VSEPR Theory

Valence Bond Theory

Molecular Orbital Theory

Molecular Geometry

What is Molecular Geometry Molecules of different substances have diverse

shapes. Atoms attach to one another in various geometric arrangements. The overall molecular shape of a molecule is determined by its bond angles in three dimensions. The shape of a molecule is very important for its physical and chemical properties

Applying the VSEPR theoryDraw a plausible Lewis structureDetermine the # of lone pairs and bonding pairs

around the central atomEstablish the geometrical orientation of the

electron pairs around the central atom as linear, trigonal planar, tetrahedral, trigonal bipyramid or octahedral

Describe the molecular geometry

The nonbonding electron pairs are as important as bonding electronpairs in determining the structure.

Nonbonding electrons take up more space in the valence shell than thebonding electrons.

If one or more of the electron pairs are lone pairs, the distribution ofelectron pair and the geometrical shape of the molecule must bedifferent.

The bond angles decrease as the number of nonbonding electron pairsIncreases.

Repulsion strengths

lone pair −lone pair lone pair − bond pair bond pair −bond pair

Structures and Formal ChargeFormal charge helps to determine which resonance

structure is most stable, as well as charges on

individual atoms.

Formal charge = [# of valence electrons] – [electrons in lone pairs + 1/2 the number of bonding electrons]

OR

Formal Charge = [# of valence electrons] – [non-bonded electrons + number of bonds]

Examples of Resonance & Formal Charge

Valence Bond TheoryThe covalent bonds are formed by overlap of atomic orbitals each of

which

contains one electron of opposite spin.

The valence bond method predicts molecule shapes from the shapes and orientation of the atomic orbitals and their overlap regions when two atoms approach.

In most cases the orbitals that overlap are reconfigured orbitals, called hybrid orbitals, having different shapes and orientations than pure orbitals.

The process of hybridization corresponds to a mathematical mixing of the valence-shell atomic orbitals.

Valence Electron Pair Geometry

Number of Orbitals

Hybrid Orbitals

Linear 2 sp

Trigonal Planar 3 sp2

Tetrahedral 4 sp3

Trigonal Bipyramidal 5 sp3d

Octahedral 6 sp3d2

Examplesfind the hybridization type and

geometry of CH4

PCl5SF6

NH3

BeF2