Concept: Molecular Orbital Theory and Bond Order

Bond order is a measure of the number of chemical bonds between two atoms. In molecular orbital theory, it is calculated as half the difference between the number of electrons in bonding molecular orbitals (N_b) and antibonding molecular orbitals (N_a).

Why (C) is correct:

1. Electron Configuration of CO: Carbon (C) has 6 electrons and Oxygen (O) has 8 electrons. Therefore, the CO molecule has a total of 6 + 8 = 14 electrons.

2. Molecular Orbital Diagram for CO (14 electrons): The molecular orbital configuration for molecules with 14 electrons (like N₂ and CO) is:

${\sigma }_{1s}^2{\sigma }_{1s}^{\ast 2}{\sigma }_{2s}^2{\sigma }_{2s}^{\ast 2}{\pi }_{2p}^4{\sigma }_{2p}^2$

3. Determine N_b and N_a:

• Number of electrons in bonding orbitals (N_b) = 2 (in ${\sigma }_{1s}$) + 2 (in ${\sigma }_{2s}$) + 4 (in ${\pi }_{2p}$) + 2 (in ${\sigma }_{2p}$) = 10
• Number of electrons in antibonding orbitals (N_a) = 2 (in ${\sigma }_{1s}^{\ast }$) + 2 (in ${\sigma }_{2s}^{\ast }$) = 4

4. Calculate Bond Order:

$\text{Bond Order}=\frac{1}{2}(N_b-N_a)=\frac{1}{2}(10-4)=\frac{1}{2}(6)=3$

Option Analysis:

• A) Zero: A bond order of zero indicates no bond exists between the atoms. This is incorrect as CO is a stable molecule.
• B) 2: A bond order of 2 would imply a double bond. This is incorrect; CO has a triple bond.
• D) 1: A bond order of 1 would imply a single bond. This is incorrect.

Correct Answer: (C)