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Before You Start

Before You Start

hollis
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Before You Start

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  1. Before You Start This lesson contains some internal hyperlinks. That is, a link that takes you to another slide in the presentation that is not the next slide in succession. To come back to your original slide (the one you were on before clicking on the internal hyperlink) always click on the following “Back” icon (don’t click on the one here, it’s not active): If for some reason you get lost, just exit the slide show mode and find your original place by glancing at the list of slides. Why don’t you try it once before starting the lesson? Go here and then click on the “Back” icon on the new slide to come back to this slide. See the next slide for one more thing to know about the internal hyperlinks before you start the lesson...

  2. Some of the internal hyperlinks in this presentation take you to a slide with an embedded interactive 3D molecule (interactive in the sense you can drag your mouse over it to rotate it and view it from different perspectives). For this to work, you need to install ChemBio3D on your computer. The instructions on how to do that follow. However, if you don’t want to download the software, I’ve provided an alternative on the next slide. 1) If you DO want to download ChemBio3D Click here to download and install a free two week trial of the ChemBio3D (requires internet connection; you will also need to register but don’t worry, they are a legit company and the software is used by chemists worldwide). You can buy the software as well, but the cost is probably prohibitive. After you’ve downloaded and installed ChemBio3D, the image below should now be an interactive 3D molecule. If you decide not to download the ChemBio3D free trial, skip the molecular geometry internal hyperlinks throughout the lesson (these are the links in the “Molecular Geometry” column of the tables that appear in the lesson). I’ve provided an alternative, described on the next page...

  3. 2) If you DON’T want to download ChemBio3D As an alternative to ChemBio3D, during the lesson I’ll also direct you to the following external site: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html At that site you can find all the same types of information that the embedded 3D molecules had. Some of the molecules at the external site are not exactly the same as the ones in this lesson, but they are the same geometries and that’s really what matters. For practice go to the link above. For this external site to be fully functional you might be prompted to install Java or give permission for Java to run. Don’t worry, Java is ok! Just give it permission to run (it probably is already running on your computer in some capacity anyway). And the source I presume is OK too. It’s from the Chemistry Department at the University of Minnesota at Duluth. If the site is working correctly on your computer, you should be able to interact with the 3D molecules by dragging your mouse over them. The default molecule is PF5 and should load on your browser looking like the following: This is just a screen shot of a portion of the external web page. It’s not an interactive molecule in the PowerPoint. Don’t hesitate to contact me should you have any difficulty with either option for viewing and interacting with the 3D molecules. With that said, let’s move on to our lesson and begin our journey of learning about Molecular Shapes...

  4. Molecular Shapes Have you ever stopped to think just how important liquid water is to life on earth? For example, water is just the right solvent to act as a medium for the essential biochemical reactions that occur in our bodies and in other plants and animals. It is the fact that water is a liquid at ordinary temperatures and pressures that is the key. If it were a solid (ice) or gas (water vapor) around room temperature or body temperature, it wouldn’t be nearly as useful. The reason water is a liquid at room temperature is determined by its molecular shape, the topic of today’s lesson. “Planetary scientists are quick to stress that it's not just water that's indispensable for life, but liquid water. The distinction is key”. Source of quote: http://www.pbs.org/wgbh/nova/evolution/liquid-of-life.html

  5. Molecular Shapes • This lesson will teach you a simple, yet powerful method for determining the shape of water and many other molecules (it will be in a later lesson that we’ll use what we learn to determine why water is a liquid at ordinary temperature and pressure). • Let’s begin by taking a look at a video in which a chemist demonstrates an analogy for what we’re about to learn. He’ll demonstrate the different shapes that form when different numbers of balloons are tied together. Key points to look for as you watch the video: • The balloons are meant to represent regions of electrons that occur in molecules • The balloons, once they are tied together, naturally assume their shapes depending on how many are tied together (2 balloons tied together have a different shape than 3 balloons tied together, and both are a different shape than 4 balloons tied together, and so on). He doesn’t force them into a certain shape. It just happens naturally. • Here is the video (requires internet connection).

  6. Molecular Shapes To understand why molecules behave in a manner similar to the balloons, it’s probably best to start with the name of the theory we’ll be using to predict molecular shapes. Valence Shell Electron Pair Repulsion (VSEPR) theory sounds complicated, but it’s really quite straightforward (you may have noticed in the video, the chemist sometimes referred to “Vee-SEPR”). The key to understanding VSEPR is to focus on the words electronandrepulsion. Your Turn #1: What is the charge of an area where electrons reside: positive or negative? (click for answer) Answer: negative Your Turn #2: If you have one negative region next to another negative region, would the regions likely attract or repel? (click for answer) Answer: repel (opposite charges attract, like charges repel)

  7. Molecular Shapes So, learning how to predict the shapes of molecules comes down to remembering two simple principles: Regions of electrons are negative. Within a molecule, these regions will repel one another and try to get as far away from each other as possible (yet still stay connected to a central atom) This is why when two balloons were tied together (each balloon representing a region of electrons), they assumed a linear arrangement because two things can’t get any farther apart (and still stay connected) then being 180o from each other. 180o Let’s begin, then, by starting with molecules that have two regions of electrons around the central atom. Try the question on the next page. We’ll explain how to arrive at the answer after you try it first...

  8. Molecular Shapes Your Turn #3: Predict which Lewis structures listed here have central atoms surrounded by TWO regions of electron pairs. (click for answer) one region Answer: second region one region second region and Use the following criteria to determine the number of regions of electron pairs around the central atom: 1) Each bondingregion counts as one region. Any type of bonding region counts as ONE region, whether it is a single, double, or triple bond. For example, for HCN, the single bond connecting the H and C counts as one region (click to emphasize), and the triple bond connecting the C and N also counts as just one region (click to emphasize). Click to emphasize the two regions for CO2. 2) Each nonbonding region counts as one region. In both HCN and CO2, the central atom, carbon, has no unshared pairs of electrons (i.e. no nonbonding electrons). Therefore…

  9. Molecular Shapes …for both HCN and CO2 there is a total of 2 regions of electron pairs: two bonding regions + zero nonbonding regions. Let’s use this information to predict the molecular shape of these compounds, starting with CO2. 180o linear linear The Electron Group Geometry is the geometry assumed by the regions of electron pairs (click to emphasize). Predict the name of the electron group geometry for when you have two regions of electron pairs (i.e. taken together, how would you describe the shape of the red lines above; click for answer). The Molecular Geometry is the geometry of the atoms only. In this case the molecular geometry is the same as the electron group geometry because the O-C-O arrangement is linear (click to add molecular geometry). Whenever you have two regions of electron pairs, the bond angle is approximately 180o because this represents the maximum distance the two regions can get from each other to minimize repulsions yet still maintain contact with the central atom. Notice that the molecular geometry is a hyperlink. It’s one of the internal hyperlinks that takes you to an embedded interactive molecule. Requires installation of ChemBio3D. If you downloaded and installed ChemBio3D, then click on the link. If you decided to not download ChemBio3D, then go to the website linked below and click on the link for “2 Bonding Electron Pairs” in the “No lone pairs” column. Be patient while the molecule loads. The example used on the external site for 2 regions of electron pairs is BeH2, but it has the exact same shape as CO2. Here is the external link: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html

  10. Molecular Shapes The electron group geometry is not always the same as the molecular geometry. When we get to an example when the two are different, it will be clearer what the distinction is. The molecular shape of HCN can be determined in a similar way leading to: 180o linear linear (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “2 Bonding Electron Pairs” in the “No lone pairs” column) Note that whenever the number of regions of electron pairs is 2, the electron group geometry is always linear. Your Turn #4: Predict which Lewis structures listed here have central atoms surrounded by THREE regions of electron pairs. (click for answer)

  11. Molecular Shapes Answer: and In the case of the BF3, the three regions come from the three single bonds (click to animate). For O3, the fact that the molecule is drawn as resonance structures is not important. You can look at either of the resonance structures to determine there are three regions of electron pairs. Let’s focus on the left resonance structure. The three regions come from the double bond (click to animate), the single bond (click to animate), and the one nonbonding pair of electrons on the center oxygen (click to animate). Your Turn #5: Predict the approximate angle between each region when you have three regions of electron pairs around a central atom. (click for answer)

  12. Molecular Shapes 120o The angle between regions is always approximately 120o when you have three regions of electron pairs. When you have three regions of electron pairs around a central atom, the electron group geometry is called trigonal planar (trigonal because if you were to connect the outer portion of each region it would form a triangular shape, click to animate; and planar because all the regions lie in the same plane). In this case, the molecular geometry is also trigonal planar because the atoms have the same configuration as the regions of electron pairs. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “3 Bonding Electron Pairs” in the “No lone pairs” column) In the case of ozone, O3, we will see our first example of where the molecular geometry is not the same as the electron group geometry. For example…

  13. Molecular Shapes < 120o As with BF3, the electron group geometry is trigonal planar because we have 3 regions of electron pairs. In this case, however, one of the regions is a nonbonding pair of electrons (the pair coded in red). This not only skews the angle from the typical 120o, but it also means the molecular geometry (which is the geometry of just the atoms) will differ from the electron group geometry. Let’s first focus on…

  14. Molecular Shapes bent (or angular) < 120o …the question of molecular geometry. To determine the molecular geometry, we need to look at the structure of the atoms only and pretend the nonbonding pair on the central atom is not there (click to get rid of the nonbonding pair). Hopefully you can see now that the when you have three regions of electron pairs and one of the regions is a nonbonding pair of electrons, the molecular geometry is calledbent or angular (click to add to table; unfortunately the external alternative site offers no 3D example of 2 bonding regions with 1 lone pair, but we’ll see this same bent molecular geometry later in another example). Now to address the issue of the bond angle…

  15. Molecular Shapes …we need to remember that electrons are in orbitals. When an electron pair is in a nonbonding orbital, it only has one nuclei to hold it in place (as opposed to a bonding orbital which has two nuclei, one from each atom on both sides of the electron pair). Therefore, nonbonding electron pairs have a much greater freedom of movement compared to bonding electron pairs. It is < 120o similar to if you were holding on to one end of a rope and swinging it. The range of motion for the other end of the rope would be quite large. However, if another person was holding on to the other end, the range of motion would now be much more limited. In the same way, nonbonding electron pairs tend to spread out more and produce more diffuse electron clouds. This results in the nonbonding pair repelling the bonding pairs more than usual and pushing them closer together as shown in the diagram above. This is why the O-O-O bond angle in ozone is less than 120o. Nonbonding electron pairs (also called unshared pairs) on central atoms cause bond angles to be smaller than the ideal angle.

  16. Molecular Shapes To summarize the main points so far: When you have two regions of electron pairs around the central atom, the electron group geometry is linear. When you have three regions of electron pairs around the central atom, the electron group geometry is trigonal planar. Your Turn #6: Predict which Lewis structures listed here have central atoms surrounded by FOUR regions of electron pairs. (click for answer) Answer: and and Methane, CH4, has four regions from the four single bonds. Phosphine, PH3, has four regions from the three single bonds plus the one nonbonding pair, and water has the four regions from the two single bonds plus the two nonbonding pairs. To determine the geometry, let’s first consider the most symmetric, CH4…

  17. Molecular Shapes Your Turn #7: Predict the approximate angle between each region when you have four regions of electron pairs around a central atom. (click for answer) Answer: 109.5o The angle, as shown above, is 109.5o. If you said 90o, that certainly makes sense (and would be correct if molecules had to be two-dimensional), but it is incorrect because most molecules are three-dimensional. By spreading out to three dimensions it minimizes repulsions between the electron pair regions on the central atom. Remember, the driving force behind VSEPR, and what makes it an effective model for predicting molecular shapes, is that regions of electron pairs will spread out as much as possible to minimize repulsions. You might be asking yourself, what are those…

  18. Molecular Shapes Your Turn #7: Predict the approximate angle between each region when you have four regions of electron pairs around a central atom. (click for answer) Answer: 109.5o tetrahedral …funny looking wedges (click to emphasize) and dashes (click to emphasize). When writing structures, chemists use wedges to indicate something coming out of the paper towards you, and dashes to indicate something going into the paper away from you. Whenever you have four regions of electron pairs around a central atom, the electron group geometry is called tetrahedral (click to add to table). The name comes from the fact that if we were to connect the outer areas…

  19. Molecular Shapes Your Turn #7: Predict the approximate angle between each region when you have four regions of electron pairs around a central atom. (click for answer) Answer: 109.5o tetrahedral …we would wind up with a four sided figure known as a tetrahedron (as shown to the right). Your Turn #8: Predict the name of the molecular geometry of CH4. (click for answer and to add to table)

  20. Molecular Shapes Your Turn #7: Predict the approximate angle between each region when you have four regions of electron pairs around a central atom. (click for answer) Answer: 109.5o tetrahedral tetrahedral The molecular geometry is also known as tetrahedral because the atoms assume the same configuration as the regions of electron pairs (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “4 Bonding Electron Pairs” in the “No lone pairs” column) . Your Turn #9: It may be too soon to ask this, but let’s try. Can you make a generalization about when the molecular geometry will be the same as the electron group geometry? (click for answer)

  21. Molecular Shapes Answer: The molecular geometry will be the same as the electron group geometry when every region of electron pairs is a bonding region (i.e. there are no nonbonding pairs of electrons on the central atom, only bonding pairs attached to atoms). Let’s now move on to our next molecule with four regions of electrons around the central atom, PH3. trigonal pyramidal < 109.5o Your Turn #10: Predict the H-P-H bond angle in PH3? (click to add answer to table) Your Turn #11: Predict the name of the molecular geometry of PH3. (hint; click to add answer to table) The angle is slightly less than 109.5o because of the effect of the nonbonding pair of electrons. Now, to determine the molecular geometry, we need to pretend the nonbonding pair is not there (click to remove the unshared pair). It is a pyramid with a triangular base, hence the name trigonal pyramidal. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “3 Bonding Electron Pairs” in the “One Lone Pair” column)

  22. Molecular Shapes For our third and final scenario in the four regions of electrons category, let’s consider water. Once again we see below that the bond angle is less than the ideal 109.5o because of the effect of the nonbonding pair of electrons. In fact the two unshared pairs (remember, unshared pairs means the same as nonbonding pairs) reduce the angle to 104.5o in this case. < 109.5o Bent (or angular) Your Turn #12: Predict the name of the molecular geometry of H2O. (hint: we’ve seen it before; click to add answer to table) (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “2 Bonding Electron Pairs” in the “Two Lone Pairs” column)

  23. Molecular Shapes It’s now time to move on to five regions of electron pairs around the central atom. Your Turn #13 Predict which Lewis structures listed here have central atoms surrounded by FIVE regions of electron pairs. (click for answer) Answer: and and and Click to highlight the five regions for XeF2, then click to do the same for PCl5, and again for ClF3, and finally SF4. Let’s now consider the geometry of the most symmetric structure of the four, PCl5….

  24. Molecular Shapes trigonal bipyramidal …remember the terms equatorial and axial think of the earth (click). The equatorial positions are analogous to the equator going around the circumference of the earth and the axial positions are similar to the imaginary line running through the center of the earth from north pole to south pole. Whenever we have 5 regions of electron pairs around a central atom, the electron group geometry is referred to as trigonal bipyramidal. This name comes from the fact that if we were to connect all the outer atoms as shown to the right, the resulting structure would resemble two triangular based pyramids, one being inverted so the bases of the two pyramids were touching. As you can see from the picture, there are two distinct angles associated with… …trigonalbipyramidal electron group geometry, 90o and 120o. To expand on this let’s use the sketch with the wedges and dashes (click). Unlike the previous three categories we’ve focused on (linear, trigonal planar, tetrahedral), trigonalbipyramidal has two distinct types of regions of electron pairs. One is called equatorial and the other axial. Equatorial positions are shown in yellow, axial in green. The angle from equatorial to equatorial is 120o. The angle from axial to equatorial is 90o. To help you… 90o 120o Your Turn #14: Predict the molecular geometry of PCl5. (click to add to table) (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “5 Bonding Electron Pairs” in the “No lone pairs” column; Can you identify the axial and equatorial positions?)

  25. Molecular Shapes Let’s move on to our next five region molecule, SF4. S It’s obvious that the sketch is not complete. The fluorines and the unshared pair of electrons are missing. The question is where do we put the unshared pair of electrons? Recall the Valence Shell Electron Pair Repulsion Theory is based upon the simple fact that each region of electrons wants to spread itself out and get as far away from other regions as possible to minimize repulsions. Your Turn #15: Also recalling that nonbonding pairs of electrons tend to spread out and take up more space then bonding pairs of electrons, predict if the nonbonding electron pair of SF4 should go in equatorial or axial positions. (click for answer)

  26. Molecular Shapes With trigonalbipyramidal electron group geometry, nonbonding electrons always go in equatorial positions. By having the nonbonding pair in an equatorial position, it means there will be only TWO relatively close 90o interactions with other regions as shown here: 90o S 90o If we were to place the nonbonding pair in an axial position, the molecule would be less stable because that would create THREE relatively close 90o interactions: 90o 90o S 90o Therefore for SF4, we place the nonbonding pair…

  27. Molecular Shapes …in an equatorial position (it doesn’t matter which of the three equatorial positions you choose). F F S see-saw F F So now the question arises what name do we assign to the molecular geometry. Let’s remove the unshared pair of electrons (click). To better understand the origin of the name, let’s now turn it sideways (click). Can you tell what it looks like yet? Here’s one more hint (click). The equatorial to equatorial bond angle (yellow to yellow) is now less than the ideal 120o and the axial to equatorial bond angle (green to yellow) is less then 90o due to the effect of the unshared electron pairs. When you have 5 regions of electron pairs and one of the regions is an unshared pair of electrons, as in SF4, the molecular geometry is known as see-saw. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “4 Bonding Electron Pairs” in the “One Lone Pair” column; see if you can rotate the molecule so it looks like a see-saw to you)

  28. Molecular Shapes Our next 5-region example is ClF3. Notice again that whenever we have trigonalbipyramidal electron group geometry, unshared pairs of electrons always go in equatorial positions. The molecular geometry is called t-shaped because if we remove the electron pairs and then turn the molecule sideways it looks like a “T” as shown to the right. Again, the axial to equatorial bond angle is less then the ideal 90o due to the unshared pair of electrons effect. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “3 Bonding Electron Pairs” in the “Two Lone Pairs” column)

  29. Molecular Shapes Our final 5-region example is XeF2. As a result of once again putting the nonbonding electrons in the equatorial positions, we are left with a linear molecular geometry. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.html and click on “2 Bonding Electron Pairs” in the “Three Lone Pairs” column) The only bond angle in this case is the axial to axial F-Xe-F bond angle (a BOND angle must be the angle between two BONDS; the unshared pairs don’t represent bonds). Your Turn #16: Predict the F-Xe-F bond angle. (click for answer)

  30. Molecular Shapes Answer: The F-Xe-F bond angle is EXACTLY 180o. This is an example where, even though we have unshared pairs of electrons which offer more repulsion than bonding pairs, the unshared pairs are symmetrically distributed in a trigonal planar arrangement so any extra repulsion offered by one of the unshared pairs is negated by the other regions. 180o To summarize the main points so far: When you have two regions of electron pairs around the central atom, the electron group geometry is linear (180o). When you have three regions of electron pairs around the central atom, the electron group geometry is trigonal planar (120o). Four regions it’s tetrahedral (109.5o), and five regions it’s trigonal bipyramidal (90o and 120o). We’re almost done! Time for our final category. From this list, the molecules which have SIX regions of electron pairs around the central atom are…

  31. Molecular Shapes and and The most symmetric is SF6: The geometry is referred to as octahedral because if we were to connect all the outer atoms we would get an eight-sided figure (click). All bond angles are 90o. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.htmland click on “6 Bonding Electron Pairs” in the “No Lone Pairs” column)

  32. Molecular Shapes The next possibility is demonstrated by BrF5: When you have octahedral electron group geometry, and one of the regions is an unshared pair of electrons, it does not matter where you put the unshared pair. All positions in the octahedral configuration are identical (there is no equatorial and axial for the octahedral arrangement). The molecular geometry is determined by imagining that the nonbonding pair is removed (click) and then connecting all the outer atoms (click). This results in a pyramid with a square base. Angles would be less than the ideal 90o due to the nonbonding electron pair effect. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.htmland click on “5 Bonding Electron Pairs” in the “One Lone Pair” column)

  33. Molecular Shapes And finally we have XeF4 to demonstrate our last example. When you have two nonbonding pairs with octahedral electron group geometry, the nonbonding pairs must go on opposite sides in order to minimize repulsions. The resulting F-Xe-F bond angle is exactly 90o for fluorines next to each other or exactly 180o for fuorines on opposite sides. The unshared pairs do not distort the angle because the extra repulsion of one is cancelled by the other. The molecular geometry is called square planar because if you imagine removing the nonbonding pairs, then connect the outer atoms, they lie in a square conformation in the same plane. (alternative 3D: http://www.d.umn.edu/~pkiprof/ChemWebV2/VSEPR/5-5.htmland click on “4 Bonding Electron Pairs” in the “Two Lone Pairs” column)

  34. Review Questions: 1. At the following link, try the online quiz which asks you to identify molecular geometries (not electron group geometries) if shown a molecular model (note that for the question with ozone, O3, the quiz incorrectly marks “bent” as a wrong answer): http://people.southwestern.edu/~footezm/GenChemTutorials/vseprquiz/vsepr_quiz2.html 2. Draw the Lewis structure, sketch the shape and determine the electron group geometry, molecular geometry, and approximate bond angles of NCl3. (click for answer) sketch electron group geometry: tetrahedral molecular geometry: trigonal pyramidal (remember: ignore lone pair; look at atoms only) bond angle: slightly less than 109.5 Lewis structure

  35. Review Questions: 3. Fill-in-the-blanks in the following statements (click for answers one at a time). With trigonalbipyramidal electron group geometry, unshared pairs of electrons go in the ___________________ positions. When there are two unshared pairs of electrons on the central atom that has octahedral electron group geometry, the two unshared pairs are located _________________________________________. 4. Complete the following table. (click for answers one at a time) equatorial on opposite sides trigonal planar 120o tetrahedral 109.5o trigonalbipyramidal 90o and 120o 90o octahedral

  36. The End

  37. Remember to click on the icon below to return to your spot in the lesson

  38. What does it look like if we connect all the atoms as shown?

  39. CO2

  40. HCN

  41. BF3

  42. O3

  43. CH4

  44. PH3

  45. H2O

  46. PCl5

  47. SF4

  48. ClF3

  49. XeF2

  50. SF6

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