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 cyanate ion, OCN-, with two double bonds, we need to follow these steps:
- Determine the total number of valence electrons:
- Oxygen (O) has 6 valence electrons.
- Carbon (C) has 4 valence electrons.
- Nitrogen (N) has 5 valence electrons.
- The negative charge (–1) adds one more electron to the total.
6+4+5+1=166 + 4 + 5 + 1 = 166+4+5+1=16 electrons. - Arrange the atoms:
Carbon is usually the central atom in such a structure because it can form more bonds than nitrogen or oxygen. Thus, we place C in the middle, bonded to O and N. - Draw the bonds:
Begin by placing a single bond between C and O, and another single bond between C and N. This uses up 4 electrons (2 electrons per bond). - Distribute the remaining electrons:
You have 12 electrons left to distribute. Oxygen and nitrogen both require 8 electrons to complete their octets. Start by placing lone pairs on O and N to complete their octets:- Oxygen needs 6 more electrons to complete its octet.
- Nitrogen needs 6 more electrons to complete its octet.
- Form the double bonds:
Now, since you have 12 electrons left, you can form double bonds. Move a lone pair from oxygen to form a double bond with carbon, and another lone pair from nitrogen to form a double bond with carbon. This satisfies the octet rule for each atom. - Assign formal charges:
Formal charge is calculated using the formula:
Formal charge=(Valence electrons)−(Lone pair electrons)−(Bonded electrons/2)\text{Formal charge} = (\text{Valence electrons}) – (\text{Lone pair electrons}) – (\text{Bonded electrons}/2)Formal charge=(Valence electrons)−(Lone pair electrons)−(Bonded electrons/2) For each atom:- Oxygen: 6 valence electrons, 4 electrons from 2 bonds, 2 electrons from 1 lone pair. Formal charge = 6−4−2=06 – 4 – 2 = 06−4−2=0.
- Carbon: 4 valence electrons, 4 electrons from 2 double bonds. Formal charge = 4−4−0=04 – 4 – 0 = 04−4−0=0.
- Nitrogen: 5 valence electrons, 4 electrons from 2 bonds, 1 lone pair. Formal charge = 5−2−3=05 – 2 – 3 = 05−2−3=0.
Thus, the formal charges on oxygen, carbon, and nitrogen are all 0.
The correct answer is d. O = 0, C = 0, N = 0.
Why this structure is correct:
- Each atom has a formal charge of 0, which is optimal for the structure’s stability.
- The Lewis structure satisfies the octet rule for all atoms and uses the available electrons efficiently.
