Molecular Orbital Theory
Overview
Molecular Orbital (MO) Theory describes bonding in terms of molecular orbitals formed by the combination of atomic orbitals.
Key Concepts
- Bonding and Antibonding Molecular Orbitals
- Bonding orbitals ($\sigma, \pi$): Lower energy, increase stability.
- Antibonding orbitals ($\sigma^*, \pi^*$): Higher energy, decrease stability.
- Molecular Orbital Diagrams
- Show energy levels of MOs.
- Used to determine bond order and magnetic properties.
Molecular Orbital Formation
- Atomic orbitals combine constructively $\rightarrow$ Bonding MO ($\sigma, \pi$).
- Atomic orbitals combine destructively $\rightarrow$ Antibonding MO ($\sigma^*, \pi^*$).
Bond Order Calculation
$\text{Bond Order} = \frac{(n_b - n_a)}{2}$ where:
- $n_b$ = number of electrons in bonding orbitals.
- $n_a$ = number of electrons in antibonding orbitals.
Examples
| Molecule | Electron Configuration | Bond Order | Magnetic? |
|---|---|---|---|
| $O_2$ | $(\sigma 2s)^2 (\sigma^* 2s)^2 (\sigma 2p)^2 (\pi 2p)^4 (\pi^* 2p)^2$ | 2 | Yes (paramagnetic) |
| $N_2$ | $(\sigma 2s)^2 (\sigma^* 2s)^2 (\sigma 2p)^2 (\pi 2p)^4$ | 3 | No (diamagnetic) |
| $F_2$ | $(\sigma 2s)^2 (\sigma^* 2s)^2 (\sigma 2p)^2 (\pi 2p)^4 (\pi^* 2p)^4$ | 1 | No (diamagnetic) |
Applications
- Predicts bond strength and magnetic properties.
- Explains delocalized bonding (e.g., in benzene).
- Used in spectroscopy and quantum chemistry.