CHAPTER 9CHAPTER 9

AP/HONORS CHEMISTRYAP/HONORS CHEMISTRY

MOLECULAR GEOMETRYMOLECULAR GEOMETRY VSEPR - valence-shell electron pair repulsionVSEPR - valence-shell electron pair repulsion IDEAL GEOMETRIESIDEAL GEOMETRIES Type geometry bond angle Type geometry bond angle ABAB22 (2 e- pairs) Linear 180 (2 e- pairs) Linear 180oo example on board example on board ABAB33 (3 e- pairs) Triangular 120 (3 e- pairs) Triangular 120oo example on board example on board

PlanarPlanar ABAB44 (4 e- pairs) Tetrahedral 109.5 (4 e- pairs) Tetrahedral 109.5oo example on board example on board ABAB55(5 e- pairs) Trigonal 120(5 e- pairs) Trigonal 120oo & 180 & 180o o example on board example on board

bipyramidal bipyramidal ABAB66(6 e- pairs) Octahedral 90(6 e- pairs) Octahedral 90oo example on board example on board

EFFECT OF UNPAIRED EFFECT OF UNPAIRED ELECTRONSELECTRONS

Type geometry bond angle Type geometry bond angle ABAB22E (2 e- pairs, Bent example on boardE (2 e- pairs, Bent example on board

1 lone pair)1 lone pair) ABAB22EE22 (2 e- pairs, Bent 105 (2 e- pairs, Bent 105oo example on board example on board

2 lone pairs)2 lone pairs) ABAB33E (3 e- pairs, Triangular 107E (3 e- pairs, Triangular 107oo example on board example on board

1 lone pair) pyramidal1 lone pair) pyramidal the electron pair geometry approximately the same as that the electron pair geometry approximately the same as that

observed when only single bonds existobserved when only single bonds exist

CONTINUEDCONTINUED

the molecular geometry is quite different the molecular geometry is quite different with lone pairs. Why?with lone pairs. Why?

lone pairs take up more space than when lone pairs take up more space than when bondedbonded

consider NHconsider NH33 it has a electronic pair it has a electronic pair

geometry of tetrahedral but the angle is geometry of tetrahedral but the angle is 107107oo

the water molecule is nonlinear because of the water molecule is nonlinear because of lone pairs 105lone pairs 105oo

OTHER LONE PAIR GEOMETRIESOTHER LONE PAIR GEOMETRIESABAB44E see-saw SFE see-saw SF44

ABAB33EE22 T-shaped ClF T-shaped ClF33

ABAB22EE22 linear XeF linear XeF22

ABAB55E square pyramidal BrFE square pyramidal BrF55

ABAB44EE22 square planar XeF square planar XeF44

when dealing with geometries multiple bonds when dealing with geometries multiple bonds behave like single bondsbehave like single bonds

CONTINUEDCONTINUED

the geometry depends upon how many the geometry depends upon how many terminal atoms the central atom has terminal atoms the central atom has around it and the number of unshared around it and the number of unshared electronselectrons

the VSEPR model can be extended to the VSEPR model can be extended to molecules which do not have a single molecules which do not have a single central atom. Ccentral atom. C22HH22 and C and C22HH44

POLARITY OF MOLECULESPOLARITY OF MOLECULES

PolarPolar large (not large enough to form ionic bonds) large (not large enough to form ionic bonds)

electronegative differenceelectronegative differenceunequal share of the electronsunequal share of the electrons

nonpolar equal sharingnonpolar equal sharing the degree of polarity is measured by its dipole the degree of polarity is measured by its dipole

moment moment ππ = Qr, Q = charge at either end, r = distance = Qr, Q = charge at either end, r = distance 1 debye = 3.33 x 101 debye = 3.33 x 10-30-30 coulomb-meters H-Cl coulomb-meters H-Cl

1.03D bond length 1.36 Ǻ1.03D bond length 1.36 Ǻ

CONTINUEDCONTINUED is it easy to determine whether a diatomic molecule is it easy to determine whether a diatomic molecule

is polar. Why?is polar. Why? check if the elements are the same or differentcheck if the elements are the same or different if a molecule contains more than two atoms, we if a molecule contains more than two atoms, we

must decide whether the whole molecule is polar or must decide whether the whole molecule is polar or nonpolarnonpolar

the are two criteria for determining the polarity of a the are two criteria for determining the polarity of a molecule: bond polarity and molecular geometry molecule: bond polarity and molecular geometry

ATOMIC ORBITALSATOMIC ORBITALS Sigma bonds Sigma bonds σσ

single lobe where the electron density is single lobe where the electron density is concentrated in the region directly between two concentrated in the region directly between two bonded atomsbonded atoms

pi bond pi bond ππ this orbital has two lobes one above the bond axis this orbital has two lobes one above the bond axis

and one below. The electron density is zeroand one below. The electron density is zero all single bonds are sigma bonds and any other all single bonds are sigma bonds and any other

bond in a double or triple bond will be pi bondsbond in a double or triple bond will be pi bonds on boardon board hybrid orbitals on boardhybrid orbitals on board

DELOCALIZE BONDINGDELOCALIZE BONDING

The bonds are given to all atoms i.e. benzene The bonds are given to all atoms i.e. benzene on board makes it stable (aromaticity)on board makes it stable (aromaticity)

magnetism - molecules with one or more magnetism - molecules with one or more paired electrons are attracted to a magnetic paired electrons are attracted to a magnetic fieldfield

the more unpaired electrons, stronger the more unpaired electrons, stronger attraction- paramagnetismattraction- paramagnetism

with no unpaired electrons, weak repulsion - with no unpaired electrons, weak repulsion - dimagnetismdimagnetism