Draw the Lewis structure of CNO’ (C-N-O) and find the formal charges on carbon, nitrogen and oxygen? (note use the lowest possible formal charge] The formal charge on C is [X], an N is [Y], and on O is [Z]. (For the formal charge write + or – sign)
The Correct Answer and Explanation is :
To determine the Lewis structure of CNO⁻ (cyanate ion), follow these steps:
Step 1: Count the total number of valence electrons
- Carbon (C) is in Group 14, so it has 4 valence electrons.
- Nitrogen (N) is in Group 15, so it has 5 valence electrons.
- Oxygen (O) is in Group 16, so it has 6 valence electrons.
- The negative charge on the ion contributes 1 additional electron.
Total valence electrons = 4 (C) + 5 (N) + 6 (O) + 1 (negative charge) = 16 electrons.
Step 2: Arrange the atoms
CNO⁻ has the following arrangement: C-N-O. Carbon is typically the central atom because it is less electronegative than nitrogen and oxygen.
Step 3: Distribute electrons
- Place a single bond between C and N, and between N and O. This uses 4 electrons (2 electrons per bond).
- Distribute the remaining electrons to complete the octets of nitrogen and oxygen, starting with oxygen because it is the most electronegative.
After distributing electrons, we are left with the following bonding:
- Carbon forms a single bond with nitrogen (2 electrons).
- Nitrogen forms a single bond with oxygen (2 electrons).
- Oxygen has 6 electrons around it (from lone pairs) and holds the negative charge.
Step 4: Calculate formal charges
The formal charge (FC) is calculated using the formula: FC=V−(L+B2)FC = V – (L + \frac{B}{2})
Where:
- VV is the number of valence electrons in the free atom.
- LL is the number of lone pair electrons on the atom in the molecule.
- BB is the number of bonding electrons shared by the atom.
Carbon (C):
- Carbon has 4 valence electrons.
- Carbon is bonded to nitrogen with one bond, so it shares 2 electrons.
- Carbon has no lone pairs.
FCC=4−(0+22)=4−1=+1FC_{C} = 4 – (0 + \frac{2}{2}) = 4 – 1 = +1
Nitrogen (N):
- Nitrogen has 5 valence electrons.
- Nitrogen is bonded to carbon and oxygen, each with a single bond (2 electrons per bond), so it shares 4 electrons.
- Nitrogen has one lone pair (2 electrons).
FCN=5−(2+42)=5−4=+1FC_{N} = 5 – (2 + \frac{4}{2}) = 5 – 4 = +1
Oxygen (O):
- Oxygen has 6 valence electrons.
- Oxygen is bonded to nitrogen with one bond (2 electrons).
- Oxygen has 3 lone pairs (6 electrons).
- Oxygen has an extra electron due to the negative charge.
FCO=6−(6+22)=6−7=−1FC_{O} = 6 – (6 + \frac{2}{2}) = 6 – 7 = -1
Final Formal Charges:
- Formal charge on Carbon (C) = +1
- Formal charge on Nitrogen (N) = 0
- Formal charge on Oxygen (O) = -1
Thus, the formal charges are:
- Carbon: +1
- Nitrogen: 0
- Oxygen: -1
Conclusion:
The Lewis structure of CNO⁻ places the negative charge on oxygen to minimize formal charges across the molecule. The formal charges on carbon and nitrogen are minimized (carbon gets +1 and nitrogen has 0 formal charge), which results in the most stable configuration. The overall charge of the molecule is -1, consistent with the ion’s charge.