What is the average bond order of the polyatomic ion CN−?

• A. 1.5
• B. 3
• C. 2
• D. 1

Answer: (B)

Verified step by step guidance

1. Recall that bond order is defined as half the difference between the number of bonding electrons and antibonding electrons: \(\text{Bond order} = \frac{\text{Number of bonding electrons} - \text{Number of antibonding electrons}}{2}\).
2. Determine the total number of valence electrons for the CN⁻ ion. Carbon has 4 valence electrons, nitrogen has 5, and the negative charge adds 1 more electron, so total electrons = 4 + 5 + 1 = 10.
3. Construct the molecular orbital (MO) diagram for CN⁻, considering the combination of atomic orbitals from carbon and nitrogen, and fill the molecular orbitals with the 10 valence electrons according to the Aufbau principle and Pauli exclusion principle.
4. Count the number of electrons in bonding molecular orbitals and antibonding molecular orbitals from the MO diagram you constructed.
5. Calculate the bond order using the formula from step 1 by substituting the numbers of bonding and antibonding electrons you found.