VSEPR THEORY

Valence Shell Electron Pair Repulsion

VSEPR THEORY

Predicts the molecular shape of a bonded molecule Electrons around the central atom arrange

themselves as far apart from each other as possible

Unshared pairs of electrons (lone pairs) on the central atom repel the most

So only look at what is connected to the central atom

6 TYPES OF MOLECULES WITH NO UNSHARED PAIRS OF ELECTRONS

LINEAR

2 atoms attached to central atom 0 unshared pairs (lone pairs)

Bond angle = 180o

Type: AB2

Ex. : BeF2

3 EXCEPTIONS TO THE OCTET RULE

Molecules with an odd number of electrons Molecules with atoms near the boundary

between metals and nonmetals will tend to have less than an octet on the central atom. (i.e. B, Be, Al, Ga)

Molecules with a central atom with electrons in the 3rd period and beyond will sometimes have more than an octet on the central atom, up to 12, called an extended or expanded octet.

LINEAR

Carbon dioxide

CO2

TRIGONAL PLANAR

3 atoms attached to central atom 0 lone pairs

Bond angle = 120o

Type: AB3

Ex. : AlF3

TRIGONAL PLANAR

Boron Trifluoride BF3

TETRAHEDRAL

4 atoms attached to central atom 0 lone pairs

Bond angle = 109.5o

Type: AB4

Ex. : CH4

TETRAHEDRAL

Carbon tetrachloride CCl4

TRIGONAL BIPYRAMIDAL 5 atoms attached to central atom 0 lone pairs

Bond angle = equatorial -> 120o

axial -> 90o

Type: AB5

Ex. : PF5

TRIGONAL BIPYRAMIDAL

Antimony Pentafluoride

SbF5

OCTAHEDRAL

6 atoms attached to central atom 0 lone pairs

Bond angle = 90o

Type: AB6

Ex. : SF6

OCTAHEDRAL

Sulfur hexafluoride

SF6

EXAMPLES OF MOLECULES WITH BOTH PAIRED AND UNSHARED (LONE) PAIRS OF ELECTRONS AROUND THE CENTRAL ATOM.

TETRAHEDRAL

Carbon tetrachloride CCl4

TRIGONAL PYRAMIDAL Tetrahedral variation #1 3 atoms attached to central atom 1 lone pair

Bond angle = 107o

Type: AB3E Ex. : NH3

TRIGONAL PYRAMIDAL

Nitrogen trifluoride NF3

BENT Tetrahedral variation #2 2 atoms attached to central atom 2 lone pairs

Bond angle = 104.5o

Type: AB2E2

Ex. : H2O

BENT

Chlorine difluoride ion ClF2

+

TRIGONAL BIPYRAMIDAL

Antimony Pentafluoride

SbF5

SEE SAW

Trigonal Bipyrimid Variation #1

Sulfur tetrafluoride SF4

T-SHAPED

Trigonal Bipyramid Variation #2

Chlorine tribromide

LINEAR

Trigonal Bipyramid Variation #3

Xenon difluoride XeF2

OCTAHEDRAL

Sulfur hexafluoride

SF6

SQUARE PYRAMIDAL

Octahedral Variation #1 Chlorine pentafluoride ClF5

SQUARE PLANAR

Octahedral Variation #2 Xenon tetrafluoride XeF4

OCTAHEDRAL

Do not need to know:T-shapeLinear

BENT

Nitrogen dioxide NO2