2513: MOLECULAR STRUCTURES
1.1 Lewis Structures
Lewis structures are two-dimensional representations of the relative position of atoms in a molecule or polyatomic ion. All valence electrons are shown distributed around the bonded atoms as either shared electron pairs (bonding pairs) or unshared electron pairs (nonbonding or lone pairs). A shared pair of electrons is represented as a short line (a single bond). Sometimes atoms can share two pairs of electrons, represented by two short lines (a double bond). Atoms can even share three pairs of electrons, represented by three short lines (a triple bond). Examples are shown for SO2 and HCN below.
1.2 Resonance Structures
Resonance refers to bonding in molecules or ions that cannot be correctly represented by a single Lewis structure. These structures are often equivalent, meaning that they contain the same number of bonds at different locations. The molecule SO2 has two such resonance forms.
Resonance structures can also be non-equivalent, in which case they will have different numbers and/or locations of bonds. Note that any valid resonance structure of a molecule can be used to determine its shape and polarity.
1.3 VSEPR Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the three-dimensional shape of molecules (or polyatomic ions). It is based on the number of electron groups around a central atom. Electron groups can be a single bond, a double bond, a triple bond, a lone pair of electrons, or a single unpaired electron. Based on VSEPR theory, the molecular shape is primarily determined by minimizing the repulsion between these electron groups. Lone pairs of electrons tend to take more space than the bonded pairs leading to somewhat distorted structures.
The difference in electronegativity of the atoms in a bond determines whether a bond is polar or nonpolar. Depending on the arrangement of the polar bonds in a three-dimensional molecular structure, the molecule can be classified as polar or nonpolar. A molecule will be nonpolar if it contains all nonpolar bonds, OR it contains polar bonds that are arranged in a symmetrical manner around the central atom. A molecule will be polar if it contains polar bonds that are arranged in a nonsymmetrical manner resulting from the presence of lone pairs on the central atom, or when there are different atoms surrounding the central atom.
linear, nonpolar linear, polar
In this experiment, students will practice drawing Lewis structures of covalent compounds and polyatomic ions. Using a molecular model kit, students will build these molecules and determine their shapes and bond angles. Students will also determine the polarity of the molecule based on the structure that they have drawn and modeled.
2.0 PRELAB ASSIGNMENT
Review (in your textbook or notes)
- the steps and rules for drawing Lewis structures, including rules for structures that follow the octet rule, and those that involve odd electrons, electron deficiencies, and expanded octets.
- the different geometries and bond angles that can be formed by different electron group arrangements around the central atom.
Molecular Model Kits
Quick Guide to using your molecular model kit:
- Each colored ball corresponds to a specific atom. Refer to the color key provided in the kit.
- Use the medium length gray links for single bonds.
- Use the short white links for lone pairs.
- Use the flexible long gray links for making double or triple bonds.
Complete the following for each of the molecules and ions on your data recording sheet.
- Calculate the total number of valence electrons of all atoms in the molecule or ion. For a polyatomic ion, add one electron for each negative charge, and subtract one for each positive charge.
- Draw the Lewis structure of the molecule or ion, including all resonance structures, if applicable.
- Build an exact geometric model of the molecule using the balls and links provided in your kit. These 3D models will serve as a visual aid to help you in drawing the perspective structure of the molecule.
- Look at your model and draw the perspective structure of the molecule or ion. Use the following guidelines:
- For bonds lying in the plane of the paper, use a regular solid line.
- For bonds that project down into the paper away from you, use a hatched wedge-shaped line.
- For bonds that project up out of the paper towards you, use a solid wedge-shaped line.
Lewis Structure Perspective Diagram
- Determine the electron group geometry around the central atom (or each central atom, if there is more than one), and the ideal bond angle(s) associated with this geometry.
- Determine the shape of the molecule based on the bonded atoms.
- Determine the polarity of the molecule. Use an arrow to show the direction of the overall dipole on the perspective drawing.
- Disassemble the models and return all balls and sticks to the model kit when finished.
5.0 DATA RECORDING SHEET
- HCN Total number of valence electrons: _____________
- CH3OH Total number of valence electrons: _____________
- SeF6 Total number of valence electrons: _____________
- NO2- Total number of valence electrons: _____________
- AsF5 Total number of valence electrons: _____________
- XeF2 Total number of valence electrons: _____________
- TeF5- Total number of valence electrons: _____________
- CH2O Total number of valence electrons: _____________
- SF4 Total number of valence electrons: _____________
- ICl2+ Total number of valence electrons: _____________
- PO43- Total number of valence electrons: _____________
- CH3CO2- Total number of valence electrons: _____________
6.0 POST-LAB QUESTIONS AND CONCLUSIONS
- The tetrahedral shape is one of the most fundamental shapes in chemical compounds. How would you describe it in words to someone who has not seen it?
- The “octet” rule appears to be a very important rule governing the structures of molecules. Considering your work with models, provide a simple explanation for the importance of 8 electrons.
- Explain in your own words why nonbonded electron pairs help determine the shapes of molecules.
- Do all of the assigned molecules obey the octet rule? If not, why (or in what way) did the octet rule fail?
- As a test of what you have learned, predict the shapes of (a) TeCl4 (b) H2S (c) Cl2O.
- Models do not necessarily have to be physical objects. They can be 2-dimensional drawings or even mental constructs. Cite one or more examples of such models encountered outside of chemistry. Can you think of models that are used in your own field of study or that you will use in your future career?