Announcements
1.
2.
3.

Exam #3: Thursday, Dec. 6th, 7:00-8:15pm
(Conflict: 5:15-6:30pm)
No calculators allowed
Activity 3: Making Models of Molecules lab write-up
due tomorrow in discussion
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.

2.
3.
Follow the duet rule for hydrogen.
The total number of electrons in your Lewis
structure must equal the total number of
valence electrons.
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
H B H
H

Even though boron does not have 8 valence electrons, this Lewis
structure is valid (boron can have only 6 valence electrons in a
molecule)
Example: BeF2



Valence electrons for Be: 2
Valence electrons for F: 7
Total valence electrons: 2 + (2*7) = 16
F

Be
F
Even though beryllium does not have 8 valence electrons, this
Lewis structure is valid (Be can have only 4 valence electrons in a
molecule)
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
Cl




Example: PCl5
P: 5 valence electrons
Cl: 7 valence electrons
Total valence e-: 40
Cl
P
Cl
Cl
Cl






Br
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
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
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
Molecules with more than one possible
structure have resonance
Usually molecules with multiple bonds
that can be drawn between more than
one atom
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
nonpolar
polar
polar
polar
nonpolar
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
polar
nonpolar
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.