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VValencealence
SShellhell
EElectronlectron
PPairair
RRepulsionepulsion
TheoryTheory
Planar triangular
Tetrahedral
Trigonal pyramidal
Bent
VSEPR theory assumes that the shape of a VSEPR theory assumes that the shape of a molecule is determined by the molecule is determined by the repulsion repulsion of of electron pairs.electron pairs.
Molecular ShapeMolecular Shape
VSEPRVSEPR Theory Theory• Based on Lewis structures
• Theory predicts shapes of compounds
• VSEPR (pronounced “vesper”) stands for Valence Shell Electron Pair Repulsion
• VSEPR predicts shapes based on electron pairs repelling (in bonds or by themselves)
• Electrons around central nucleus repel each other. So, structures have atoms maximally spread out
VSEPR overviewVSEPR overview• Each shape has a name
• Names of Shapes:• tetrahedral
• trigonal pyramidal
• Bent
• Linear
• trigonal planar
Triangular PlanarTetrahedral
Trigonal pyramidalLinear
Bent or V
Models
C
H
H
H
H methane, CHmethane, CH44
Bonds are all evenly spaced Bonds are all evenly spaced electrons electrons
109.5°109.5°
C
H
H
H
H
TetrahedralTetrahedral
NH H
H
C
H
H
H
H
Less repulsion between the bonding pairs of Less repulsion between the bonding pairs of electronselectrons
.
.ammoniammoni
aa
NHNH33
. ...
.
.
Trigonal Trigonal PyramidalPyramidal
C
H
H
H
H NH H
H
OH H.. ..
..
109.5° (109.5° (109.5109.5°)°) 109.5° (109.5° (107107°)°) 109.5° (109.5° (104.5°)104.5°)
water, water, HH22OO
H
CH H
HN
H HH
OH H
Bent or VBent or V
2 unshared pairs of e’s at top of O repel bonds and force them to bend
Molecule Lewis Structure Number of electron pairs
CH4
NH3
SHAPE
Tetrahedral
Trigonal Pyramidal
4
4
(3 shared
1 lone pair)
Molecule Lewis Structure Number of electron pairs
H2O
CO2
SHAPE
Bent
4
(2 shared 2 lone pairs)
2
Linear
Molecule Lewis Structure Number of electron pairs
BeCl2
BF3
SHAPE
2
3
Linear
Trigonal Planar
Hybrid Orbitals• VSEPR works well for shapes/geometry,
but not for describing the types of bonds formed
• Hybridization: the mixture of atomic orbitals to form the same number of new orbitals
• Carbon is the most common element that undergoes hybridization
Methane Hybrid Orbitals
• Electron configuration of methane: [He]2s22p2
• One s and three p orbitals hybridize to form four sp3 orbitals
Molecular Shape/Hybrid Orbitals:
Linear/sp
Trigonal planar/sp2
Tetrahedral/sp3
Trigonal pyramidal/sp3
Bent/sp3
Phosphorus trihydride• Total number of valence electrons: 8
• Lewis structure: three single bonds and one lone pair
• Shape: trigonal pyramidal
• Four bonding positions = sp3 hybrid
Intermolecular Forces
• Forces that hold together identical particles such as water molecules in a drop of water
• Three such forces are: dispersion forces, dipole-dipole forces, and hydrogen bonds
Dispersion Forces• Also known as London forces
• Weak forces that result from a temporary shift in the density of electrons in electron clouds
• For example, if two nonpolar molecules collide, the electron clouds of one molecule repels the electron cloud of the other molecule, creating a greater electron density in on region of each electron cloud
Dipole-Dipole Forces• Attractions between oppositely charged regions
of polar molecules
• Since dipoles in polar molecules are permanent, dipole-dipole forces are stronger than dispersion as long as the molecules are similar in mass.
Hydrogen Bonds• Type of dipole-dipole attraction
• Occurs between molecules containing a hydrogen atom bonded to a small, highly electronegative atom with at least one lone pair (i.e., fluorine, oxygen, or nitrogen)
• F, O, and N are electronegative enough to cause a large partial positive charge on the H, but small enough to allow their lone pairs to come close to H atoms