Announcements 1. Exam #3: Thursday, Dec. 6th, 7:00-8:15pm

(Conflict: 5:15-6:30pm)

No calculators allowed

2. Activity 3: Making Models of Molecules lab write-up due tomorrow in discussion

3. Lon-capa HW Remaining 1. Homework 7 – Type 1 due Monday, 12/3 by 7pm 2. Homework 8 – Type 1 due Tuesday, 12/4 by 7pm 3. Review Questions for Exam III due Wed, 12/5 by

10pm

Room assignments for Exam III and TA Exam Review Sessions are posted on website Look under Exam Info link

TA Exam Review Sessions

General Rules for Lewis Structures 1. Follow the octet rule.

Follow the duet rule for hydrogen.

2. The total number of electrons in your Lewis structure must equal the total number of valence electrons.

3. The least electronegative atom is typically the central atom (except for hydrogen).

Clicker #1 Which of the following supports why Lewis structures are not a completely accurate way to draw molecules? A) The Lewis structure for O2 shows all electrons as paired but when liquid O2 is poured onto a magnet, it behaves as a molecule with an unpaired electron. B) There are sometimes multiple valid Lewis structures for a single chemical species. C) When adding up the number of valence electrons for a molecule (such as nitrogen monoxide), it is possible to get an odd number which would make it impossible to satisfy the octet rule for all atoms. D) All of the above statements support why Lewis structures are not a completely accurate way to draw molecules. E) Lewis structures are the most accurate way to draw molecules and are completely correct.

Exceptions to the rules There are problems with model of Lewis structures

As we saw with the paramagnetism of O2 last time

We don’t know that electrons are exactly where we placed them (placement based on probability)

Because the model is not perfect, there are exceptions to the rules

Example: BH3 Valence electrons for B: 3 Valence electrons for H: 1 Total valence electrons: 3 + (1*3) = 6

Even though boron does not have 8 valence electrons, this Lewis structure is valid (boron can have only 6 valence electrons in a molecule)

H H B

H

Example: BeF2 Valence electrons for Be: 2 Valence electrons for F: 7 Total valence electrons: 2 + (2*7) = 16

Even though beryllium does not have 8 valence electrons, this Lewis structure is valid (Be can have only 4 valence electrons in a molecule)

F F Be

Exceeding the octet rule If central atom is in third row or below, can exceed 4 bonds (8

electrons) if needed

Theorized that d orbitals can contribute to expanded bonding

Example: PCl5

P: 5 valence electrons Cl: 7 valence electrons Total valence e-: 40

Cl Cl P

Cl

Cl Cl

Example: XeBr4

Xe: 8 valence electrons Br: 7 valence electrons Total valence electrons: 36

Noble gases can sometimes exceed the octet rule as well

Br Br Xe

Br

Br

Determining Polarity Determining if a molecule is polar is very important

because it helps us to understand why it has certain properties (for example, why H2O is a liquid but H2 and O2 are gases at room temperature)

Polar uneven distribution of electron pairs

Molecule overall is asymmetrical (not symmetrical)

Nonpolar even distribution of electron pairs

Molecule overall is symmetrical

Determining Polarity First: draw Lewis structure for molecule

Second: determine geometry (how atoms and lone pairs are arranged around central atom)

Third: determine shape of molecule (how does molecule look with just atoms, not including lone pairs)

Fourth: Ask yourself - is the molecule symmetrical?

Terminology Effective pairs: composed of all valence electron

pairs around central atom (bonding pairs and lone pairs) Multiple bonds are treated like single bonds, so they only

count as one effective pair

Lone pairs: do not participate in bonding

Geometry: how effective pairs are arranged around central atom to minimize electron-electron repulsion 5 types of geometry

Shape: how bonded atoms are arranged (does not

include lone pairs!)

To determine geometry and shape, refer to charts in lab book, pp. 59-60 You need to memorize these charts for the

exam!!!

Thing that drives determining geometry and shape is Valence Shell Electron Pair Repulsion theory (VSEPR)

The key is to minimize repulsion between electron pairs by moving them as far apart as possible This results in lower energy = more stable!

Geometry terms Linear

Number of effective pairs: 2 Bond angles: 180°

Examples: CO, CO2

Geometry terms Trigonal planar

Number of effective pairs: 3 Bond angles: 120°

Examples: BF3, NO3-, SO2

Geometry terms Tetrahedral

Number of effective pairs: 4 Bond angles: 109.5°

Examples: CH4, NH3, H2O, HCl

Geometry terms Trigonal bipyramid

Number of effective pairs: 5 Bond angles: 90, 120, 180°

Examples: PCl5, SF4, ClF3, I3-

Geometry terms Octahedral

Number of effective pairs: 6 Bond angles: 90, 180°

Examples: SF6, ICl5, XeF4

Geometry and Shapes

Molecules with more than one possible structure have resonance Usually molecules with multiple bonds that can be drawn between more than one atom

While C-O bond is polar, both pull equally in opposite directions; nonpolar C-O bond is more polar than C-S, uneven “pull” on center atom; polar Lone pair does not “pull”, and C-O bond is polar; polar

Geometry and Shapes

While B-F bond is polar, all three pull equally in opposite directions; nonpolar B-F bond is more polar than B-H, uneven “pull” on center atom; polar Lone pair does not “pull”, and Se-O bond is polar; polar

Clicker #2 Does TeS2 have an overall net dipole moment (is it polar overall)?

A) Yes, the electrons are found more towards the sulfurs.

B) Yes, the electrons are found more towards the tellurium.

C) No, the electron distribution is symmetrical around the central atom.

Geometry and Shapes

polar nonpolar

polar

polar

polar

Clicker #3 Does CCl4 have an overall net dipole moment (is it polar overall)?

A) Yes, the electrons are found more towards carbon.

B) Yes, the electrons are found more towards the chlorines.

C) No, the electron distribution is symmetrical around the central atom.

Geometry and Shapes

polar

polar

nonpolar

nonpolar polar

Clicker #4 Does ICl2

– have an overall net dipole moment (is it polar overall)?

A) Yes, the electrons are found more towards the central iodine atom.

B) Yes, the electrons are found more towards the outer chlorine atoms.

C) No, the electron distribution is symmetrical around the central atom.

Geometry and Shapes

nonpolar

polar

nonpolar

polar

polar

Clicker #5 Does SF6 have an overall net dipole moment (is it polar overall)?

A) Yes, the electrons are found more towards sulfur.

B) Yes, the electrons are found more towards the fluorines.

C) No, the electron distribution is symmetrical around the central atom.