Valence Bond Theory

Valence Bonding Theory

Incorporates atomic orbitals of atoms, use of quantum theory

Dealing ONLY with orbitals involved in a covalent bond

Electron clouds of individual atoms OVERLAP when atoms form chemical bond

Valence Bonding Theory (cont.)

Covalent bond formation involves 2 electrons of opposite spins paired in an overlap of atomic orbitals with high electron density (p. 403)

Strong chemical bonds with more atomic overlap

Optimal orbital overlap---gives maximum bond strength

Key Points to Valence Bond Theory

1) Electrons generally like to stay in their normal orbitals (as if in isolated atom).

2) Electrons involved in a chemical bond hang out in an atomic orbital overlap.

3) The orientation of the atomic orbitals of the central atom determine its molecular geometry

4) Atomic orbitals with more than one axis (p orbital)– maximum overlap found when imaginary line connecting the atoms passes through the maximum overlap.

Hybridization

Electron configuration of Carbon (show only valence electrons)

**Atoms in a chemical bond do NOT have the same orbitals as in isolation **

Hybridization—General

Combines/Mixes atomic orbitals

Not an actual process, a method of visualizing and explaining molecular structure

Hybrid orbitals—

Mix of 2 types of orbitals

Back to Carbon

Combine/mix together “s ” and “p” orbitals

Result is 4 orbitals with same shape and energy

Hybridization—General (cont.)

Applies mostly to central atoms

# hybrid orbitals = # of combined orbitals

Hybrid orbitals can interact/overlap with regular or hybrid orbitals

Hybrid orbitals have different shapes and orientations from regular orbitals

Hybrid Orbitals

1) sp3 hybrid orbitals

Mixture of 1 s orbital and 3 p orbitals

Results in 4 hybrid orbitals

Tetrahedral structure, 109.5° bond angle

Ex. CH4—C

NH3---N

Hybrid Orbitals

2) sp2 hybrid orbitals

Seen a lot with double bonds

Mixture of 1 s orbital and 2 p orbitals

results in 3 hybrid orbitals

Trigonal planar structure, 120° bond angle

Ex. BF3

Hybrid Orbitals

3) sp hybrid orbitals

Seen a lot with triple bonds

Mixture of 1 s orbital and 1 p orbital

Results in 2 hybrid orbitals, 2 empty p orbitals

Linear structure, 180° bond angle

Ex. BeCl2

How do we discuss hybridization????

Focus on central atom

Each atomic orbital results in a hybrid orbital in chemical bond

ALL hybrid orbitals have electrons (lone or bonded pair)

Hybrid orbital orientation = electron group geometry

1) Determine electron group geometry

2) Find appropriate hybridization

3) Describe how the orbitals overlap to give molecular geometry

Electron Group

Electron Group Geometry

Hybridization

2 Linear sp

3 Trigonal planar

sp2

4 Tetrahedral

sp3

Ex. 1 Determine the hybridization of the central atom in…..

H3O+

Ex. 2 Determine the hybridization of the central atom in…..

NH3

Ex. 3 Determine the hybridization of the central atom in…..

CO2

Ex. 4 Determine the hybridization of the central atom in…..

O2

Homework Molecular Geometry (p. 427 #21, 23, 25,

27)

Hybridization (Read pp. 402-408, p. 428 #45-46)