Importance: One of the most advanced theories in present-day chemistry used to explain various kinds of points that previous theories couldn't.
Study Focus: Primarily focuses on diatomic molecules, relevant for IIT JEE Main and Advanced syllabus.
Molecular Orbital (MO) Theory
Basis: According to the Molecular Orbital Theory (MOT), molecules are expected to form when atomic orbitals combine.
Example of Molecular Oxygen (O2)
Bond Formation: Oxygen molecule (O2) with two unpaired electrons in its atomic state; MOT explains its paramagnetic nature.
Valence Bond (VB) Theory vs. MOT: VB Theory fails to explain the paramagnetic behavior of O2, which MOT successfully does using molecular orbitals.
Key Points of MOT
Appropriate Combination of Atomic Orbitals
Linear Combination of Atomic Orbitals (LCAO): The concept where atomic orbitals combine to form molecular orbitals.
Combination Rules: Atomic orbitals combine in a constructive (bonding) and destructive (anti-bonding) manner forming equal numbers of molecular orbitals.
Characteristics of Molecular Orbitals
Existence: Only molecular orbitals exist after combination; atomic orbitals lose their existence.
Electron Distribution: Electrons in molecular orbitals are under the influence of all nuclei in the molecule, unlike atomic orbitals.
Terms: Molecular orbitals are called 'polycentric' because the electron probability is influenced by multiple nuclei.
Types of Molecular Orbitals
Bonding Molecular Orbitals (BMO)
Lower energy, more stable from which it's formed.
Increased electron density between nuclei, leading to stronger bonding.
Anti-bonding Molecular Orbitals (ABMO) / Anti-bonding MOs
Higher energy, less stable.
Decreased electron density between nuclei, leading to weaker bonding.
Combination of Atomic Orbitals
Constructive Interference: Forms Bonding Molecular Orbitals (BMOs), indicated by wave functions combining in phase.
Destructive Interference: Forms Anti-bonding Molecular Orbitals (ABMO), indicated by wave functions combining out of phase, reducing electron density between nuclei.
Energy Levels
Bonding MOs: More stable, lower energy compared to atomic orbitals.
Anti-bonding MOs: Less stable, higher energy compared to atomic orbitals.
Conditions for Combination
Symmetry and Energy: Only atomic orbitals of similar energy and symmetry will combine efficiently.
Practical Examples: Linear combinations like sigma (σ) and pi (π) bonds formed from s-p and p-p orbital combinations.
Conclusion
Next Lecture: Detailed discussion on sigma and pi bonding in Molecular Orbital Theory.
Remember: Molecular Orbital Theory better explains phenomena such as magnetism and electron distribution in molecules.