Q. Are we ready for complex covalent molecules yet?
A. Almost.
Astaxanthin, a member of the
carotenoid family, is part of a nutritious
diet—for lobsters and shrimp. These
crustaceans bind astaxanthin to the protein
β-crustacyanin, which changes the shape of
the astaxanthin molecules, turning them a
gray-blue color. Cooking releases the
molecules, returning them to their original
bright red color.
cook
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So far, covalent bonding has been
described as totally equal sharing.
Sometimes, sharing is 60-40 or 70-30.
H
+
Cl
H Cl
Cl kinda hogs
the electron.
H gets 2 (not 8
because H can’t handle 8)
2
Many bonds are polarized.
H
Cl
d+
d-
Electrons
preferentially
circulate
around the
chlorine atom,
taking up
more space
there.
What about the sex life of bowling pins?
3
Magnetic Fleas
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Covalent molecules with more than 2
atoms are slightly more complex.
Example: CO2
Bonds: polar
Molecule: not polar.
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The shape of a molecule is determined
by electron-electron repulsion.
O
C
O
120 degrees
The first bond (gray) is
All there is to VSEPR theory:
normal; the second one
How can electrons be as
(orange: it’s one bond in two
far apart as possible?
parts) lies above & below the
first bond—think of
p -type orbitals.
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Bungee Time: Are those C-O
bonds polar?
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If there are more than 2 atoms in the
molecule, the existence of polarized
bonds does not necessarily make the
whole molecule polarized.
It depends on symmetry.
8
The individual C-O bonds are polar, but
the effect cancels due to symmetry when
we consider the whole CO2 molecule.
The CO2 molecule is not polar.
Despite pretty high mass
(44x hydrogen) it is still a gas at normal
temperatures.
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If there are more than 2 atoms in the molecule, the
existence of polarized bonds does not necessarily
make the whole molecule polarized.
F
Always 2
questions:
B
F
120o
1. Are the bonds polar?
2. Is the molecule polar?
F
This is configuration of atoms is called trigonal planar.
BF3 is a toxic gas, useful for synthesizing various chemicals.
This molecule violates the octet rule. Oh, well.
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Let’s try methane, CH4
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How can we spread out 4
hydrogens and their electrons?
90o
H
H
C
H
H
?
Right idea, but we forgot an important detail. Nothing
constrains the 4 electron clouds to exist in 2D!
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Let’s illustrate this with models.
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Tetrahedron: to draw this thing, we learn to draw a cube first. Then
locate the lower and upper “crossed” vertices.
This shape is important!
H
H
C
14
H
H
109.5 degrees in 3D gives the electrons
more space than 90 degrees in 2D
Water is the most amazing
covalent molecule.
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What is the shape of water?
Nonbonding e- cloud
H
O
H
Right idea, but we forgot an important detail. Nothing
constrains those electron clouds to exist in 2D!
16
Tetrahedron: to draw this thing, we learn to draw a cube first. Then
locate the lower and upper “crossed” vertices.
This shape is important!
O
H
H
109.5 degrees gives more
space to the electrons 17
So water isn’t straight!
Electrons
comin’ at ya!
Electrons
Going away from you!
O
H
H
Electrons in the plane
of the blackboard…
umm…screen.
109.5 degrees
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It’s great not to be straight!*
Despite its low mass
(18 x hydrogen) water is
a liquid at normal
temperatures. Dipoledipole forces!
O
H
H
Explains: high boiling
high heat capacity,
surface tension.
*This is not a social commentary.
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http://www.graphicmaps.com/webimage/
countrys/namerica/usstates/artwork/rivers/
uslayout.htm 
http://www.airphotona.com/image.asp?
imageid=810&catnum=11200
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Let’s do a thought experiment
(gedanken experiment).
What happens if three people aim water cannons at a ball?
Can the ball be suspended in mid-air?
http://chronicle.augusta.com/images/headlines/030403/Continental_water_cannons.jpg
http://www.billythekidsnewmexico.com/TheMotionPictureCo.com/Fireboat%20water%20cannons2b.jpg
See also: http://www.youtube.com/watch?v=RqrdcVCZ794
This is a good time to introduce
Concept Maps
http://classes.aces.uiuc.edu/ACES100/Mind/C
Map.html
http://classes.aces.uiuc.edu/ACES100/Mind/C
Map.html
This concept map addresses the following question:
Are molecules polar?
Electronegative elements (Northeast elements) Suck Electrons
Atoms identical
Atoms NOT identical
Bond Not Polar
Example: H2
Bond Polar
Molecule
not polar
Symmetrical?
Yes
No
Example: CH4
Why: all 4 H atoms pull electrons equally
and Symmetrically from C so forces balance
Molecule
polar
Example: CFH3
Why: the 3 H atoms pull electrons down, but
not as hard as the F atom pulls up.
Result: CFH3 has higher melting point (-142) than CH4 (-182) than H2 (-253).
Happy St. Patrick’s Day (Late)
Green: l ~ 530 nm
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More complex molecules
OOPS!
OOPS!
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Sudan I dye: a.k.a. Solvent Orange R
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Some covalent structures make ions; after that,
treat them as ions.
So far, we have seen only atomic ions Na2O, NaCl, CaCl2
There are also "molecular ions" - covalent molecules
can have a charge !
e.g. Sulfate ion
SO42How many valence electrons ?
S:6x1=6
O : 6 x 4 = 24
Charge : 2- = 2
32 e- total
If it looks like a duck,
swims like a duck,
and quacks like a duck,
then it probably is a duck.
Molecular ions pass the duck test.
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What does this molecular ion look like?
You need to be told that the sulfur is in center. (When in
doubt, assume the least prevalent atom is centered.)
put electrons in to bond O to S (takes 8)
satisfy octet for oxygen (takes 24)
total used: 32 electrons = OK
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Treat the molecular ion similar to an
atomic one. Sulfate has –2 charge, so
behaves like O2-
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Do another: phosphate
Let's do another: phosphate PO43P : 5 valence e4 x O : 24 valence echarge : 3 valence e32 total electrons
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Why is phosphate –3 while
sulfate is only –2?
Answer: One less proton:
compare P (z = 15) to S (z = 16)
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Take cyanide. No—not literally!
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Formal charge is not used much in this class, but
it is described in the notes on the web.
You can get a formal charge for each atom in the molecule.
FC =
Number of Protons in atom
- electrons in inner core of atom
- Half the Number of electrons in bonds
- any nonbonded electrons.
Nature hates to produce charges, so the “best” Lewis
structure has low formal charge.
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Return of our old nemesis:
Nonmolecular vs. Molecular Compounds
…is largely the difference between
ionic and covalent!
Covalent: mutual attraction to
shared ion cloud.
Ionic: One has electrons, the other
gave electrons.
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Liquids are a very complex and
rare* state of matter.
Biggest difference: cohesion.
Why cohesion?
Permanent dipoles—e.g. water.
Induced dipoles—e.g. Cl2 or He (at very cold temps!)
*The liquid state is remarkably rare; in particular,
few planets have liquid water. Some have liquid methane.
Nonpolar molecules and atoms generate
temporary dipoles. The associated field travels to a
neighbor almost instantaneously, causing it to form
a complementary dipole. This leads to a weak
attraction, visible at low temperatures.
d+
d_
d+
d_
This plus charge is only a partial charge AND it lasts just
a very, very short time, but that is long enough to induce
a neighbor to generate a charge. The two dipoles then
attract, very weakly.
Magnet/Nail Demo
Water is a strange, strange liquid!
•Very high melting and boiling points for such a light
molecule (due to high polarity and H bonds)
•Solid density is less than liquid density. Very unusual!
•High heat capacity: it takes a lot of energy to raise the
temperature of water. This is important determining the
weather and stability of ocean temperature, etc.
•High heat of vaporization: boiling water takes lots of
energy; it really is attracted to itself! Can you think of
social groups that are like this?
Other solids lighter than their
corresponding liquid, like water.
CAUTION: This is just a Yahoo Answers List!
== Less Dense in Solid Form ==
confirmed as solid less dense than liquid:
gallium - 5.91 (solid) vs 6.095 (liquid)
bismuth - 9.78 (solid) vs 10.05 (liquid)
germanium - 5.323 (solid) vs 5.60 (liquid)
silicon - 2.3290 (solid) vs 2.57 (liquid)
water - 0.917 (solid) vs 0.998 (liquid)
claimed but probably false:
acetic acid - 1.266 (solid) vs 1.049 (liquid)
antimony - 6.697 (solid) vs 6.53 (liquid) (this "error" is repeated in many
places, inc wikipedia) Water is not always less dense in solid form. Depending
on how the water crystals are formed, it may actually be more dense.
Examples include HDA and VHDA.
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More about water
•High surface tension. The
amount of energy
required to expand the
surface area by one unit
of area is called the
surface tension.
Figure 7.9 Surface Tension
Hydrogen “bonds”—how real?
Things containing
-OH (e.g. CH3OH)
-NH (e.g. CH3NH2)
HF
H-bonding & permanent dipoles
make water very high-boiling.
Water & Oil Demos
Talc / Wax Demos
Soap Demos
Solids
--crystalline:
•a regular array of atoms or molecules exists
•We see x-ray diffraction
examples: sugar, salt, ice
--amorphous:
•atoms or molecules in no real order, yet
more or less immobilized.
•examples: glass, very rapidly cooled water
or rapidly cooled polymers
Metals: things with distributed electrons
(not just shared—but smeared out!)
Metals can be liquid (mercury) or solid (gold).
Usually elemental, but covalent molecular
metals and "near-metals" do exist.
The nuclei of elemental metals are usually said
to be afloat in a "sea of electrons".
Metallic Properties: all related
to that sea of electrons
•Luster--they shine!
•High electrical conductivity--use metals for wires.
•High thermal conductivity--cold metal will feel much colder than
cold wood, for example. The heat rushes out of metal quickly. You
may see someone walk on fiery hot coals, but not on sunbaked train
tracks!
•Ductility and malleability: metals can be drawn (into wire) or
hammered (into leaf).
•Insolubility in water. Well...this is a tricky one. Metals do not dissolve
with water--unless they react with it to produce a metal salt.
Semiconductors are metalloids
with deliberate impurities.
Before semiconductors, we used
tubes to control the flow of
electrons.
Transistors do the same thing as tubes, but
we can make them much smaller and at
lower cost.
The Eniac (Electronic Numerical
Integrator and Computer)
Special types of materials—a little bit
more is said in the notes on-line.
Supercritical fluids ... are sort of half-gas/half-liquid substances.
Gels ... are mechanical solids made mostly from liquids!
Aerogels
Superconductors
Liquid Crystals
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