Welcome to CHM 102!

• Instructor: Jason Whitehead• Phone (office): 910-962-7602• Phone (cell): 800-NO-WAYYY• Email: jrw7046@uncw.edu• Alternate Email: Jason.Whitehead@Duke.edu• Office Hours: Monday-Thursday, 9:45-11:00 am,

or by appointment (call or email)

The Basics

• Grading Breakdown: 3 exams (50%), lab(15%), 4 quizzes(15%), and the final(20%).

• If you’re more than 10 min late for the first half 8:00 am – 8:50 am, you’ll have to wait for the second half to start at 9:00 am to come in.

• Attendance will not be taken in lecture. Students who miss lecture should consult with other class members to obtain notes.

• ATTEND LAB!?! Miss more than 4 labs and receive an F for CHM 102!

Part I:VSEPR Theory and

Lewis Structure Review

CHM 102 LectureTuesday, June 26th

What does VSEPR stand for?

V - valence

S - shell

E - electron

P - pair

R - repulsion

What does it do?

• VSEPR Theory gives us a 3-dimensional idea of how our molecules are structured.

• The key to VSEPR lies in the words electron pair repulsion. Opposites attract, but like charges repel each other! Electrons in bonds and lone pairs (valence electrons) that are around atoms try and arrange themselves to get as far away from each other as possible!

Some shapes to know:

The basics…

• To use VSEPR Theory, we have to start with these two terms:

• bonding electron domains - electrons shared between two atoms in a bond.

• nonbonding electron domains - electrons found located on one atom (lone pair).

*Multiple bonds, single bonds, and lone pairs, each count for one electron domain.

How many domains of each type?

Molecular vs. Electron Domain Geometry

• There are two types of geometry: Molecular Geometry - spatial

arrangement of atoms in a molecule (deals centrally in bonding domains).

Electron Domain Geometry-spatial arrangement of electron domains (bonding AND nonbonding) around an atom.

• When no nonbonding domains (lone pairs) are present, molecular geometry IS THE SAME as electron domain geometry.

Lewis Structures!

• Lewis Structures such as the one shown to the right aid in determining which electron domains are bonding and which are nonbonding.

• Understanding Lewis structures is an important piece to solving a VSEPR puzzle!

4 Steps of Drawing Lewis Structures

1. Count up your total valence electrons. (i.e. Oxygen has 6, Nitrogen 5, Carbon 4, Halogens 7, etc).

2. If your structure is an ion and has an overall plus or minus charge (i.e. NH4

+) you have to subtract an electron for each (+) charge and add an electron for each (-) charge.

4 Steps… (cont’d)

3. Write your central atom down, and arrange the surrounding atoms connected to it.

4. Make sure all of the atoms have an octet, save for your noted exceptions! (i.e. BF3, SF6, READ THIS SECTION!!).

Enter the VSEPR Theory

• Does ammonium (NH4)+ really look flat and cross- shaped as shown above to the right?

• NO! You can count 4 bonding electron pairs, and 0 nonbonding electron pairs. This gives ammonium a tetrahedral molecular geometry AND tetrahedral electron domain geometry.

Drawing Ammonia, NH3.

• With 1 Nitrogen and 3 Hydrogens, with no charge on our molecule, you should count up 8 total valence electrons.

• Following through the same way as before:

VSEPR Theory Strikes Again• We’ve got the lewis

structure for ammonia now.

• How many nonbonding electron domains? How many bonding electron domains?

• This results in tetrahedral electron domain geometry (A) but trigonal pyramidal molecular geometry (B) with ~109.5o H-N-H.

Tackling a larger molecule…

• Let’s do carbonate (CO3)2-

• Each oxygen has 6 v.e., each carbon has 4 v.e., and we have to add 2 additional electrons to the total count (to account for the 2- charge. Total v.e. count: 24.

What’s the geometry of carbonate?

• Once again we have a lewis structure.

• How many nonbonding domains? How many bonding?

• What should the molecular geometry be? Electron domain geometry?

Part II:Molecular Polarity and Hybridization

CHM 102 LectureTuesday, June 26th

What is molecular polarity?

• A molecule is considered polar if it has a non-zero dipole moment.

• A dipole moment is a measure of the strength and direction of the separation of partial negative and positive charges in a molecule.

• The molecular dipole moment for water is shown to the right, with the arrow pointing towards the higher electron density (partial negative) and the tail of the arrow being the partially positive “pole.”

How do you determine polarity?• The two most important contributing factors to finding

molecular polarity are looking at geometry, as well as individual bond polarity.

• The arrow for a bond dipole works the same way as for a molecular dipole, with the arrow pointing to the more electronegative element.

• Shown below is the polar H-F bond, which points to fluorine, which is the most electronegative element.

Recall the trend in electronegativity

Comparing bond polarities

• Which is more polar…. the C-O, B-O, or N-O

bond?

• When looking at the O-H bond, which direction does the dipole arrow go?

• Compare the N2, Br2, and Cl2 bonds. Which is more polar (if they are indeed polar)?

Returning to Water and Polarity

• Note that the bond dipoles for water point towards the oxygen. Because of the way they are oriented, the overall dipole ends up going straight up through the molecule.

Molecular Polarity and Geometry

• “Polarity Tug o’ War”- Try and think of the potential molecular polarity of a molecule as a tug of war game.

• If bond dipoles are pulling with equal and opposite force, they cancel one another out.

• The examples shown right, CO2 (top), and CCl4 (bottom) contain polar bonds, but are nonpolar (have an overall zero dipole moment).

Symmetry and Polarity

• Note the high degrees of symmetry of atoms around a central atom tends to make bond dipoles “cancel.”

• This is in part what is meant by the “Geometry” consideration in finding molecular polarity.

Is CCl3F polar or nonpolar?

Is SF5Br polar or nonpolar?

Is SF4Br2 polar or nonpolar?

Is BeClF polar or nonpolar?

Hybridization

• Hybridization is often used to explain geometries in polyatomic molecules.

• It involves the combination of individual atomic orbitals upon bonding to form “hybridized” orbitals.

• The shape and characteristics of the orbitals change upon hybridization.

sp-hybridization

• Linear molecules such as BeCl2 and CO2 show sp hybridization.

sp2-hybridization

• Molecules or portions of molecules such as the carbon on the molecule of acetone below, exhibit trigonal planar electron domain geometry and have sp2-hybridization.

sp3

• Molecules showing tetrahedral electron domain geometry such as methane (CH4) have sp3 hybridization.

Do we see a trend here?

• sp3d

• Sp3d2

• Count the electron domains around the central atom for an easy way to find the hybridization!!