Covalent Bonding Pt 2: VSEPR Theory

Introduction To 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.

Considering Molecular Geometry If we draw the Lewis structures of water without considering geometry, we would derive the following: δ- δ+ δ+ H O H 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 different

Considering Molecular Geometry The actual geometry of water is shown below: H O δ+ δ- + = 104.5o 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?

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 other’s way” as best they can. These arrangements maximize the distance between the balloon centers. Electrons behave the same exact way.

VSEPR In the valence-shell electron-pair repulsion theory (VSEPR), the electron domains around a central atom: are arranged as far apart from each other as possible experience the least amount of repulsion have a geometry around the central atom that determines molecular shape

Using VSEPR To Predict Geometry STEP 1 Figure out the Lewis structure of the molecule.

Determining Polarity Once we know the correct geometry of a molecule, 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 has a separation of positive and negative partial charges, called a dipole, indicated with + and – has dipoles that do not cancel (not symmetrical)

C H C O - - - - + Cl Cl Cl Cl Nonpolar Molecules B A nonpolar molecule contains nonpolar bonds, as determined from differences in electronegativity, or… contains polar bonds, but the molecule is symmetrical (no lone pair on central atom), so the dipoles cancel! C H C O δ + - Cl Non-polar bonds Symmetrical Symmetrical Polar bonds - Non-polar bonds Symmetrical Cl δ Symmetrical, polar bonds - - δ B Cl Cl + δ OVERALL DIPOLE = 0 δ

Domain Geometry Around Total Electron Domains Domain Geometry Around Central Atom Bonding Domains Lone Pair MOLECULAR GEOMETRY B A B 2 2 Linear Ex. CO2 A B Trigonal planar 3 3 Ex. BH3 A B •• Bent 104.5o 2 1 Ex. NO2-

Just a note Any molecule containing only two atoms must be linear. There is no other possible arrangement. Ex. H2, HCl, CO, etc.

Examples Give the Lewis structures and geometries of the following molecules. Label each as polar or nonpolar: SO3 SO2 F2

4-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.

Domain Geometry Around Total Electron Domains Domain Geometry Around Central Atom Bonding Domains Lone Pair MOLECULAR GEOMETRY A B Tetrahedral 4 Ex. CH4 A B •• Trigonal Pyramidal 4 3 1 Ex. NH3 A B •• Bent 104.5o •• 2 2 Ex. H2O

Examples Give the chemical structures and geometries of the following molecules. Label each as polar or nonpolar. SOF2 PF3 OF2 CCl4

Expanded Electron Domains As 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 Z X Axial position (z axis) Equatorial position (x-y plane) Y Five coordinate molecules assume some variation of the trigonal bipyramidal configuration shown to the right. If you have a 5 coordinate molecule which contains a lone pair, like SF4, the lone pair will go in an equatorial position. Lone pair want to be as far away from other electron domains as possible, AND SHOULD ALWAYS BE PLACED IN EQUITORIAL POSITIONS !!!!

Domain Geometry Around Total Electron Domains Domain Geometry Around Central Atom Bonding Domains Lone Pair MOLECULAR GEOMETRY Trigonal Bipyramidal A B 5 Ex. PCl5 A B •• Seesaw 4 1 5 Ex. SF4 T-shaped A B •• 3 2 Ex. ClF3

A Five-Coordinate molecule with 3 Lone pairs is LINEAR Symmetrical about the central atom. Ex. XeF2 Note: In this chapter, you will find examples of noble gases acting as central atoms.

Examples Give the chemical structures and geometries of the following molecules. Label each as polar or nonpolar. PBr5 TeCl4 *IOF2- (ions are not polar because of the formal charge)

Six-Coordinate Molecules Take on an Octahedral Geometry Unlike 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 If a second lone pair exists, it is placed the maximum distance (180o) from the 1st pair First electron pair is placed in an axial position

Domain Geometry Around Total Electron Domains Domain Geometry Around Central Atom Bonding Domains Lone Pair MOLECULAR GEOMETRY Octahedral B B B 6 A B B B Ex. SF6 6 Square pyramidal 5 1 •• B B A B B B Ex. BrF5

Domain Geometry Around Total Electron Domains Domain Geometry Around Central Atom Bonding Domains Lone Pair MOLECULAR GEOMETRY Square Planar •• 6 4 2 B B A B B •• Ex. XeF4

Examples Give the chemical structures and geometries of the following molecules. Label each as polar or nonpolar. SeF6 ICl5 XeF4