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