Molecular ShapesMolecular Shapes

Molecular ShapesMolecular Shapes

In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion.

We call this process Valence Shell Electron Pair Repulsion (VSEPR) theory.

The VSEPR Model – common Mol. Geo.The VSEPR Model – common Mol. Geo.

Electron-Domain Geo.Electron-Domain Geo.

The VSEPR ModelThe VSEPR Model

Predicting Molecular GeometriesPredicting Molecular Geometries

The VSEPR ModelThe VSEPR Model

Predicting Molecular GeometriesPredicting Molecular Geometries

The VSEPR ModelThe VSEPR Model

Predicting Molecular GeometriesPredicting Molecular Geometries

The VSEPR ModelThe VSEPR Model

Molecules with Expanded Valence ShellsMolecules with Expanded Valence Shells

The VSEPR ModelThe VSEPR Model

Molecules with Expanded Valence ShellsMolecules with Expanded Valence Shells

The VSEPR ModelThe VSEPR Model•We determine the electron domain geometry by looking at electrons around the central atom.•We name the molecular geometry by the positions of atoms.•We ignore lone pairs in the molecular geometry.

The VSEPR ModelThe VSEPR ModelThe Effect of Nonbonding Electrons and The Effect of Nonbonding Electrons and Multiple Bonds on Bond AnglesMultiple Bonds on Bond AnglesBy experiment, the H-X-H bond angle decreases on moving from C to N to O:

Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs.Therefore, the bond angle decreases as the number of lone pairs increase.

OHH

104.5O107O

NHH

HC

H

HHH109.5O

The VSEPR ModelThe VSEPR Model

The Effect of Nonbonding Electrons and The Effect of Nonbonding Electrons and Multiple Bonds on Bond AnglesMultiple Bonds on Bond AnglesSimilarly, electrons in multiple bonds repel more than electrons in single bonds.

C OCl

Cl111.4o

124.3o

The VSEPR ModelThe VSEPR ModelMolecules with More than One Central AtomMolecules with More than One Central AtomIn acetic acid, CH3COOH, there are three central atoms.We assign the geometry about each central atom separately.

Polarity of MoleculesPolarity of MoleculesPolar molecules interact with electric fields.If the centers of negative and positive charge do not coincide, then the molecule is polar.

Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic MoleculesExample: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent.

Polarity of MoleculesPolarity of MoleculesDipole Moments of Polyatomic MoleculesDipole Moments of Polyatomic Molecules

Covalent Bonding and Orbital OverlapCovalent Bonding and Orbital Overlap

Lewis structures and VSEPR do not explain why a bond forms.

How do we account for shape in terms of quantum mechanics?

What are the orbitals that are involved in bonding?

We use Valence Bond Theory:•Bonds form when orbitals on atoms overlap.•There are two electrons of opposite spin in the orbital overlap.

Covalent Bonding and Orbital OverlapCovalent Bonding and Orbital Overlap

Covalent Bonding and Orbital OverlapCovalent Bonding and Orbital Overlap

Hybrid OrbitalsHybrid Orbitalsspsp Hybrid Orbitals Hybrid OrbitalsConsider the BeF2 molecule (experimentally known to exist):•Be has a 1s22s2 electron configuration.•There is no unpaired electron available for bonding.•We conclude that the atomic orbitals are not adequate to describe orbitals in molecules.

We know that the F-Be-F bond angle is 180 (VSEPR theory).

We also know that one electron from Be is shared with each one of the unpaired electrons from F.

Formation of sp Hybrid OrbitalFormation of sp Hybrid Orbital

FG09_013.JPG

Formation of spFormation of sp2 2 OrbitalsOrbitalsFG09_015.JPG

Formation of spFormation of sp3 3 OrbitalsOrbitalsFG09_016.JPG

Bonding in Bonding in HH22OO

FG09_017.JPG

Hybridization in Ethylene Hybridization in Ethylene FG09_021.JPG

Table 9.4Table 9.4p 366p 366

TB09_005.JPG

Hybrid OrbitalsHybrid OrbitalsSummarySummaryTo assign hybridization:•draw a Lewis structure;•assign the electron pair geometry using VSEPR theory;•from the electron pair geometry, determine the hybridization; and •name the molecular geometry by the positions of the atoms.

Pi Bond Formation in Ethylene Pi Bond Formation in Ethylene FG09_022.JPG

Triple Bond in Acetylene Triple Bond in Acetylene FG09_023.JPG

Bonding in Benzene Bonding in Benzene FG09_028.JPG

Orbitals of Benzene Orbitals of Benzene FG09_029.JPG

Molecular OrbitalsMolecular OrbitalsSome aspects of bonding are not explained by Lewis structures, VSEPR theory and hybridization. (E.g. why does O2 interact with a magnetic field?; Why are some molecules colored?)

For these molecules, we use Molecular Orbital (MO) Theory.Just as electrons in atoms are found in atomic orbitals, electrons in molecules are found in molecular orbitals.Molecular orbitals:

each contain a maximum of two electrons;have definite energies;can be visualized with contour diagrams;•are associated with an entire molecule.

Molecular OrbitalsMolecular OrbitalsThe Hydrogen Molecule The Hydrogen Molecule

MO Electron ConfigurationsMO Electron ConfigurationsFG09_039.JPG