Humboldt State University ® Department of Chemistry

Richard A. Paselk

Science 331
Fall 2004 Lecture/Activity Office: SA560a
Notes: 13 September Phone: x 5719
Home: 822-1116
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.

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

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):

 

Abbreviated VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

  1. Draw a correct Lewis Structure.
  2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs.
  3. 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.

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.

  1. How many geometries can you find?
  2. What are they?
  3. Sketch each geometry

 

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.

  1. How many molecular geometries can you find for each electronic geometry?
  2. What are they?
  3. Sketch each geometry

Worked example molecules: CO, CO2, NH3, CH3+, H2O, CH2O, CH4

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.


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© R A Paselk

Last modified 15 September 2004