an mo diagram for a certain diatomic molecule shows that the molecule has 8 electrons in bonding orbitals and 3 electrons in antibonding orbitals. what is the bond order of this molecule?

The bond order of this molecule is 2.5.

How to see it step by step

For molecular orbital (MO) theory, bond order is given by the standard formula:

Bond order=(number of bonding electrons)−(number of antibonding electrons)2\text{Bond order}=\frac{(\text{number of bonding electrons})-(\text{number of antibonding electrons})}{2}Bond order=2(number of bonding electrons)−(number of antibonding electrons)​

You are told:

• Bonding electrons = 8
• Antibonding electrons = 3

So:

Bond order=8−32=52=2.5\text{Bond order}=\frac{8-3}{2}=\frac{5}{2}=2.5Bond order=28−3​=25​=2.5

Quick intuition

• More electrons in bonding orbitals strengthen the bond.
• Electrons in antibonding orbitals weaken it.
• A bond order of 2.5 means the bond is stronger than a double bond but not as strong as a triple bond.

TL;DR: With 8 bonding and 3 antibonding electrons, bond order = (8−3)/2=2.5(8-3)/2=2.5(8−3)/2=2.5.