5: Lewis Structures

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Note: This PRE-LAB ASSIGNMENT MUST BE COMPLETED BEFORE COMING TO LAB. In your lab notebook, draw a large picture (Lewis structure) of all the molecules (such that you can write distances above the bond). You will use these structures label bond lengths and angles as you proceed through the experiment.

Learning Objectives

To learn how to use one type of computer modeling
To look at what affects bond lengths and bond angles.

Experiment:

We will be using a program called Spartan to construct and examine molecules in during lab.

Part I:

Construct the molecules in the table below using Spartan. Minimize each structure after constructing it, by using the Minimize button. This must be completed after building each molecule.

Bond Order	Terminal Atoms	Hetero Atoms
CH₄	CF₄	H₃CNH₂
H₃C – CH₃	CCl₄	H₃COH
H₂C = CH₂	CBr₄	H₃CPH₂
HC ≡ CH	CI₄	H₃CSH

Step 1

Spartan Part I Step 1.jpg Open Spartan and click on new build.

Step 2

Spartan Part I Step 2.jpg Click on the carbon atom with four bonds.

Step 3

Spartan Part I Step 3.jpg Double click on the screen. You will see a carbon atom with four bonds appear.

Step 4

Spartan Part I Step 4.jpg Click the minimize button. This will place hydrogen atoms on any open bonds. Reminder this must be completed after each and every molecule is built regardless if hydrogen needs to be added or not.

Step 5

Spartan Part I Step 5.jpg Click measure distance under the Geometry menu.

Step 6

Spartan Part I Step 6.jpg Click the two atoms on either side of the bond that you would like to measure or click on the bond. The distance measurement will be in the lower right hand corner.

Step 7

Spartan Part I Step 7.jpg To create a bond between two carbons, double click the end of one of the bonds.

Step 8

Spartan Part I Step 8.jpg Click the minimize button. Measure bond lengths.

Step 9

Spartan Part I Step 9.jpg To create a double bond between two carbon atoms, click the carbon with the double bond in the atom menu. Double click on the screen.

Step 10

Spartan Part I Step 10.jpg To attach the second carbon atom, double click the end of the double bond.

Step 11

Spartan Part I Step 11.jpg Click minimize. Measure bond lengths.

Step 12

Spartan Part I Step 12.jpg To attach a different atom to a carbon, first click the carbon with four bonds and then double click on the screen. Then click the atom you would like to attach in the atom menu and double click the end of one of the bonds on the carbon that you just created on the screen.

Application of Part I:

Step 1

Using the methods discussed in the pre-lab and your data from Part I-1, predict the lengths of the following bonds. Explain your answers.

N-F, N-O, N-Cl, H-F

Step 2

Construct a molecule containing one or more of these bonds so that you can measure the bond lengths on Spartan. You must be certain that you can compare the bonds. This means that they must all have the same bond order (all single bonds). Do your predictions agree with Spartan?

Part II:

Construct the following molecules and measure all the bond angles in the molecule. Be sure that you select the atoms in the order of the angle being measured. i.e. H – C – H for CH₄. Record these drawings and your bond angles in your notebook.

CH₄	NH₃	H₂O
H₂C = CH₂	H₂C = O	HC ≡ CH

Step 1

Spartan Part II Step 1.jpg Build CH₄ molecule. To measure the angle, click measure angle under the geometry menu.

Step 2

Spartan Part II Step 2.jpg Click the atoms in the proper order, hydrogen then carbon followed by the last hydrogen in the bond angle to be measured. The bond angle will be in the lower right hand corner.

Step 3

Spartan Part II Step 3.jpg To build a benzene ring, click rings in the atom menu.

Step 4

Spartan Part II Step 4.jpg The benzene ring will appear in the top box.

Step 5

Spartan Part II Step 5.jpg Double click on the screen. Click the minimize button and measure the angles of the carbon, carbon, carbon bond as well as the hydrogen, carbon, carbon bond.

Part III:

Some practice on Lewis Structures

Complete the steps below for the molecules, NO₂⁺N₂O NO₂Cl NH₂^- Note: the underlined atom is the central atom.

Step 1

Draw the complete, correct Lewis structure

Step 2

Write all formal charges that are different from zero (0)

Step 3

Indicate the total number of electronic domains around the central or all internal atom(s) (underlined), specifying whether they are bonding or non-bonding.

Step 4

Predict the electronic and molecular shapes of the central atom in the ions and molecules below. It is up to you to determine if you use multiple bonding of the oxygen atoms to the nitrogen atom but you must explain your choices in your report.

Discussion:

Summarize your data for the experiment in a table.
What factors control the length of a bond?
What factors control the angle of a bond?
Did you see any relationship between bond order and bond angle?