Draw a Lewis structure for OCN- with 2 double bonds. What is the formal charge on each atom? a. O = -1 C = -1 N = +1 b. O = -1 C = 0 N = 0 c. O = 0 C = 0 N = -1 d. O = 0 C = 0 N = 0 e. O = +1 C = 0 N = -2
The Correct Answer and Explanation is:
To draw the Lewis structure for the OCN⁻ ion (cyanate ion) with 2 double bonds, we follow these steps:
Step 1: Count total valence electrons
- Oxygen (O): 6 electrons
- Carbon (C): 4 electrons
- Nitrogen (N): 5 electrons
- Extra negative charge: +1 electron
Total = 6 + 4 + 5 + 1 = 16 valence electrons
Step 2: Skeleton structure
Atoms are arranged in the order O–C–N. Try to form two double bonds:
O=C=N⁻
Step 3: Distribute electrons
- Carbon has 4 valence electrons and makes 4 bonds (2 double bonds), so it has 0 formal charge.
- Oxygen has 6 valence electrons. In the structure O=C=N⁻, oxygen forms a double bond and has 4 non-bonding electrons.
Formal charge on O = 6 – (4 non-bonding + 2 bonding) = 0 - Nitrogen has 5 valence electrons. It has 4 bonding electrons (2 bonds) and 2 non-bonding electrons.
Formal charge on N = 5 – (2 non-bonding + 4 bonding) = -1
Final Lewis Structure with 2 double bonds:
O=C=N⁻
Step 4: Formal charges
- Oxygen (O): 0
- Carbon (C): 0
- Nitrogen (N): -1
So, the correct answer is:
c. O = 0, C = 0, N = -1
Explanation
The cyanate ion, OCN⁻, consists of three atoms: oxygen, carbon, and nitrogen. It has a total of 16 valence electrons. When drawing the Lewis structure with two double bonds, we arrange the atoms as O=C=N⁻. This structure satisfies the octet rule for each atom and uses the total number of available electrons correctly.
Carbon is placed in the center since it can form four bonds. We assign double bonds between carbon and both oxygen and nitrogen. After placing the double bonds, we distribute the remaining electrons to complete octets. This gives oxygen and nitrogen each a lone pair (two non-bonding electrons), while carbon has none.
We then calculate the formal charges:
- For oxygen: 6 valence electrons − (4 non-bonding + 2 in bonds) = 0
- For carbon: 4 valence electrons − (0 non-bonding + 4 in bonds) = 0
- For nitrogen: 5 valence electrons − (2 non-bonding + 4 in bonds) = −1
This structure places the negative charge on nitrogen, which is acceptable since nitrogen is more electronegative than carbon but less than oxygen. This version is one of the resonance forms of OCN⁻ and is important for understanding the electron distribution.
Thus, the correct formal charges for the O=C=N⁻ Lewis structure are:
O = 0, C = 0, N = -1
Correct answer: c. O = 0, C = 0, N = -1.
