Valence Bond Theory

Why is the Valence Bond theory useful?

1. Provides qualitative picture of bonding

2. Useful for considering a series of molecules w/ one part in common (eg, functional group)

3. Used in infrared of large molecules

What is resonance in VB theory?

Used in VB theory when no single Lewis-like structure represents the bonding in a molecule

How do resonance forms contribute to the covalent bonding? (in H2)

H-H covalent form most important

Ionic forms EQUALLY important

How does VB theory work for H2?

1s orbitals between H atoms interact, forms WAVEFUNCTION for the molecule

Interactions causes a σ-bond orbital in region between the 2 atoms

How is the wave function expressed for 2 H atoms that are NOT interacting?

How is the wavefunction expressed for 2 H atoms that are close together?

How is the wavefunction of a molecule expressed when ionic forms are included?

How would VB theory work for a molecule like N2?

1. Interaction between 2p orbital

2. Three p orbitals on each atom

3. Each contain 1 electron

4. 3 covalent bonds formed

What is hybridisation?

Mixing of atomic orbitals w/ diff ANGULAR MOMENTUM (2ndary) quantum numbers

Occurs in N2

How does hybridisation occur between s and p orbitals?

How does hybridisation occur in N2?

Greater overlap between atomic orbitals lead to more STABLE bonding orbitals

How is the VB wavefunction expressed for a HF molecule?

What are the disadvantages to the VB theory?

1. Works best for organic molecules

2. Doesn’t account for changes in energy for electrons in different orbitals

3. Assumes all electrons are localised in specific regions