 = +1
(BUT if it has 2 lone pairs: 5 – (4 + 2) = -1) ← This is true in N⁻–C≡O⁺ - Carbon:
4 – (0 lone + 4 bonding) = 0 - Oxygen (triple bonded, 2 lone pairs):
6 – (4 + 3×2 = 6) = 6 – 7 = -1 → too negative. But in this case:
Oxygen has 4 non-bonding electrons (2 pairs), and 3 bonds = 6 electrons
6 – (4 + 3) = -1 But we made a mistake! Let’s double-check for best structure.
Best Lewis Structure:
Structure: N=C=O⁻
- N double bonded to C, which is double bonded to O
- One lone pair on N, two on C, three on O
Formal Charges:
- Nitrogen: 5 – (2 lone + 2 bonds) = 5 – 4 = +1
- Carbon: 4 – 4 = 0
- Oxygen: 6 – (6 lone + 2 bonding) = 6 – 7 = -1
This makes formal charges:
- N: +1
- C: 0
- O: –1
Net charge: +1 + 0 – 1 = 0 → not consistent with NCO⁻
So try structure N–C≡O⁻:
- N has one lone pair, single bonded to C
- C triple bonded to O
Formal charges:
- N: 5 – (2 + 2) = +1
- C: 4 – 4 = 0
- O: 6 – (4 + 3) = –1
Now total: +1 + 0 –1 = 0
Still no. The most accurate best Lewis structure that adds to –1 is:
:N≡C–O:⁻
- N triple bonded to C, single bonded to O⁻
Formal charges:
- N: 5 – (2 + 3) = 0
- C: 4 – (4) = 0
- O: 6 – (6 + 1) = –1
This is the best Lewis structure.
✅ Correct Answer: D. In the best Lewis structure the formal charge of the nitrogen is -1
However, based on above corrected formal charge:
Actually this is incorrect, because in the best structure, nitrogen has 0 formal charge.
Let’s re-evaluate options:
- Final Lewis structure:
:N≡C–O:⁻ - Formal charges: N = 0, C = 0, O = –1
So, the correct statement is:
✅ B. In the best Lewis structure the formal charge of the oxygen is –1
✅ Final Answer: B
✅ Correct Statement: In the best Lewis structure, the formal charge of the oxygen is –1.
This matches the total –1 charge of the ion, and places the negative charge on the most electronegative atom, oxygen, which makes the structure most stable.