Chemistry 101 : Chap. 9Chemistry 101 : Chap. 9

Molecular Geometry and Bonding Theories

(1) Molecular Shape

(2) The VSEPR Model

(3) Molecular Shape and Molecular Polarity

(4) Covalent Bonding and Orbital Overlap

(5) Hybrid Orbitals

(6) Multiple Bonds

Molecular ShapeMolecular Shape

3-dimensional rendering of a molecule (example = CH4)

Structural Formula

Ball-and-stick model

Perspective drawing

Space-filling model

tetrahedral

Molecular ShapeMolecular Shape

Five fundamental shapes of ABn

bond angle

Molecular ShapeMolecular Shape

Derivatives from the ABn geometries :

Additional molecular shapes can be obtained by removing

corner atoms from the basic shape

VSEPR ModelVSEPR Model

Valence-Shell Electron-Pair Repulsion :

The geometry around a central atom is determined by the

number of bonding pair of electrons and non-bonding pair

(lone pair) electrons surrounding the atom.

Electron DomainElectron Domain

Electron Domain:

A region surrounding the central atom in which the electrons

(bonding or nonbonding pair) are likely to be found

VSEPR and Electron Domain :

The electron domains in an atom get as far apart from

each other as possible

The best arrangement of electron domains is the one that

minimizes the repulsions among them.

Electron DomainElectron Domain

Example : How many electron domains do NH3 and O3 have?

Electron Domain GeometryElectron Domain Geometry

Electron-Domain Geometry : The arrangement of electron

domains about the central atom of a molecule or ion

Electron Domain GeometryElectron Domain Geometry

Molecular GeometryMolecular Geometry

Molecular Geometry : The arrangement of only the atoms around

the central atom

(1) If all the electron domains arises from bonding pair,

electron-domain geometry = molecular geometry

(2) If one or more electron-domains arise from nonbonding pairs,

ignore such domain to predict the molecular geometry

Molecular GeometryMolecular Geometry

Procedure to determine a molecular structure

(1) Draw a Lewis structure and count the total number of

electron domains around the central atom

(2) Determine the electron-domain geometry by arranging the

electron domains to minimize the repulsions among them.

(3) Use the arrangement of the bonded atoms to determine

the molecular geometry

Molecular GeometryMolecular Geometry

Example : Use VSEPR model to predict the molecular geometry of

O3, H2O, and NH4+

Molecular GeometryMolecular Geometry

Effect of multiple bonding on bond angle

Because multiple bonds contain higher electron-charge density than single bonds, multiple bonds represent larger electron domain

Effect of nonbonding electrons on bond angle

Nonbonding pairexperience less nuclear attractionlarger electron domain

Molecular GeometryMolecular Geometry

Example : Predict the geometry of IF5 and SF4