Chemical Bonding: Covalent Systems Written by Rebecca Sunderman, Ph.D Week 1, Winter 2012, Matter & Motion

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1 Chemical Bonding: Covalent Systems Written by Rebecca Sunderman, Ph.D Week 1, Winter 2012, Matter & Motion A covalent bond is a bond formed due to a sharing of electrons. Lewis structures provide a description of the atoms and bonding within a covalent compound as well as the valence electrons. VSEPR theory utilizes electronic groups, from Lewis structures, and proposes electronic and molecular geometries based on the repulsion interactions between these electronic groups. An electronic geometry takes into account the number of electronic groups around a central atom. A species with four electronic groups would have an electronic geometry of tetrahedral. The true geometry, or molecular geometry, examines the number of electron pairs around the central atom. The molecular geometry will be a variation of the electronic geometry. For example, a central atom with four electronic groups, one of which being a lone pair of electrons, would have a molecular geometry of trigonal pyramidal. Using models is an effective way to study VSEPR theory and the resulting electronic and molecular geometries. Appendix I provides a chart to assist with electronic and molecular geometry determination. Appendix II includes diagrams of some common molecular geometries In this laboratory experience you will determine Lewis structures and make VSEPR theory models of the compounds, elements and ions found in table 1. In addition you will build several larger organic molecules and examine VSEPR geometry within the compound. Name Formula carbon dioxide CO 2 nitrogen N 2 oxygen difluoride OF 2 hydronium ion H 3 O + phosphorus trichloride PCl 3 hydrogen peroxide H 2 O 2 ozone O 3 hexafluorosilicate(iv) ion 2 SiF 6 nitrate ion NO 3 formaldehyde CH 2 O xenon tetrafluoride XeF 4 iodine pentafluoride IF 5 sulfur tetrafluoride SF 4 triiodide ion I 3 Table 1: Compounds, Elements and Polyatomic Ions 1

2 Prelab Assignment: Complete the following items prior the start of lab. 1) Prepare your lab notebook for recording data Complete the title, name, intro BE SURE TO INCLUDE APPENDIX 1 & APPENDIX 2 INFORMATION and purpose as usual. Leave space for the procedure (The procedure section will be very short for this experiment.). Beginning on the bottom half of a page or on the next full page divide up each page into fourths until fourteen sections are created. (Each section should take up one quarter of a page.) See figures 1 and 2. Name and formula Lewis Structure VSEPR Sketch Electronic Geometry : Figure 1: Notebook Pages divisions Molecular Geometry : Figure 2: Section Divisions Each small section will need room to contain the compound, element or polyatomic ion name, formula, Lewis structure, VSEPR sketch, electronic and molecular geometry. The order does not matter. (If you need more space, divide your page into two blocks instead of four.) 2) Fill in the name, formula and Lewis structure for each compound, element or polyatomic ion from Table 1. Procedures and Data Collection: Model Kits: We will be using several different types of model kits for this experiment. Each has its use and each can help you visualize molecular shape. Use balloons (model #1), Styrofoam (model #2), and Organic kits (model #3) for each species you examine: A minimum of three models will be constructed for each species. In the Styrofoam model kits, each toothpick represents two electrons. It is important to include all electron pairs on the central atom. 2

3 Small Molecules: Compare Lewis structures with your partner and reach an agreement for each of the fourteen species in Table 1. Use model kits to make VSEPR theory models of these compounds, elements and ions. For each model, obtain an instructors initials or check your work with another team (get their initials) before taking the model apart. Larger Organic Molecules: Larger systems have a more complicated geometry. VSEPR theory can still be applied. When using VSEPR theory we examine geometry at each atomic center. Use the Organic model kits for this portion of the experiment. 1) Build the compound 3,5dimethylheptane. For each carbon center identify the VSEPR geometry. Provide the molecular formula for this compound. Draw the line structure for this compound. CH 3 CH 3 CH 3 CH 2 CH CH 2 CH CH 2 CH 3 2) Build the compound methylcyclopentane. For each carbon center identify the VSEPR geometry. Provide the molecular formula for this compound. Draw the line structure for this compound. CH 3 H 2 C CH CH 2 CH 2 CH 2 3) Build the compound 2methyl1,3butadiene. For each carbon center identify the VSEPR geometry. Provide the molecular formula for this compound. Draw the line structure for this compound. CH 2 =CCH=CH 2 CH 3 4) Build the following set of three compounds: C 2 H 6, C 2 H 4, and C 2 H 2. Draw Lewis structures and diagram observed geometries. For each carbon center identify the VSEPR geometry. 3

4 Data Analysis & Additional Questions: These questions can be completed outside of lab time. Be sure to write the question and your answer to the question in your lab notebook. Some of these questions can best be answered while you still have a model to examine. 1. Which compounds, elements and/or ions violate the octet rule? 2. Define the term allotrope. What allotropic element was examined in this experiment? Provide an additional allotrope of this element. 3. Phosphorous is another example of an allotropic element. In the white phosphorous elemental form, P 4, each phosphorus atom has identical bonding. Draw a Lewis structure for this molecule. Diagram the geometry of this molecule. 4. Single bonded carbontocarbon systems have full rotation around the bond. As bond order increases to either a double or triple carbontocarbon bond, that rotation is restricted. Why? Explain. References: Brady, Russell & Holum; Chemistry: Matter and Its Changes, 3 rd Ed. Wiley & Sons (2000) Holtzclaw, Robinson & Odom; General Chemistry with Qualitative Analysis, 9 th Ed. D.C. Heath and Company (1991) 4

5 Appendix 1: Partial VSEPR Geometry Determination Chart # electronic groups Electronic Geometry Angle (degrees) # unbonded electron pairs Molecular Geometry Example 2 Linear Linear HCN 3 Planar Planar BF 3 3 Planar 1 Bent NO 2 4 Tetrahedral Tetrahedral CH 4 4 Tetrahedral 1 Pyramidal NH 3 4 Tetrahedral 2 Bent H 2 O 5 5 Bipyramidal Bipyramidal 90, Bipyramidal PCl 5 1 See Saw SF 4 6 Octahedral 90, 90 0 Octahedral SF 6 6 Octahedral 1 Square Pyramidal IF 5 6 Octahedral 2 Square Planar ICl 4 5

6 Appendix 2: Diagrams of Some Common VSEPR Molecular Geometries Cut out each geometry diagram and secure it, with its appropriate name, in your lab notebook. (If you would prefer to draw each diagram instead of cutting and pasting, you may.) INCLUDE THESE GEOMETRIES WITH THEIR NAMES AS A PART OF YOUR INTRODUCTION TO THIS EXPERIMENT. Bent Linear Octahedral See Saw Square Planar Square Pyramidal Tetrahedral Bipyramidal Planar Pyramidal 6