Draw Lewis structures and MO diagrams for CN+, CN, and CN-. According to the Lewis model, which species is most stable? According to MO theory, which species is most stable? Do the two theories agree?
The correct answer and explanation is:
Correct Answer:
According to the Lewis model, CN⁻ is the most stable.
According to Molecular Orbital (MO) theory, CN is the most stable.
The two theories do not agree.
Explanation (300 words):
To compare CN⁺, CN, and CN⁻, we first draw their Lewis structures and analyze their Molecular Orbital (MO) diagrams.
Lewis Structures:
The total valence electrons are calculated by summing the valence electrons of carbon (4) and nitrogen (5), and adjusting for charge:
- CN⁺: 4 + 5 – 1 = 8 electrons
- CN: 4 + 5 = 9 electrons
- CN⁻: 4 + 5 + 1 = 10 electrons
Lewis Structures:
- CN⁺: C≡N⁺ with no lone pairs on carbon, and two lone pairs on nitrogen
- CN: C≡N with one unpaired electron
- CN⁻: C≡N⁻ with a lone pair on carbon and two lone pairs on nitrogen
In the Lewis model, structures with complete octets and minimal formal charge are more stable. CN⁻ has a full octet on both atoms and minimized formal charges, making it the most stable by this model.
Molecular Orbital (MO) Diagrams:
The MO diagram for diatomic species with fewer than 14 total electrons follows the 2s–2s*–2p–2pπ–2pσ order.
- CN⁺: 13 electrons → bond order = (8 bonding – 5 antibonding)/2 = 1.5
- CN: 14 electrons → bond order = (8 bonding – 4 antibonding)/2 = 2
- CN⁻: 15 electrons → bond order = (8 bonding – 5 antibonding)/2 = 1.5
The highest bond order implies the strongest bond and highest stability. According to MO theory, CN is the most stable because it has the highest bond order of 2.
Comparison:
The Lewis theory predicts CN⁻ as most stable due to full octets and charge distribution.
The MO theory predicts CN as most stable due to the strongest bond.
Therefore, the two models do not agree in this case.