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Dr. Harris9/20/12HW: Ch 8: 19, 23, 29, 33Lecture 10: VSEPR Theory (Ch 8)IntroductionTo date, we have learned about the Lewis structures of covalent bonds

Lewis structures give insight into how atoms are bonded within a molecule, but does NOT tell us about the shape (molecular geometry), of the molecule

Molecular geometry plays a major role in the properties of a substance. This is particularly true with biochemical reactions.

Importance of Molecular Shape and StructureThalidomide was a popular drug for the treatment of morning sickness. The chemical formula of Thalidomide is C13H10N2O4

If the synthetic procedure is not properly controlled, either of the two optical isomers (mirror images) of the drug can formRelieves morning sicknessSevere birth defectsConsidering Molecular GeometryIf we draw the Lewis structures of water without considering geometry, we would derive the following:HOH++-The Lewis structure suggests that water is a linear (straight) molecule.

However, if this were true, then the dipoles moments would be in opposite directions, as described above, and water would be a nonpolar molecule

If this were the case, life as we know it would be very differentConsidering Molecular GeometryThe actual geometry of water is shown below:HOH++-This is a bent geometry. The angle between the atoms is 104.5o. In this geometry, the molecule has a net dipole moment directed upward, which is why water is polar.

How do we determine the geometry?104.5o+=VSEPR Theory

The images below show balloons tied together at their ends.

There is an optimum geometry for each number of balloons, and the balloons spontaneously attain the lowest-energy arrangement.

In other words, the balloons try to get out of each others way as best they can. These arrangements maximize the distance between the balloon centers. Electrons behave the same exact way.6VSEPRIn the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom:

are arranged as far apart from each other as possible

have the least amount of repulsion of the negatively charged electrons

have a geometry around the central atom that determines molecular shapeUsing VSEPR To Predict GeometrySTEP 1Figure out the Lewis dot structure of the molecule.

22Total Electron DomainsDomain Geometry Around Central AtomBonding DomainsLone Pair DomainsMOLECULAR GEOMETRY0BBALinearABBBTrigonalplanarABBEx. BH3Ex. CO2Ex. NO2-Bent30213

104.5oJust a noteAny molecule containing only two atoms must be linear. There is no other possible arrangement. Ex. H2, HCl, CO, etc.ExamplesGive the chemical structures and geometries of the following:SO3BeCl2HCNSO2F24-Coordinate Molecules Have a Tetrahedral Arrangement

A tetrahedron is a shape consisting of 4 triangular faces. The vertices are separated by an angle of 109.5o, and each position is equivalent.

Another way to view a tetrahedron is to imagine a cube with atoms at opposite corners, with the central atom at the center of the cube.

Total Electron DomainsDomain Geometry Around Central AtomBonding DomainsLone Pair DomainsMOLECULAR GEOMETRY4403122ABBBBTetrahedralEx. CH4ABBBTrigonal PyramidalEx. NH3ABBEx. H2OBent104.5oExamplesGive the chemical structures and geometries of the following:SO42-PF3OF2Expanded Electron DomainsAs stated in the previous lecture, central atoms with a principal quantum number of n>3 can accommodate more than 8 valence electrons.

In many instances, there will be 5 or 6 bonds around these central atoms

The regions occupied by the constituent atoms in a 5-coordinate structure are not equivalent. The constituent atoms may be either equatorial or axial. Five-Coordinate Molecules

If you have a 5 coordinate molecule which contains a lone pair, like SF4, the lone pair will go in an equatorial position. Equatorial position (x-y plane)Axial position (z axis)XYZLone pair want to be as far away from other electron domains as possible

Five coordinate molecules assume some variation of the trigonal bipyramidal configuration shown to the left.Axial vs. Equatorial Lone PairIn the top arrangement, we have placed the lone pair in an equatorial position. Here, the lone pair has two nearby neighbors that are 90o, and two distant neighbors 120o away

In the bottom arrangement, the lone pair is in an axial position. The lone pair has three nearby neighbors 90o away and one distant neighbor 180o away.

The top arrangement is preferred because the lone pair has less nearby neighborsSFFFFSFFFF

EEEAATotal Electron DomainsDomain Geometry Around Central AtomBonding DomainsLone Pair DomainsMOLECULAR GEOMETRY5ABBBBBABBBB5041Trigonal BipyramidalEx. PCl5SeesawEx. SF4ABBBT-shapedEx. ClF332

A Five-Coordinate molecule with 3 Lone pairs is LINEAR

Symmetrical about the central atom.Ex. XeF2 Note: In this chapter, you will find that Xe is actually able to make chemical bonds. ExamplesGive the chemical structures and geometries of the following:PBr5PF4-TeCl4Six-Coordinate Molecules Take on an Octahedral GeometryUnlike a trigonal bipyramid, the equatorial and axial positions in an octahedral are equivalent.

When placing lone pairs in the structure, we must still maximize their distance. It is customary to first place lone pair in the axial positions.

Lone Pairs Migrate As Far Away From One Another As Possible

First electron pair is placed in an axial positionIf a second lone pair exists, it is placed the maximum distance (180o) from the 1st pair22Total Electron DomainsDomain Geometry Around Central AtomBonding DomainsLone Pair DomainsMOLECULAR GEOMETRY660ABBBBBBOctahedralEx. SF651Square pyramidalABBBBBEx. BrF5

Total Electron DomainsDomain Geometry Around Central AtomBonding DomainsLone Pair DomainsMOLECULAR GEOMETRY642

ABBBBSquare PlanarEx. XeF4ExamplesGive the chemical structures and geometries of the following:SeF6ICl5NiCl42-

Determining PolarityNow that we know the geometry of molecules, we can determine whether or not the molecule is polar (has an overall dipole moment)A polar molecule contains polar bonds, as determined from differences in electronegativity (lecture 14)has a separation of positive and negative partial charges, called a dipole, indicated with + and has dipoles that do not cancel (not symmetrical)Polar MoleculesOCS+HCl-+--Overall Dipole moment = Overall Dipole moment = N-+++HHHOverall Dipole moment = Nonpolar MoleculesA nonpolar molecule contains nonpolar bonds, as determined from differences in electronegativityOr may be symmetrical Or dipoles cancel

ClClHHSymmetrical C-+++HHHCOO+--H+OVERALL DIPOLE = 0