ChemViz Lab: Properties of Bonds
Created by: Rebecca A. Kruse
Pre-lab Assignment:
Read “Making Animations with Waltz.”
Pre-lab Discussion:
Atoms interact with one another by transferring or sharing their valence electrons. Valence electrons are the
electrons that lie furthest from the nucleus and govern the chemical properties of the element, including chemical
bonding. When valence electrons are transferred from one atom to another, positive and negative ions form which
are attracted to one another through electrostatic forces called ionic bonds. Covalent bonds form when one or
more pairs of valence electrons are shared between atoms to achieve an octet. Nonpolar covalent bonds occur
when electrons are shared equally such that electron density is found at a point midway between the two atoms in
the bond. Polar covalent bonds occur when electron density is displaced toward one atom in the bond.
Electronegativity is often used to predict whether a bond is ionic, nonpolar covalent, or polar covalent. These
properties of bonds were demonstrated last week using electron density surfaces and electrostatic potential maps
in Spartan.
In addition, there are three important characteristics of chemical bonds that your textbook does not cover in great
detail. These characteristics are critical for understanding many chemical and physical properties of
compounds/molecules. First, each chemical bond is associated with a particular amount of energy, known as the
bond energy, which depends on the type of atoms bonded and the number of electrons transferred or shared
between those atoms. As we will see today, when plotted against distance, the bond energy can also be used to
find the bond length between two atoms. In such graphs, the bond length corresponds to the lowest bond energy
(always remember, lowest energy equals most stable!) The more electron pairs shared between two atoms, the
more bonds between them, or the higher the bond order. Bond order is also related to bond energy and bond
length (do you what these relationships are?) and a simple way to determine bond order is to draw the Lewis
Structure.
In this lab you will be performing electron density animations of diatomic molecules forming or breaking apart. You
will use the supercomputer generated energy graphs to determine the bond length of the animated molecule. You
will then compare your calculated bond lengths with theoretical values found in reference books (your textbook) or
using online resources. You should develop an understanding of how bond energy, bond length, and bond order
are interrelated. You may wish to work ahead, writing detailed instructions to yourself.
Purpose:
In this exercise you will use electron density animations generated in ChemViz to investigate: 1) the bond length as
it relates to bond energy, 2) bond order as it relates to bond length and bond energy, and 3) how well your
calculations of bond length agree with theoretical values.
Lab Procedure:
Follow the directions provided in “Making Animations with Waltz” to do the following:
1. Logon to the Waltz interface at the ChemViz web page at http://chemviz.ncsa.uiuc.edu.
Bond Length
2. Choose one of these seven elemental diatomic molecules (H 2, N2, O2, F2, Cl2, Br2, I2).
3. Determine the atomic radius of your element at http://chemicool.com/ Note: you must convert to Å.
4. Create a 10-step electron density animation of the two atoms, moving them from 1 atomic radius apart
to several (3 or 4) atomic radii in Å along the x-axis.
5. In the Energy File below the images, locate the bond energy minimum and the corresponding bond
length.
6. Create a second 10-step electron density animation “focused” around the bond energy minimum. The
intent here is to improve the accuracy of the bond length calculation.
7. In the Energy File below the images, locate the new bond energy minimum and the corresponding
bond length.
8. In the Animation Frames below the images, find the image that corresponds to this bond length.
9. Using the Print Scrn button copy, paste, and crop the following into a single PowerPoint slide:
1) the energy plot with data table and 2) the image corresponding to the calculated bond length.
Be sure to include your molecule in the title, and include a printed copy of this slide in your laboratory notebook.
Bond Order
10. Draw the Lewis Structure for your molecule to approximate the bond order.
Percent Error
11. Using a reference text or online resource, find the theoretical bond length for your molecule with the
bond order determined in 10.
12. Find the percent error of your calculated bond length with respect to the theoretical bond length
determined in 11.
If time permits, continue to “focus” your calculation to improve bond length accuracy and hence, % error.
Questions:
1. What molecule did you study?
2. What did you determine the bond length to be?
3. How does this calculated bond length compare with the accepted value?
4. What is your calculated percent error?
Suggestions for Further Work:
Use a table of electronegativities to create a diatomic molecule/compound that is polar covalent and another that is
ionic then complete the following:
a. Determine the atomic radii of your elements at http://chemicool.com/.
b. Use Waltz to make animations of these molecules/compounds to determine their bond lengths.
c. Compare these bond lengths with the accepted values and calculate the percent error.
d. What trends or relationships do you notice pertaining to atomic radii, bond length, and bond energy for
nonpolar covalent, polar covalent, and ionic molecules/compounds.