Valence Shell Electron Pair Repulsion Theory 06/10/99 Planar triangular Valence Shell Electron Pair Repulsion Theory Tetrahedral Trigonal pyramidal Bent

Molecular Shape 06/10/99 VSEPR theory assumes that the shape of a molecule is determined by the repulsion of electron pairs.

VSEPR Theory Based on Lewis structures 06/10/99 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 overview Names of Shapes: tetrahedral trigonal pyramidal Bent 06/10/99 Each shape has a name Names of Shapes: tetrahedral trigonal pyramidal Bent Linear trigonal planar

Models Tetrahedral Triangular Planar Bent or V Linear 06/10/99 Models Tetrahedral Triangular Planar Bent or V Linear Trigonal pyramidal

methane, CH4 Tetrahedral Bonds are all evenly spaced electrons 109.5° 06/10/99 methane, CH4 Tetrahedral 109.5° Bonds are all evenly spaced electrons

ammonia NH3 .. .. .. .. Trigonal Pyramidal 06/10/99 .. ammonia NH3 .. .. .. Trigonal Pyramidal Less repulsion between the bonding pairs of electrons

06/10/99 .. .. water, H2O 109.5° (109.5°) 109.5° (107°) 109.5° (104.5°)

06/10/99 Bent or V 2 unshared pairs of e’s at top of O repel bonds and force them to bend

Number of electron pairs 06/10/99 Molecule Lewis Structure Number of electron pairs CH4 NH3 SHAPE Tetrahedral 4 Trigonal Pyramidal 4 (3 shared 1 lone pair)

Number of electron pairs 06/10/99 Molecule Lewis Structure Number of electron pairs H2O SHAPE Bent 4 (2 shared 2 lone pairs) Linear CO2 2

Number of electron pairs 06/10/99 Molecule Lewis Structure Number of electron pairs BeCl2 BF3 SHAPE Linear 2 Trigonal Planar 3

06/10/99 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 06/10/99 Methane Hybrid Orbitals Electron configuration of methane: [He]2s22p2 One s and three p orbitals hybridize to form four sp3 orbitals

06/10/99 Molecular Shape/Hybrid Orbitals: Linear/sp Trigonal planar/sp2 Tetrahedral/sp3 Trigonal pyramidal/sp3 Bent/sp3

Phosphorus trihydride 06/10/99 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 06/10/99 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 06/10/99 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

06/10/99 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 06/10/99 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