Draw the Lewis structure for the OCN- molecules and determine if each molecule is polar or nonpolar.
The Correct Answer and Explanation is :
To draw the Lewis structure of the cyanate ion (OCN⁻) and determine its polarity, let’s go through the steps systematically.
Step 1: Count the Total Valence Electrons
- Oxygen (O) has 6 valence electrons.
- Carbon (C) has 4 valence electrons.
- Nitrogen (N) has 5 valence electrons.
- The cyanate ion has a negative charge (⁻), so we add 1 electron to the total count.
The total number of valence electrons is:
[ 6 (\text{O}) + 4 (\text{C}) + 5 (\text{N}) + 1 (\text{negative charge}) = 16 \text{ electrons} ]
Step 2: Arrange the Atoms
The general structure of OCN⁻ suggests that oxygen is bonded to carbon, and carbon is bonded to nitrogen, with the negative charge located on oxygen in many resonance forms. A reasonable initial structure would have:
- A single bond between O and C.
- A triple bond between C and N.
- The extra electron would likely be placed on the oxygen atom, which is more electronegative.
Step 3: Distribute the Electrons
The Lewis structure would look like this:
O=C≡N⁻- Oxygen has two lone pairs of electrons.
- Carbon has no lone pairs, as it’s involved in bonds with both oxygen and nitrogen.
- Nitrogen has one lone pair.
Step 4: Check Formal Charges
To confirm this structure, we check the formal charges:
- Oxygen: 6 valence electrons – (2 lone pairs + 1 bond) = -1.
- Carbon: 4 valence electrons – (0 lone pairs + 4 bonding electrons) = 0.
- Nitrogen: 5 valence electrons – (1 lone pair + 3 bonding electrons) = 0.
This structure satisfies the octet rule and places the negative charge on oxygen, as expected.
Step 5: Determine Polarity
The molecule has a linear structure with oxygen at one end, carbon in the middle, and nitrogen at the other. The electronegativity difference between oxygen (3.44) and nitrogen (3.04) creates a dipole, with oxygen being partially negative and nitrogen partially positive. The molecule’s linear geometry ensures that the dipoles do not cancel out, making the molecule polar.
Conclusion
The Lewis structure for OCN⁻ is linear, with a negative charge on the oxygen atom. The molecule is polar because the electronegativity differences between oxygen and nitrogen create a dipole, and the linear shape ensures that the dipoles do not cancel.