Concept: Bond order is a fundamental concept in molecular orbital theory that describes the number of chemical bonds between a pair of atoms. It provides an indication of the stability of a molecule.

Why (D) is correct: According to Molecular Orbital Theory (MOT), the bond order of a molecule is defined as half the difference between the number of electrons in bonding molecular orbitals (N_b) and the number of electrons in antibonding molecular orbitals (N_a). A higher bond order generally indicates greater bond strength and shorter bond length.

The formula for bond order is:

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

Where:

• $N_b$ = Number of electrons in bonding molecular orbitals
• $N_a$ = Number of electrons in antibonding molecular orbitals

Option Analysis:

• A) The difference between the number of electrons in bonding and antibonding orbitals: This would be $N_b-N_a$, which is twice the bond order, not the bond order itself.
• B) Total number of electrons in bonding and antibonding orbitals: This sum ($N_b+N_a$) does not represent the bond order; it represents the total number of valence electrons involved in molecular orbitals.
• C) Twice the difference between the number of electrons in bonding and antibonding orbitals: This would be $2(N_b-N_a)$, which is four times the bond order.
• D) Half the difference between number of electrons in bonding and antibonding orbitals: This is the correct definition of bond order as per Molecular Orbital Theory.

Correct Answer: (D)