Modeling Molecular Shapes
Chemistry Laboratory
Honors Chemistry (820), 2017-18

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Last update: Tuesday, September 16, 2008 7:08 AM


The purpose of this experiment is to develop a better understanding of the shape of molecules, and the role that the valence electrons play in determining molecular shape.


Most of our studies have focused around the composition and physical properties of elements and compounds, and the reactions that elements and compounds undergo. It turns out that a number of the properties of compounds are determined by the very shape of the molecules comprising the compound. Properties such as physical state (solid, liquid, and gas), as well as freezing point, boiling point, viscosity, and many others, are actually related to the shape of molecules. Indeed, the shape of molecules also determines the shape of the world around us, in that all macroscopic objects are comprised of molecules of particular shapes and sizes. Nature is amazing.

In this laboratory you will predict the shape of molecules using a physical model for molecular structure called the “ball and stick” model. The ball and stick model represents atoms as hard spheres with holes drilled in them, and represents bonds by sticks that connect the wooden “atom” spheres together. We must recognize that all models have limitations. Clearly, atoms are not hard spheres, and bonds are not sticks, however, the ball and stick model can still be used very effectively to show approximate bond angles and bond lengths. Since bond angle and bond length are at the very heart of molecular structure, we can gain great insight into the shapes of molecules from this primitive model. In addition, this simple model can predict if a molecule is polar or non-polar by the evaluation of the electronegativity of the elements in each of the molecular bonds, coupled with the geometry of the molecule.

The geometry of small molecules can be predicted by examining the number of electron pairs that surround the central atom using the Valence Shell Electron Pair Repulsion (VSEPR) rule. Simply stated, this rule tells us that since the valence shell electron pairs surrounding a central atom repel one another, they will position themselves as far apart from each other as possible. This rule yields the shape of the central atom, from which the molecular geometry is based. The molecular geometry is determined by identifying the number of bonds that are formed by the central atom, along with and the number of unshared electron pairs. Unshared electron pairs (called lone pairs) have greater repulsion than do bonding pairs, thus the lone pairs will reside in the positions around the central atom that allow for the greatest space. The molecular geometry, however, is determined only by the atoms that are bonded to the central atom, thus the shape of the bonding electron pairs dictates the molecular shape.

Once the shape of the molecule has been determined, the hybridization of the central atom or atoms can be predicted. Also, the covalent bonds of the molecule can be examined individually to see which, if any, are polar covalent bonds. If the molecule contains polar covalent bonds, then whether the molecule is polar or non-polar can be determined from the location of the polar bonds within the geometry of the molecule.


Assemble the following before commencing work on the activity.

  • A suitable "ball and stick" modeling kit.
  • A table or chart of electronegativity values.
  • A flat, clean workspace.

Safety and Waste Management

Safety eyewear and protective clothing are not required for this activity. The modeling kits contain small parts that are easily lost. Take care not to dispose of any kit parts accidentally as waste!

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Pre-activity Questions

Provide written responses to the following items before starting the activity. You may type and print your responses, or write them by hand on a separate piece of notebook paper. Your responses are due at the start of your laboratory period when this activity is scheduled.

  1. What is meant by molecular shape?
  2. Name and describe three characteristics of molecules (and the elements comprising the molecules) that are responsible for the physical dimensions and shape of the molecule that forms.
  3. Name and describe at least three other models that are used to depict or describe molecules. Explain how each differs from each other.

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  1. Prepare your workspace. Clean the lab bench top, if needed.
  2. Obtain a model kit – check the contents against the printed list that accompanies the box.
  3. Construct the first molecule on the list.
  4. Fill out the appropriate information in the Data Table you prepared in advance in your laboratory notebook, or use the Data Table handout per the instructions of your teacher.
  5. Repeat steps 4 and 5 for each of the molecules listed. Be sure to construct each molecule.
  6. When you have completed your data table, disassemble the models and return the parts to the model kit box.
  7. Verify the contents of the model kit against the list of parts that comes with each kit.



For each molecule listed by formula, draw a Lewis dot structure, determine the geometry of the central atom(s) according to VSEPR, determine the hybridization of the central atoms, and the overall shape of the molecule. List any polar bonds, determine if the overall molecule is polar, then draw a reasonable 3-D depiction of the model you have created. Record the information listed in the results section for each molecular formula listed either in your laboratory notebook, or by completing the appropriate row in the Data Table according to the instructions of your teacher.


Examine each completed structure carefully. Draw a reasonable 3-D representation of each structure in the space allocated on the Data Table, or in your laboratory notebook according to the instructions of your teacher.


As needed, calculate the electronegativity difference between the two atoms forming a bond to determine if the bond is non-polar, polar, or ionic.

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Post-activity Questions

Answer the following questions. Show your work and results either in your laboratory notebook, or on the report form provided, as instructed by your teacher.

  1. What is an electric dipole?
  2. What is a covalent bond?
  3. What is a polar covalent bond?
  4. What two factors determine if a molecule is polar or not?
  5. List five different molecular geometries that you studied in this experiment.
  6. Calculate the electronegativity difference (ΔEN), then predict the type of bond that will form for each of the following:
    1. Na – Cl
    2. C – H
    3. F – Cl
    4. S – O
    5. C – F
  7. Explain how you used molecular geometry to predict the polarity of molecules. Support your answer with examples from this investigation.

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Summarize how you think using physical models to depict the shape of molecules is useful. Use complete sentences in your summary. Confine your response to no more than one double spaced page.


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Copyright© 2005 – 2014, and 2015 by Alan Crosby (adcrosby@nshs-science.org)
Newton South High School, Newton Centre, MA 02459
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