Based on molecular orbital theory, which of the following molecules has a bond order of zero and is therefore predicted to be unstable?

• A. F2
• B. N2
• C. He2
• D. O2

Answer: (C)

Verified step by step guidance

1. Recall that bond order in molecular orbital theory is calculated using the formula: \(\text{Bond Order} = \frac{(\text{number of bonding electrons}) - (\text{number of antibonding electrons})}{2}\).
2. Determine the total number of valence electrons for the molecule in question. For He2, each helium atom has 2 valence electrons, so total electrons = 4.
3. Fill the molecular orbitals with these electrons according to the molecular orbital energy diagram for diatomic molecules, placing electrons first in bonding orbitals and then in antibonding orbitals.
4. Count the number of electrons in bonding orbitals and antibonding orbitals for He2. Since He2 has 4 electrons, they fill the \(\sigma_{1s}\) bonding orbital and the \(\sigma_{1s}^*\) antibonding orbital equally.
5. Calculate the bond order using the formula from step 1. For He2, the number of bonding and antibonding electrons are equal, resulting in a bond order of zero, which predicts that He2 is unstable.