Molecular Orbital (MO) Theory is a way to explain how atoms combine to form molecules by looking at electrons in the whole molecule, not just between two atoms.
Here’s the explanation:
1. Key Idea
- Atomic orbitals from different atoms combine to form molecular orbitals.
- These molecular orbitals belong to the entire molecule, not a single atom.
2. Types of Molecular Orbitals
- Bonding orbitals (σ, π) → lower energy → electrons here stabilize the molecule.
- Antibonding orbitals (σ, π)** → higher energy → electrons here destabilize the molecule.
3. Rules for Electron Filling
- Aufbau principle → fill from lowest energy to highest.
- Pauli exclusion principle → max 2 electrons per orbital, opposite spins.
- Hund’s rule → fill degenerate orbitals singly first.
4. Bond Order
- Bond order = (electrons in bonding − electrons in antibonding) ÷ 2
- Positive bond order → stable molecule
- Zero bond order → molecule does not exist
5. Why It’s Useful
- Explains bond strength and stability.
- Explains magnetism (paramagnetic vs diamagnetic).
- Can predict existence of molecules that Lewis structures can’t.
In short:
Molecular Orbital Theory says electrons are spread over the whole molecule in molecular orbitals, and their arrangement explains bond strength, stability, and magnetic properties.