Draw the OCN- Lewis structure: One way of writing the Lewis structure of the cyanate ion OCN- places one double bond between the carbon atom and the oxygen atom and another double bond between the carbon atom and the nitrogen atom; Don’t forget the lone pair of electrons. What are the formal charges on the oxygen, carbon, and nitrogen atoms, respectively, for this structure? Select one:
a. 0, 0, -1
b. -1, 0,0
c. -2, 1, 0
d. none of these
The Correct Answer and Explanation is:
Step 1: Count the total valence electrons
- Oxygen (O): 6 valence electrons
- Carbon (C): 4 valence electrons
- Nitrogen (N): 5 valence electrons
- Charge (⁻1): Add 1 electron
Total = 6 + 4 + 5 + 1 = 16 valence electrons
Step 2: Draw the structure
We’re told to consider a structure with:
- A double bond between carbon and oxygen (C=O)
- A double bond between carbon and nitrogen (C=N)
So the skeletal structure is:
mathematicaCopyEditO = C = N⁻
Now, distribute the remaining electrons to complete octets and calculate formal charges.
Step 3: Assign lone pairs
In this structure:
- Oxygen has two bonds → 4 electrons in bonds → needs 4 more (2 lone pairs)
- Carbon has 4 bonds → complete octet → no lone pairs
- Nitrogen has two bonds → 4 electrons in bonds → needs 4 more (2 lone pairs)
So now:
- O: 2 lone pairs (4 electrons)
- C: 0 lone pairs
- N: 2 lone pairs (4 electrons)
Step 4: Calculate formal charges
Formal charge = Valence electrons – (Lone pair electrons + ½ Bonding electrons)
Oxygen:
- Valence: 6
- Lone pairs: 4 electrons
- Bonding: 4 electrons (2 bonds)
- FC = 6 – (4 + 2) = 0
Carbon:
- Valence: 4
- Lone pairs: 0
- Bonding: 8 electrons (4 bonds)
- FC = 4 – (0 + 4) = 0
Nitrogen:
- Valence: 5
- Lone pairs: 4
- Bonding: 4
- FC = 5 – (4 + 2) = -1
Answer:
O: 0, C: 0, N: -1
✅ Correct choice: a. 0, 0, -1
300-Word Explanation:
The Lewis structure of the cyanate ion (OCN⁻) helps us understand the bonding and charge distribution in the molecule. OCN⁻ consists of an oxygen atom, a carbon atom, and a nitrogen atom. Since it’s an ion with a charge of -1, we must account for one extra electron when distributing valence electrons.
We are told to evaluate the structure where the carbon forms double bonds with both oxygen and nitrogen (O=C=N⁻). This satisfies the octet rule for all atoms and uses up all 16 valence electrons efficiently.
Formal charge calculations help identify the most stable structure and verify if the charges align with the overall charge of the ion. For this double-bonded structure:
- Oxygen has 6 valence electrons. With two lone pairs and two bonds, its formal charge is 0.
- Carbon shares 4 bonds and has no lone pairs, making its formal charge also 0.
- Nitrogen has two lone pairs and two bonds, giving it a formal charge of -1.
The sum of formal charges (0 + 0 + -1) equals the total ionic charge of -1, confirming the stability and correctness of this Lewis structure.
Thus, the formal charges on the oxygen, carbon, and nitrogen atoms are 0, 0, and -1, respectively. This matches choice a, making it the correct answer. The use of formal charges is essential in choosing the most representative resonance structure for polyatomic ions like OCN⁻.
