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Science 331 |
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| Fall 2004 |
Lecture/Activity |
Office: SA560a |
| Notes: 13 September |
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Phone: x 5719
Home: 822-1116 |
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e-mail: rap1 |
Chemical Bonds
Last time we talked about chemical bonds. In particular we
discussed the two types of strong bonds:
Covalent vs. Ionic compounds: There are two kinds of
strong bonds: ionic bonds and covalent bonds.
- In covalent compounds atoms have a definite relationship
to each other, they are "married." Thus for water,
H2O, the smallest particle is a water molecule
containing one oxygen and two hydrogens.
- In ionic compounds ions of opposite charge attract each other,
but there is no definite attachment. Thus in sodium chloride
crystals each sodium ion is surrounded by six chloride ions and
each chloride ion is surrounded by six sodium ions and they are
equally attracted by each - there is no one-to-one relationship.
One of the most important ideas of chemistry is that the properties
of substances depend on the arrangements of particular atoms in
those substances and how they are attached to each other. Bonding
describes how they are attached. What I want to look at next is
shape.
Molecular Geometry
The importance of molecular shape: recognition at the
molecular level in organisms. Shape and electron density are extraordinarily
important to the interaction of biomolecules - Examples
- Sarin (nerve gas)
- "drugs"
- estrogen mimics ("feminization" of various animal
populations - birth control complication)
Lewis Structures enable us to predict bonding patterns for
molecules of the representative elements, but how can we predict
their shapes? We will add another tool, VSEPR Theory, to our chemical
toolbox - a simple way to predict the geometry of bonds around
a central atom (for larger molecules predict one center at a time).
VSEPR (Valence Shell Electron Pair
Repulsion) Theory is based on three main assumptions
(there are more advanced versions). We are going to look at an
even more abbreviated version with just two assumptions (it will
still give us good predictions for common molecules):
- Electron pairs will orient around a central point to minimize
repulsion.
- Repulsion is strong at 90° and weaker at 120° (weakest
at 180°).
Abbreviated VSEPR predicts geometry based on these assumptions
in a few simple, sequential, steps:
- Draw a correct Lewis Structure.
- Determine the Steric Number = the number of bonded
atoms + the number of lone pairs.
- Maximize the angles between electron pairs, placing the lone
(unbonded) pairs at the extremes.
With this in mind, let's explore the geometries possible
for molecules with a single central atom and an octet of electrons.
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Use your "molecular model kit" (gum drops, grapes
etc. and toothpicks) to build the possible electronic geometries
for atoms with two bonded atoms and no lone pairs, three bonded
atoms and no lone pairs, one bonded atom and one lone pair, one
bonded atom and two lone pairs, four bonded atoms, etc.
- How many geometries
can you find?
- What are they?
- Sketch each geometry
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Let's now explore the molecular geometries possible
with these electroninc geometries. Build examples of the three
electronic geometries above (a gumdrop/grape central atom and
toothpick electron bonds). Now determine how many different molecular
geometries you can make by adding one or more atoms(gumdrops
or grapes) to the electron pairs (toothpicks). Widely used colors
for different elements are: Black = Carbon, Red = Oxygen, Blue
= Nitrogen, Yellow = Sulfur, Orange = Bromine, Green = Chlorine,
Purple = Iodine. Or you can be creative.
- How many molecular geometries can you find for each electronic
geometry?
- What are they?
- Sketch each geometry
Worked example
molecules: CO, CO2, NH3, CH3+,
H2O, CH2O, CH4
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Other geometries occur when there are more than eight electrons
(as frequently occurs with transition metals), but we won't worry
about them in this course.
© R A Paselk
Last modified 15 September 2004
