Post on 16-Dec-2015
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
Q. Are we ready for complex covalent molecules yet?A. Almost.
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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 hogsthe electron.
H gets 2 (not 8because H can’t handle 8)
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Many bonds are polarized.
Cl
HElectrons preferentially circulate around the chlorine atom, taking up more space there.
+
-
What about the sex life of bowling pins?
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Magnetic Fleas
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Covalent molecules with more than 2 atoms are slightly more complex.Example: CO2
Bonds: polarMolecule: not polar.
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The shape of a molecule is determined by electron-electron repulsion.
C OO120 degrees The first bond (gray) is
normal; the second one (orange: it’s one bond in two parts) lies above & below the first bond—think of p -type orbitals. 6
All there is to VSEPR theory:How can electrons be as far apart as possible?
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.
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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.
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Always 2 questions:
1. Are the bonds polar?2. Is the molecule polar?
This is configuration of atoms is called trigonal planar.
BF3 is a toxic gas, useful for synthesizing various chemicals.
F
FB
F120o
This molecule violates the octet rule. Oh, well.
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Let’s try methane, CH4
How can we spread out 4 hydrogens and their
electrons?
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C H
H
H
H 90o
?Right idea, but we forgot an important detail. Nothing constrains the 4 electron clouds to exist in 2D!
Let’s illustrate this with models.
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H
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
109.5 degrees in 3D gives the electrons more space than 90 degrees in 2D
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H
Water is the most amazing covalent molecule.
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What is the shape of water?
H HO
Right idea, but we forgot an important detail. Nothing constrains those electron clouds to exist in 2D!
Nonbonding e- cloud
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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
O
109.5 degrees gives more space to the electrons 17
So water isn’t straight!
O
H H
Electrons comin’ at ya!
Electrons Going away from you!
Electrons in the plane of the blackboard…umm…screen.
109.5 degrees
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It’s great not to be straight!*
*This is not a social commentary.
H H
O
Despite its low mass (18 x hydrogen) water is a liquid at normal temperatures. Dipole-dipole forces!
Explains: high boilinghigh heat capacity,surface tension. 19
http://www.airphotona.com/image.asp?imageid=810&catnum=11200
Missouri-Yellowstone confluence
http://www.graphicmaps.com/webimage/countrys/namerica/usstates/artwork/rivers/uslayout.htm
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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.jpghttp://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/CMap.html
http://classes.aces.uiuc.edu/
ACES100/Mind/CMap.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 Bond Polar
Moleculenot polar
Molecule polarExample: H2
Example: CH4 Example: CFH3
Why: all 4 H atoms pull electrons equally and Symmetrically from C so forces balance
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).
Symmetrical?Yes No
Happy St. Patrick’s Day (Late)
Green: ~ 530 nm
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More complex molecules
OOPS!
OOPS!
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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
SO42-
How many valence electrons ?
S : 6 x 1 = 6 O : 6 x 4 = 24 Charge : 2- = 2 32 e- total
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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.
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: phosphateLet's do another: phosphate PO4
3- P : 5 valence e-4 x O : 24 valence e- charge : 3 valence e- 32 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, butit 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” Lewisstructure 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.
+ __
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.
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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.
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.
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 diffractionexamples: 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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