Write the correct Lewis structure and assign a formal charge to each atom in fulminate ion, (CNO)
The Correct Answer and Explanation is:
Lewis Structure and Formal Charges for Fulminate Ion (CNO⁻)
Correct Lewis Structure of CNO⁻:
⁻
C ≡ N⁺–O⁻
This structure consists of:
- A carbon (C) triple bonded to nitrogen (N),
- Nitrogen (N) single bonded to oxygen (O),
- A negative formal charge on oxygen and a positive formal charge on nitrogen, for a net charge of –1.
Formal Charge Calculation:
Use the formula: Formal charge=Valence electrons−(Lone pair electrons+12×Bonding electrons)\text{Formal charge} = \text{Valence electrons} – (\text{Lone pair electrons} + \frac{1}{2} \times \text{Bonding electrons})
- Carbon (C):
- Valence electrons = 4
- Bonds: triple with N (6 electrons), no lone pairs
- FC = 4 – 0 – (6/2) = 4 – 3 = +1
- But in this structure, C has a single bond to O and triple to N, giving:
- 4 – 0 – (8/2) = 4 – 4 = 0
- Nitrogen (N):
- Valence electrons = 5
- Bonds: triple to C (6 electrons), single to O (2 electrons)
- FC = 5 – 0 – (8/2) = 5 – 4 = +1
- Oxygen (O):
- Valence electrons = 6
- Bonds: single to N (2 electrons), 6 non-bonding electrons
- FC = 6 – 6 – (2/2) = 6 – 6 – 1 = –1
Explanation
The fulminate ion (CNO⁻) is a linear anion composed of carbon, nitrogen, and oxygen atoms. Drawing its correct Lewis structure requires balancing octet rules, total valence electrons, and minimizing formal charges. The molecule has 16 valence electrons: 4 (C) + 5 (N) + 6 (O) + 1 (extra for the negative charge).
Multiple resonance forms are possible, but the most stable structure has carbon triple bonded to nitrogen and nitrogen single bonded to oxygen. This structure satisfies the octet rule for each atom and distributes the formal charges in a way that the total equals –1, consistent with the ion’s overall charge.
In this arrangement:
- Carbon (C) shares 8 electrons via one triple bond and one single bond, yielding a formal charge of 0.
- Nitrogen (N), bonded to both C and O, ends up with a formal charge of +1.
- Oxygen (O), with three lone pairs and a single bond, carries a –1 formal charge.
Though nitrogen having a +1 formal charge might seem unstable, this is balanced by the more electronegative oxygen carrying the negative charge. This structure is further supported by experimental data and molecular orbital calculations, showing that fulminate ion tends to exist with a negative charge on oxygen, not nitrogen.
This structure is important in understanding the chemistry of explosive compounds like mercury fulminate, where this ion plays a central role. Its instability is partly due to this polar formal charge distribution and high electron density on oxygen, contributing to its explosive character.
