Chapter 6: Bonding Theories

Bonding Theories

Provide descriptions of how bonds are formed in molecules or polyatomic ions. Based on Quantum Mechanical approaches. Electrons reside in quantum mechanical orbitals.

Valence Bond Theory

Atomic based orbitals. Easily visualized orbital interactions. The valence electrons of the atoms in a molecule reside in atomic orbitals (s, p, d or hybridized orbitals).

Molecular Orbital Theory

Molecular based orbitals. More complex but provides better electronic description.

What does the Valence Bond Theory state?

A chemical bond results from overlap of two atomic or hybrid orbitals. Typically, each orbital contributes 1 electron. Each atom retains its own orbitals, but the electron pair in the overlapping orbitals is shared by both atoms. It is possible for an empty orbital and a filled orbital to overlap, but two filled orbitals will not overlap. The greater the amount of orbital overlap, the stronger the bond. The shape of the molecule is determined by the geometry of the overlapping orbitals.

Hybridization of Orbitals

Pauling found that wave functions of s and p orbitals (and other orbitals) could be combined to form ________ ________. ________ ________ minimize the energy of the molecule by maximizing the orbital overlap in a bond. Hybrid orbital formation is driven by energy considerations.

The hybridization of the central atom

The molecule’s electron geometry indicates…..

The number of hybrid orbitals present

The steric number indicates…..

sp³d² hybridization. Steric number = 6.

Octahedral Electron Geometry

sp³d hybridization. steric number = 5

trigonal bipyramidal electron geometry

sp³ hybridization. steric number = 4. Hybrid orbitals are degenerate in energy.

Tetrahedral electron geometry

sp² hybridization. steric number = 4.

Trigonal Planar electron geometry

Sigma (σ) bonds

Head-to-head overlap of orbitals. Between s-to-s, p-to-p, hybrid-to-hybrid, s-to-hybrid, etc. Strongest type of covalent bond

Pi (π) bonds

Sideways overlap of orbitals. Between unhybridized parallel p orbitals. Hybrid orbitals do not form __ bonds. Weaker than a σ bond. Like a p orbital, a __ has two lobes. Found associated with sigma bonds.

Example of sigma bond

C—C bond dissociation energy: 347 kJ/mol

Example of sigma + pi bond

C=C bond dissociation energy: 611 kJ/mol

sp hybridization. Steric number = 2.

Linear Electron Geometry

Linear Electron Geometry (sp Hybrid Orbital)

Unhybridized p orbitals form pi bonds via side-to-side overlap. Atoms with two sp orbitals can form two pi bonds. These are two unhybridized p orbitals on a sp-hybridized atom.

Molecular Orbital Theory

applies well to delocalized pi bonds, such as the double bond in ozone. The electrons are not confined to being shared by just two atoms but instead are spread across the entire molecule. Does not need resonance structures which involve delocalized electrons.