Chemistry 101 Laboratory

• Read in your textbook, pages Chp 7.5 through 7.9 concerning the quantum theory of atoms.
• Be sure you understand the relationship between the principle (n), angular momentum (l) and magnetic (m) quantum numbers before coming into the lab

 

 
 
 

1. If the principle quantum number is 4, what are possible values for the angular momentum quantum number?  State the general relationship between the principle and angular momentum quantum numbers.

2. The quantum numbers for the first 2 rows of the periodic table are n, l, m_l  and m_s. What does the last one represent? What are possible values for it?  How many possible, different orbitals can there be for Carbon where n=2 ?

Learning the practice of chemistry not only involves learning how to use glassware, tongs and burners, but also learning the use of some more high-tech tools. Chemists today have many tools at their disposal, and one of the most useful of these is the computer. The computer allows us to model things that we cannot see in the physical world, whether it be the far reaches of the universe, the depths of the sea or the inner workings of the atom. Today you'll be using a program designed to allow a scientist to visualize modern atomic orbitals, where electrons are located. Computational science is the emerging new field that integrates computer science, mathematics, and science.  Using web- and local based programs, you will explore the variation of the "size" and "shape" of atomic orbitals of the first few elements in the periodic table.

Instructions

A. The Internet as a resource

 

1. The experiments can be carried out with your lab partner or on your own.  The experiments can also be done either in the lab or at home.  The "transportability" of Computational Chemistry experiments ( and research ) is a common feature to numerically intensive scientific investigations.

2. Sit down at a computer and open your browser to a search engine. Some popular ones include Lycos, Yahoo!, Dogpile, etc.

3. In the search box, type in "atomic orbitals".

4. You should be presented with a list of web sites that are related to "atomic orbitals". From this list, find two web sites that have good information on atomic orbitals. (The web sites you choose can't be the same as anyone else in the class, nor can they be the site that the orbital viewer is on www.orbitals.com )

5. For each of your web sites: record the URL of the site and print out the first page of the site for permanent copy. present a one paragraph summary of the site and the information you gained on atomic orbitals at the site.

6. If you are having trouble finding web sites, you can try a different search engine or a different phrase, such as "s p d f orbitals".  Many of the search engines have "focus" or common groups of information (usually "TV, Shopping, etc.") but also have scientific groups.  The latter have listings under "Computational XYZ" , where the "XYZ" part varies with the search engine.  Most have information on General Chemistry and atomic orbitals.

B. Visualization of Atomic Orbitals

 

1. The program "Orbital Viewer" is installed on all of the computers in the General Chemistry Lab.  Alternatively, you may download and use it at home.  If you decide to do the latter:  In your browser's navigation bar, type in http://www.orbitals.com/orb/ov.htm. This will take you to the Orbital Viewer homage. (The Orbital Viewer is a copyright of David Manthey)

2. Download the Orbital Viewer by clicking on the link and following directions.  This is not necessary on the computers in the General Chemistry lab, where OV is loaded on all of the computers.

3. Install the Orbital Viewer to the default directory and open the program.  Again, not necessary in the General Chemistry lab.



The Orbital Viewer main screen


 

4. Under the file menu, click on "New", and observe what happens. (You should be seeing an f orbital defined by the quantum numbers 4,3,0.  These are the n, l, and m quantum numbers for this orbital.)

5. On the toolbar, click the button representing Schrödinger's Wave function (Y). This opens the menu where you can change the values of n, l, and ml. (Note that ms isn't listed on this screen. Why not?)


This button lets you change the quantum numbers


 

6. Try a few different combinations of quantum numbers and describe the orbitals you see. What happens if you put in a combination that cannot exist? (ex. 2,3,4.)

7. Explore the other functions of the orbital viewer. Decide on a function you think is interesting and explain it in your lab book (again, one paragraph is sufficient.). A few possible projects include: making your model appear in stereo; changing the lighting effects around your model; exploring the different ways a model can be displayed (points, polygons, raytraced); etc.  In terms of the efficient use of time, polygons are always faster and their use is suggested for the experiments you do in this lab.

8. If you have time, you may want to build a simple molecule by combining two atoms. For example, H2 has a bond length of  74 pm and can be easily made using the program. (Ask for help on this if you have trouble doing it yourself.  As a hint, what you need to do is to displace the two atoms +37 pm along the x-axis and -37 pm along the x-axis so that they are separated by the proper distance.  Use of 1s orbitals for each then generates the molecule.

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A short explanation of Orbital Viewer's toolbar


 

A. This button opens a menu that lets you change how your orbital is displayed.	I. This button lets you change the position, angle and zoom of your virtual camera.
B. This button lets you change how many points are calculated and displayed.	J. This button lets you make cutaway models of your orbitals.
C. This button opens a menu of polygon rendering options.	K. This button lets you change the colors of your orbital model.
D. This button lets you change the way an orbital is modeled in ray trace mode.	L. This button resets the camera position to (0,0,default)(x,y,z), the starting position.
E. This button lets you change the various options for adding asymptotes to the model.	M. When used with the reset camera position, this changes the model size to fit the screen.
F. This menu lets you change the default settings for the orbital that you are viewing.	N. This button lets you display multiple open windows in a tile format.
G. This button lets you change the lighting of your model.	O. This button lets you display multiple open windows in a cascading format.
H. This button lets you render your model in various 3D or stereoscopic modes.	P. This button opens the online help file.

Post-Lab Questions

1.  If you compare orbitals with different principle quantum numbers but the same angular momentum and magnetic quantum numbers, what are two very specific differences between them?  (Other than the "l" and "m" quantum numbers of course.)

2.  As the central charge of the nucleus increases for higher atomic weight atoms, the actual size of the nucleus is still much smaller than that of the orbitals containing the electrons.  If we compare a 2s and 4s electron in lithium and potassium, explain two reasons why it is easier to remove the 4s electron than the 2s electron from the atom.  You should be able to graphically demonstrate this using the Orbital Viewer Program.  Remember when doing this and other atoms besides hydrogen that you have to change the number of protons in the nucleus to account for the effect of the nuclear charge on the orbital.  As a reminder, the number of protons equals the atomic number of the element in question.

3.  Suppose that there are two atoms of carbon which are separated from one another by 120 pm along the x axis, with the y-axis in the same plane as the x-axis and the z-axis perpendicular to the plane.  It is possible for the 2s orbitals of the carbons to overlap and form a bond between them.  What other orbitals can form a bond between the carbons and along the x axis?  Can any other bonds form.  You should illustrate this using the orbital viewer program.

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