Two Bonding Theories Valence Bond Theory (localized electron model) –Electrons in a molecule still occupy orbitals of individual atoms. Molecular Orbital Theory –Says that atomic orbitals no longer exist. Molecular orbitals are available for occupation by electrons.

Valence Bond Theory Electrons in a molecule still occupy orbitals of individual ATOMS. Half-filled orbitals of two atoms overlap.

VB Theory: Formation of H 2 S

VB Theory: Carbon 2s2p Would suggest only two bonds possible

VB Theory: Carbon 2s2p What if we just bump an electron up? Would suggest two different types of bonds form (one with an s orbital and three with p orbitals)

Valence Bond Theory Atomic Orbitals Hybridize. These are still atomic orbitals. Two or more nonequivalent orbitals of same atom combine. When X number of atomic orbitals hybridize, X number of hybrid orbitals will result.

2s2p Four atomic orbitals (one s and three p) hybridize into four degenerate hybrid orbitals, as a group called sp 3 2sp 3 VB Theory: Carbon

How will four degenerate orbitals exist around the central atom?

VB Theory: NH 3

VB Theory: sp 3 hybridization

VB Theory: sp 2 hybridization

C 2 H 4, sp 2 hybridization

Valence Bond Theory Hybridization is directly connected to the Electron-pair Geometry sp 3 sp 2 sp = linear sp 3 d = trig bipryramidal sp 3 d 2 = octahedral = tetrahedral = trigonal planar

Molecular Orbital Theory Says that atomic orbitals no longer exist. Molecular orbitals are available for occupation by electrons. Atomic orbitals from all atoms overlap and become an equivalent number of molecular orbitals.

Molecular Orbital Theory Constructive vs. Destructive Interference

Molecular Orbital Theory

Molecular Orbital Energy Diagram for H 2

Molecular Orbital Energy Diagram for He 2

Molecular Orbital Theory

Sigma Bond – electron density along line joining two nuclei - from the side, would look like an s orbital - first bond that forms between two nuclei (single bond) bonding:  1s antibonding:  * 1s  * 2p  2p

Molecular Orbital Theory Pi Bond – formed by sideways overlapping of orbitals, electron density above and below plane of nuclei – second / third bond that forms between two nuclei (double / triple bonds) - less overlap means weaker than sigma bonds

Molecular Orbital Theory The simplest example, H 2 : 1s orbitals from each hydrogen overlap Form one bonding molecular orbital And one antibonding molecular orbital Two orbitals overlap to form a bonding and an antibonding molecular orbital

from Susan Piepho, Sweet Briar College