Here’s a explanation of Molecular Orbital Theory (MOT):
1. Definition
- Molecular Orbital Theory (MOT) explains how atomic orbitals combine to form molecular orbitals when atoms bond together.
- These molecular orbitals belong to the whole molecule, not just to a single atom.
- Electrons in these orbitals are spread over the entire molecule rather than being confined between two atoms.
2. Key Points
- Combination of Atomic Orbitals (AOs):
- Atomic orbitals from bonded atoms combine constructively or destructively.
- Constructive combination → bonding molecular orbital (lower energy)
- Destructive combination → antibonding molecular orbital (higher energy)
- Electron Distribution:
- Electrons fill molecular orbitals according to:
- Pauli Exclusion Principle (max 2 electrons per orbital, opposite spins)
- Hund’s Rule (fill degenerate orbitals singly first)
- Electrons fill molecular orbitals according to:
- Types of Molecular Orbitals:
- Bonding (σ, π) → stabilize the molecule
- Antibonding (σ, π)** → destabilize the molecule
- Non-bonding (n) → electrons stay on one atom, neither stabilizing nor destabilizing
3. Importance of MOT
- Explains bond order (strength and stability of bonds)
- Predicts magnetic properties (paramagnetic vs diamagnetic)
- Explains electron delocalization in molecules with resonance
- Helps understand spectroscopy and chemical reactivity
Example:
- In H₂, the 1s orbitals of two hydrogen atoms combine to form:
- A σ₁s bonding orbital → electrons stabilize the molecule
- A σ*₁s antibonding orbital → empty in H₂, so it doesn’t destabilize