Molecular Orbital Theory (MOT)
Describes bond formation using molecular orbitals, energy level diagrams, bond order, and magnetic properties.
What is Molecular Orbital Theory (MOT)?
Orbitals formed by the linear combination of atomic orbitals, which are delocalized over the entire molecule.
Key formula / rule: Bond Order
Key points
- Understand the formation of molecular orbitals.
- Differentiate between bonding and antibonding molecular orbitals.
- Construct MO diagrams for simple diatomic molecules.
- Calculate bond order and relate it to bond strength.
Common exam trap
Confusing atomic orbitals with molecular orbitals.
Definitions
- Term
Molecular Orbitals (MOs)
- Meaning
Orbitals formed by the linear combination of atomic orbitals, which are delocalized over the entire molecule.
- Term
Bonding Molecular Orbital
- Meaning
A molecular orbital formed by constructive interference of atomic orbitals, resulting in increased electron density between nuclei and lower energy.
- Term
Antibonding Molecular Orbital
- Meaning
A molecular orbital formed by destructive interference of atomic orbitals, resulting in a node between nuclei and higher energy.
- Term
Bond Order
- Meaning
A measure of the number of chemical bonds between two atoms, calculated as half the difference between the number of electrons in bonding and antibonding molecular orbitals.
- Term
Paramagnetism
- Meaning
A property of substances that are weakly attracted by an external magnetic field, due to the presence of unpaired electrons.
- Term
Diamagnetism
- Meaning
A property of substances that are weakly repelled by an external magnetic field, due to the absence of unpaired electrons.
Learning objectives
Understand the formation of molecular orbitals.
Differentiate between bonding and antibonding molecular orbitals.
Construct MO diagrams for simple diatomic molecules.
Calculate bond order and relate it to bond strength.
Predict magnetic properties of molecules using MOT.
Formulae
- Name
Bond Order
- Note
Nb = number of electrons in bonding molecular orbitals, Na = number of electrons in antibonding molecular orbitals.
- Expression
B.O. = 1/2 (Nb - Na)
Prerequisites
Atomic Structure
Electronic Configuration
Atomic Orbitals (s, p, d)
Linear Combination of Atomic Orbitals (LCAO)
Common mistakes
Confusing atomic orbitals with molecular orbitals.
Incorrectly applying the s-p mixing order for MO energy levels.
Miscalculating bond order by not counting bonding and antibonding electrons correctly.
Assuming all molecules with even numbers of electrons are diamagnetic.
Keywords
Molecular Orbital Theory
MOT
Molecular Orbitals
Bonding Orbitals
Antibonding Orbitals
Bond Order
Paramagnetism
Diamagnetism
MO Diagram
LCAO
s-p mixing
Practice preview
What is the bond order of the O2 molecule according to MOT?…
easy
Which of the following species is paramagnetic based on MOT?…
medium
If the bond order of a molecule M2 is 1, what is the electronic configuration of its bonding and antibonding molecular orbitals?…
hard