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