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:
The molecule OCN⁻, also known as the cyanate ion, consists of oxygen (O), carbon (C), and nitrogen (N) atoms bonded in the order: O–C–N. Let’s break down the process of drawing the Lewis structure for OCN⁻:
Step 1: Count 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 (OCN⁻) means there is an extra electron, adding one more.
Total = 6 (O) + 4 (C) + 5 (N) + 1 (extra electron) = 16 valence electrons.
Step 2: Determine the bonding structure.
Since O, C, and N are bonded in that specific order, we can draw the skeleton structure with a single bond between O and C, and C and N.
Step 3: Distribute electrons to complete the octets.
- First, place a pair of electrons (a bond) between each pair of atoms. This uses 6 electrons, leaving us with 10 electrons to place.
- Now, give each atom enough electrons to complete their octet.
- Oxygen needs 6 more electrons (it will form a double bond with carbon in the final structure).
- Carbon needs 2 more electrons to complete its octet.
- Nitrogen needs 2 more electrons (it will also form a triple bond with carbon).
Step 4: Complete the octets and account for formal charges.
- Oxygen has 4 valence electrons and 2 lone pairs, forming a double bond with carbon.
- Carbon has 4 electrons from bonding (it needs 4 electrons total). It forms a triple bond with nitrogen, which completes its octet.
- Nitrogen has 5 valence electrons and 1 lone pair, forming a triple bond with carbon. This also completes its octet.
Formal Charges:
- Oxygen: In the final structure, oxygen has 2 lone pairs and 1 double bond with carbon. Formal charge on oxygen = 6 (valence electrons) – 4 (electrons around it in bonds) = -1.
- Carbon: Carbon has 0 lone pairs and is involved in two bonds (a double bond with oxygen and a triple bond with nitrogen). Formal charge on carbon = 4 (valence electrons) – 4 (electrons around it in bonds) = 0.
- Nitrogen: Nitrogen has 1 lone pair and is involved in a triple bond with carbon. Formal charge on nitrogen = 5 (valence electrons) – 4 (electrons around it in bonds) = 0.
Step 5: Identify resonance structures.
There are two major resonance structures for OCN⁻:
- Structure 1 (Major Contributor): A double bond between oxygen and carbon, and a triple bond between carbon and nitrogen. Oxygen has a negative charge.
- Structure 2 (Minor Contributor): A single bond between oxygen and carbon, and a triple bond between carbon and nitrogen. Oxygen has a negative charge, but this structure is less stable due to an incomplete octet on oxygen.
Step 6: Conclusion.
The major contributor is Structure 1, where oxygen has a double bond with carbon and a negative charge, while nitrogen and carbon form a triple bond. This structure has the most stable distribution of electrons and formal charges, with the negative charge on the most electronegative atom (oxygen).
