. 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 cyanate (OCN), we will first count the total number of valence electrons for the atoms involved and then follow the rules to assign bonds, lone pairs, and formal charges.
Step 1: Count the Valence Electrons
- Oxygen (O) is in group 16, so it has 6 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.
Total valence electrons = 6 (O) + 4 (C) + 5 (N) = 15 electrons
Step 2: Arrange the Atoms
- The atoms are bonded in the order: O–C–N. Carbon (C) will be the central atom because it has the lowest electronegativity among the three.
Step 3: Draw the Basic Skeleton Structure
- Connect the atoms with single bonds: O–C–N.
- Each single bond represents two electrons, so we use 2 electrons for each bond (total of 4 electrons).
Step 4: Distribute the Remaining Electrons
After the bonds, we have 11 electrons remaining (15 total – 4 used in bonds). Now, we distribute them as lone pairs:
- Oxygen will take 6 electrons to complete its octet, which leaves it with two lone pairs.
- Nitrogen will take 6 electrons to complete its octet, which leaves it with one lone pair.
- Carbon, as the central atom, will complete its octet with bonds, but it will have no lone pairs.
Step 5: Formal Charges and Resonance Structures
To minimize formal charges, let’s evaluate the formal charges for each atom. Formal charge (FC) is calculated using the formula: FC=Valence electrons−Lone pair electrons−12×Bonding electronsFC = \text{Valence electrons} – \text{Lone pair electrons} – \frac{1}{2} \times \text{Bonding electrons}FC=Valence electrons−Lone pair electrons−21×Bonding electrons
For the structure, we have three possibilities:
- Structure 1: Double Bond Between C and O, Single Bond Between C and N
- Oxygen: Formal charge = 6 – 4 (lone pair electrons) – 2 (shared electrons) = 0
- Carbon: Formal charge = 4 – 0 (lone pair electrons) – 6 (shared electrons) = 0
- Nitrogen: Formal charge = 5 – 2 (lone pair electrons) – 2 (shared electrons) = +1
- Structure 2: Single Bond Between C and O, Triple Bond Between C and N
- Oxygen: Formal charge = 6 – 6 (lone pair electrons) – 2 (shared electrons) = 0
- Carbon: Formal charge = 4 – 0 (lone pair electrons) – 6 (shared electrons) = 0
- Nitrogen: Formal charge = 5 – 2 (lone pair electrons) – 6 (shared electrons) = -1
- Structure 3: Triple Bond Between C and O, Single Bond Between C and N
- Oxygen: Formal charge = 6 – 2 (lone pair electrons) – 6 (shared electrons) = -1
- Carbon: Formal charge = 4 – 0 (lone pair electrons) – 6 (shared electrons) = 0
- Nitrogen: Formal charge = 5 – 2 (lone pair electrons) – 2 (shared electrons) = +1
Step 6: Identify the Major Contributor
The major contributor will be the structure where the formal charges are closest to zero. Structure 1 (with the double bond between O and C and the single bond between C and N) minimizes formal charges, with formal charges of 0 on O, C, and N, making it the most stable and the most likely resonance form.
Final Answer:
- The major contributor to the resonance is Structure 1.
- The Lewis structure for cyanate (OCN) has formal charges of 0 on oxygen, carbon, and nitrogen, and is stabilized by the double bond between oxygen and carbon, and the single bond between carbon and nitrogen.
