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Postulates of Molecular Orbital Theory

Molecular Orbital Theory (MOT) describes molecular bonding through the wave nature of electrons, emphasizing the combination of atomic orbitals to form molecular orbitals. Key postulates include the Linear Combination of Atomic Orbitals (LCAO), the types of molecular orbitals (bonding, antibonding, non-bonding), and the principles governing electron filling. The energy order of molecular orbitals varies with atomic number, affecting bond strength and magnetic properties based on electron configuration.

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252 views11 pages

Postulates of Molecular Orbital Theory

Molecular Orbital Theory (MOT) describes molecular bonding through the wave nature of electrons, emphasizing the combination of atomic orbitals to form molecular orbitals. Key postulates include the Linear Combination of Atomic Orbitals (LCAO), the types of molecular orbitals (bonding, antibonding, non-bonding), and the principles governing electron filling. The energy order of molecular orbitals varies with atomic number, affecting bond strength and magnetic properties based on electron configuration.

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Postulates of Molecular

Orbital Theory
Molecular Orbital Theory
(MOT) explains bonding in
molecules on the basis of
wave nature of electrons. It
considers that atomic
orbitals of atoms combine to
form molecular orbitals
which belong to the whole
molecule.

Main Postulates:
1. Linear Combination of
Atomic Orbitals (LCAO):
Atomic orbitals of
comparable energy and
proper symmetry combine to
form molecular orbitals.
2. Number of Molecular
Orbitals: Combination of 'n'
atomic orbitals gives 'n'
molecular orbitals.
3. Types of Molecular
Orbitals:
- Bonding orbital: Lower
energy, increases stability.
- Antibonding orbital:
Higher energy, decreases
stability.
- Non-bonding orbital:
Energy remains same as
atomic orbital.
4. Shape of Molecular
Orbitals: Depending on
symmetry, orbitals are called
sigma (σ) and pi (π).
5. Filling of Molecular
Orbitals: Electrons are filled
according to:
- Aufbau Principle
- Pauli Exclusion Principle
- Hund’s Rule of Maximum
Multiplicity.
Energy Order of Molecular
Orbitals (Increasing Order)
The relative energy of
molecular orbitals is not the
same for all molecules. It
depends on atomic number
(Z).

For molecules with Z ≤ 7 (H₂


to N₂):
σ(1s) < σ*(1s) < σ(2s) <
σ*(2s) < π(2px) ≈ π(2py) <
σ(2pz) < π*(2px) ≈ π*(2py) <
σ*(2pz)
For molecules with Z ≥ 8 (O₂
to Ne₂):
σ(1s) < σ*(1s) < σ(2s) <
σ*(2s) < σ(2pz) < π(2px) ≈
π(2py) < π*(2px) ≈ π*(2py) <
σ*(2pz)

Difference between Atomic


Orbital and Molecular Orbital
Atomic Orbital:
- Represents region of space
around nucleus of single
atom.
- Formed by wave function of
one electron.
- Can hold maximum 2
electrons with opposite
spins.

Molecular Orbital:
- Represents region of space
around nuclei of bonded
atoms.
- Formed by linear
combination of atomic
orbitals.
- Can hold maximum 2
electrons with opposite
spins.
Difference between Bonding
and Non-Bonding Electrons
Bonding Electrons:
- Participate in formation of
bond.
- Present in molecular
bonding orbitals.
- Increase stability of
molecule.

Non-Bonding Electrons:
- Do not participate in bond
formation.
- Present in orbitals localized
on single atom.
- Do not affect bond order
directly.

Molecular Electronic
Configuration of Homo-
Diatomic Molecules
Bond Order

Bond Order = (Number of


bonding electrons - Number
of antibonding electrons) / 2

- Higher bond order means


greater bond strength and
shorter bond length.
- If bond order = 0,
molecule is unstable.

Magnetic Property

- If unpaired electrons are


present → Paramagnetic.
- If all electrons are paired →
Diamagnetic.

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