lecture 10: covalent bonding pt 2: vsepr theory ( ch 8)

Lecture 10: Covalent Bonding Pt 2: VSEPR Theory (Ch 8)

Suggested HW: (Ch 8) 19, 23(a, c and d only), 28, 29, 34* Bond angles are not required. Label the geometries of each molecule. Label molecules as either polar or nonpolar.

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:

• 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

H HOδ+ δ+δ-

Considering Molecular Geometry The actual geometry of water is shown below:

HO

Hδ+ δ+

δ-

• 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 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 1Figure out the Lewis dot 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 (lecture 9)

– has a separation of positive and negative partial charges, called a dipole, indicated with + and –

– has dipoles that do not cancel (not symmetrical)

Nonpolar Molecules• 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)

Cl Cl CO Oδ+

δ- δ- CH

H H

H

OVERALL DIPOLE = 0

BCl ClSymmetrical, polar bonds

Non-polar bondsSymmetrica

l

Symmetrical

Polar bonds Non-polar bondsSymmetrica

l

Clδ-

δ-δ-+δ

2 2

Total Electron Domains

Domain Geometry Around

Central AtomBonding Domains

Lone Pair Domains

MOLECULAR GEOMETRY

0BB A

Linear

A

B

B B

Trigonalplanar

AB B

••

Ex. BH3

Ex. CO2

Ex. NO2-

Bent

3 0

2 1

3

104.5o

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.

Total Electron Domains

Domain Geometry Around

Central AtomBonding Domains

Lone Pair Domains

MOLECULAR GEOMETRY

4

4 0

3 1

2 2

A

B

BB

B

Tetrahedral

Ex. CH4

AB

BB

•• Trigonal Pyramidal

Ex. NH3

AB

B

••

••

Ex. H2O

Bent104.5o

Examples

Give the chemical structures and geometries of the following molecules. Label each as polar or nonpolar.SO4

2-

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

• 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) X

Y

Z

LONE PAIR WANT TO BE AS FAR AWAY FROM OTHER ELECTRON DOMAINS AS POSSIBLE, AND SHOULD

ALWAYS BE PLACED IN EQUITORIAL POSITIONS !!!!

• Five coordinate molecules assume some variation of the trigonal bipyramidal configuration shown to the right.

Total Electron Domains

Domain Geometry Around

Central AtomBonding Domains

Lone Pair Domains

MOLECULAR GEOMETRY

5

A

BB

BB

B

A

B

BB

B

••

5 0

4 1

Trigonal Bipyramidal

Ex. PCl5

Seesaw

Ex. SF4

A

B

B

B

••••

T-shaped

Ex. ClF3

3 2

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-

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

First electron pair is placed in an axial position

If a second lone pair exists, it is placed the maximum distance (180o) from the 1st pair

Total Electron Domains

Domain Geometry Around

Central AtomBonding Domains

Lone Pair Domains

MOLECULAR GEOMETRY

6

6 0 A

BB

BB

B

B

Octahedral

Ex. SF6

5 1

Square pyramidal

AB

BB

B

B

••

Ex. BrF5

Total Electron Domains

Domain Geometry Around

Central AtomBonding Domains

Lone Pair Domains

MOLECULAR GEOMETRY

6 4 2 AB

B

B

B

••

••

Square Planar

Ex. XeF4

Examples

• Give the chemical structures and geometries of the following molecules. Label each as polar or nonpolar.– SeF6

– ICl5

– XeF4