VValencealence

SShellhell

EElectronlectron

PPairair

RRepulsionepulsion

TheoryTheory

Planar triangular

Tetrahedral

Trigonal bipyramidal

Octahedral

VSEPR TheoryVSEPR Theory• Based on Lewis structures we can know the

shape or “geometry” of molecules• VSEPR (pronounced “vesper”) stands for

Valence Shell Electron Pair Repulsion• VSEPR, as the name suggests, predicts

geometry based on the repulsion of electron pairs (in bonds or by themselves)

• Electrons around the central nucleus repel each other. Thus, resulting structures have atoms maximally spread out

VSEPR overviewVSEPR overview• 2 Principles of the Theory

– Electron pairs repel each other– 2 kinds of pairs

• Bonded pairs (shared between atoms)• Lone pairs (NOT shared)

• Molecules are represented by AXY, where A is the central atom and Y is the # of peripheral atoms

• AX2 = linear• AX3 = planar triangular• AX4 = tetrahedral (tetra = 4 faces)• AX5 = trigonal bipyramidal (2 pyramids)• AX6 = octahedral (octa = 8 faces)

Planar triangular Tetrahedral

Trigonal bipyramidal Octahedral

Lone pairsLone pairs• Thus far we have considered (built) only

structures where there are no free electrons around the central atom

• These electrons that are not involved in bonds are called “lone pairs”

• Essentially, they have the same influence on molecular structure as electron pairs in bonds

• The result is some weird shapes and names…

orVs.

orH C

H

H

H

H C H

H

H

H N H

H

H N H

H

Variations on Tetrahedral MoleculeVariations on Tetrahedral Molecule• The tetrahedral molecule is AX4

• Lone pairs can be indicated with AXYEZ, where Z is the number of lone pairs

• By replacing 1 bond with a lone pair the tetrahedral shape becomes “trigonal pyramidal”

• AX3E

• By replacing two bonds with lone pairs we get a “bent” (non-linear) shape (AX2E2 ) (Fig. 3.26, pg. 87)

Variations on Trigonal BipyramidalVariations on Trigonal Bipyramidal• AX5 is trigonal

bipyramidal

• AX4E is unsymmetrical tetrahedron

• AX3E2 is T-shaped

• AX2E3 is linear

See Fig. 3.37 on pg. 91

Variations on Octahedral ShapeVariations on Octahedral Shape• AX6 is octahedral

• AX5E is square pyramidal

• AX4E2 is square planar