Name(s) Lewis Dot Structures, VSEPR, and Geometries of Centers in Molecules/Ions

Transcription

1 Name(s) Lewis Dot Structures, VSEPR, and Geometries of Centers in Molecules/Ions Vocabulary and General Information 1. A center (or central atom) in a molecule or polyatomic ion (PAI) is defined to be an atom that is covalently bonded to more than one other atom. If an atom is bonded to only one other atom, it is called a terminal atom. Note that is always a terminal atom (and thus never a center) since it can only form one covalent bond. 2. A lone pair (also called a nonbonding pair) is a pair of electrons that is not shared by two atoms. It has a single atom to which it belongs. 3. A bonding pair (or shared pair) is a pair of electrons that is shared by two atoms, and therefore comprises a single covalent bond between the two atoms involved. 4. A bond angle refers to the angle formed by the axes of two covalent bonds originating from a single center in a molecule or PAI. Note that exactly three atoms must be involved; the two atoms bonded to the center, and the center itself. ne can therefore specify the bond angle by naming the three atoms, putting the center in the middle (e.g., a NC bond angle would refer to the angle formed by a C-N and C- bond, where C is a center to which both and N are bonded). NTE: you cannot have a bond angle between two lone pairs or a bonding pair and a lone pair! You can only have a bond angle between two bonding pairs (i.e., there must be atoms at the end of both pairs of electrons!). 5. An electron cloud around a center is a term used to refer to either a lone pair of electrons R any number of electrons involved in bonding between the center and another atom. That is, a single bond, a double bond, and a triple bond are all counted as one electron cloud each. 1 It is absolutely critical that you count clouds correctly. Re-read, please, and practice this on the prelab assignment (very carefully)! 6. For VSEPR purposes, a geometry is a term that describes the relative positions in space of things around a center. In one type of geometry, the things are electron clouds (see below). In the other, the things are atoms. (a) An electron-cloud geometry (ECG) is the geometry of the electron clouds around a center in a molecule or PAI. It describes the number of electron clouds that surround a center, as well as the angles between the axes that define the positions of the electron clouds around that center. There is a unique electron-cloud geometry corresponding to the number of electron clouds around a center: linear (two clouds), trigonal planar (three clouds), tetrahedral (four clouds), trigonal bipyramidal (five clouds), and octahedral (six clouds). See the images in Section 10.2 in Tro to see what these look like. Since there are typically more than one AG (see below) associated with each ECG, I think of each ECG as representing a family of AG s. (b) An atom geometry (AG) [in my class] is the geometry of the atoms around [and sometimes including] a center in a molecule or PAI. It describes the number and relative positions in space of all the atoms bonded to (and sometimes including) the center. If there is only one center in a molecule, this kind of geometry describes the shape of the molecule, and so it is often referred to as a molecular geometry or molecular shape. Unfortunately, most chemistry texts also use molecular geometry or shape (rather than atom geometry) to describe the geometry around each center in a molecule having more than one center. I refuse to do that because the actual shape of a large molecule is not described accurately by any single atom geometry. Nonetheless, you should be aware of this convention. There are eleven types of distinct AG s. Five of these have the same names as the five ECG s (see above). Each of these AG s has no lone pairs. The other six are called bent, trigonal pyramidal, see-saw, T-shaped, square pyramidal, and square planar. (A couple of these can be arrived at by more than one way.) Table 10.1 in Tro (p. 436) shows all of these. All of these models have at least one lone pair, and they can be grouped into families based on their ECG. See page 8 of this handout to see these families. 7. Analytical Process: In analyzing a center to assess geometrical aspects, I find it useful to follow this process: 1) Count the electron clouds, 2) Determine the electron cloud geometry, which sets the basic bond angle information (and thus also sets the family ) [NTE: Later, this will also determine what type of hybrid orbitals are used by 1 Note: It is more traditional to use the term electron pair in place of electron cloud because VSEPR stands for valence shell electron pair repulsion. Many authors are switching to cloud or group because although we treat a double bond or a triple bond as one thing around a center in VSEPR theory, neither a double or triple bond contains only one pair of electrons! (It should be called VSECR or VSEGR! )

2 the central atom for forming covalent bonds or housing lone pairs!]. 3) nce the family is determined, one need only consider how many lone pairs are around the center if one want to determine the AG. This is so because an AG differentiates between a lone pair (which does not have an atom at the end of it) and a bonding pair (which does have an atom at the end of it); an ECG does not differentiate these. 8. Bottom line, and an example. The bottom line of all this? The number of electron clouds (and the ECG) is the key for angular information (and hybrid orbitals, later). If you want the AG, just additionally count the number of lone pairs and see where you end up : If all of the electron clouds are bonding clouds (i.e., no lone pairs!), then the name of the AG will be identical to the electron-cloud geometry that corresponds to that specific number of clouds. But if any of the electron clouds are lone pairs, the atom geometry will have a different name, which describes the geometry of the atoms around the center (but the angles of the ECG). As a result, with a little thought and practice, these names can generally be reasoned out/memorized. Example (ECG of trigonal planar): If there are three electron clouds surrounding a center, the ECG is trigonal planar (with 120 angles), but there will be two possible AG s: 1) The first AG, the one with no lone pairs, will be the AG having the same name as the ECG, which in this case is trigonal planar (no lone pairs here just means all three electron clouds have an atom at the end of it!); 2) The second one, which has one of the three electron clouds being a lone pair (meaning the remaining two electron clouds have atoms at the end of them) is called bent. [Imagine just pulling off one of the three atoms in a trigonal planar geometry and look at what is left over!]. Although the names are different for these two AG s, the bond angles will be nearly the same (120 here) because of the three (total) electron clouds. If you pull off a second atom from trigonal planar, however, you end up without a center, so that s why there are only two AG s for the trigonal planar family. If you do the same thing starting with tetrahedral, you can see why there are three possible AG s, and with trigonal bipyramidal, there are four possible AG s (see back page of handout and/or Table 10.1 in Tro). Although there are, in principle, five possible AG s for an octahedral ECG, the last two are not observed in nature, so only three are considered here. What does this exercise actually involve? Models and Model Parts: You will be given a container containing pre-formed models of the AG s that correspond to the families associated with 3, 4, 5, and 6 electron clouds (e.g., trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral). These models will have white pegs (representing atoms) at the end of each green (or gray) tube (representing bonding electrons). A multi-pronged peg will be in the center of each model, giving it its basic shape. Never dismantle these parent AG s they should be put back into the boxes intact at the end of the exercise. The container will also contain free green (or gray) tubes with colored pegs on them (representing atoms), some free red tubes, which represent lone pairs, and a some 3-, 4-, 5-, and 6-pronged pegs (representing central atoms). These parts will be used by you in Part B to build all of the non-parent AG s in the four families noted above. General Procedures: In Part A, you will first examine the parent AG structures (i.e., no lone pairs) to familiarize yourself with the names, angles, and overall look of these geometries. In Part B, you will build all of the models of AG s that have at least one lone pair in them. You will do this one family at a time, and you will physically organize these models on the benchtop by family so you can see the similarities and differences. You will also draw a wedge and dash sketch of each geometry/model/ag to represent the geometry faithfully in three-dimensions. You will also compare each sketch of a geometry/model/ag to a Lewis dot structure (LDS) representation of a center with that given AG, so you can see how the two map to one another (and see how an LDS generally does not show proper geometric information that is not its purpose!). In Part C, you will either start with or generate LDS s for various molecules having only one center, and use VSEPR ideas to answer geometrical questions about them and draw geometrically "faithful 3-D sketches of them. 2

3 Although this can be done without the models present, the idea here is to use the models, if present, to help you visualize what you draw. In Part D, you will answer similar questions regarding angles and AG s about molecules with more than one center, and do so with fewer explicit steps (i.e., less guiding you along the way). The idea here is that it is best to think through all of the steps as in Part C problems, even if you are not asked specifically to state the number of electron clouds or the ECG. (The Q s at the very end of the worksheet are on topics that come after VSEPR; answer them later ) ===================================================================================== Part A. Examining and familiarizing yourself with the basic atom geometries (the ones having no lone pairs and the same names as the electron-cloud geometries). 1. Remove the pre-made single-centered structures from your container. For the sake of these exercises, each green stick represents a bonding cloud of electrons (imagine that there s an atom at the other end). Red sticks are used to represent lone ( nonbonding ) pairs later when needed. 2. Find the structure that matches each sketch below, and write the proper name of the atom geometry below the sketch (use your text or class notes to get the names). If you are uncertain about why the name is what it is, ask me! It is important for you to become very familiar with these names, and they D have some logic to them! (NTE: I have left out linear here, but you are responsible for that as well). 3. Look closely at the way the dashed and solid wedges attempt to show the three-dimensional arrangement of the structures. old up the model to each drawing and rotate it around until you can see how to interpret the wedges and dashes so that you can visualize the 3D shape that the drawing is intending for you to see. nce you have done this matching for each structure, check off the box indicated. Note: You will need to be able to draw such structures yourself in this exercise, as well as on an exam, so use these drawings as models when you need to draw these geometries in the exercise today. 3. Write down the value(s) for the bond angles that characterize the geometry in each structure below (consult your text), and then examine the actual model to see if it appears to match the angle it is supposed to have. Write the name of the atom geometry depicted: I have physically and visually matched the model with the drawing so I can see how the wedges indicate threedimensionality. (Check each box as you go.) Write the bond angle(s) in this atom geometry: 3

4 Part B. Generating and analyzing the atom geometries that involve lone pairs. NTE: D NT DISMANTLE ANY MDEL UNLESS SPECIFICALLY TLD T D S. AT TE END F PART C, YU SULD AVE TWELVE CMPLETED MDELS T ANALYZE/ANDLE. 1. In the Trigonal Planar Family (3 charge clouds): "Bent". Use the extra pieces provided to make a replica of the trigonal planar model. Remove one of the green sticks from this trigonal planar model, and replace it with a red stick (lone pair). This represents the atom geometry called bent (or angular or V-shaped ). (Can you see why?) Remember that the atom geometry name generally arises from the look of the ATMS rather than the lone pairs. Sketch a picture of this atom geometry, including the lone pair. Draw the Lewis dot structure (LDS) for S2, which has this atom geometry, and see how it corresponds to your sketch of "bent". Sketch of Bent, with lone pairs and angles shown Lewis Structure Asked For 2. In the Tetrahedral Family (4 charge clouds): "Trigonal Pyramidal" and "Bent" To start off, build two additional tetrahedral models (use extra pieces). a. Remove one green stick (bonding pair) from one of the tetrahedral models and replace it with a short red stick (lone pair). This model now represents the atom geometry called trigonal pyramidal. (Can you see why?) Sketch this atom geometry. Draw the LDS for N3, which has this atom geometry, and see how it corresponds to your sketch of "trigonal pyramidal". Sketch of Trigonal Pyramidal, with lone pairs and angles Lewis Structure Asked For b. Remove two green sticks off of the second tetrahedral model, and replace them with red sticks. This atomic arrangement should look familiar--it is called bent! owever, in this "version" of "bent", the bond angle is not 120, because there are four charge clouds instead of three. Sketch this geometry, showing the two lone pairs. Draw the LDS for 2, which has this atom geometry, and see how it corresponds to your sketch of "bent". Sketch of Bent (as others) Lewis Structure Asked For 4

5 3. In the Trigonal Bipyramidal Family (5 charge clouds): "See Saw", "T-Shaped", and "Linear" To start off, build three additional models of the trigonal bipyramidal geometry. a. Are all the bonding clouds identical in this geometry? You should see that they are not! There are three equatorial bonding clouds and two axial ones. Remove an axial bonding cloud from one of the replicas and replace it with a lone pair. Using a second replica, remove an equatorial bonding cloud and replace it with a lone pair. Compare the two models that you just made (that each have a single lone pair). Given that a lone pair is held more closely to the center and therefore pushes harder on the other electron clouds, which model represents the one with the least amount of repulsion? Note that it is the 90 interactions that cause the worst repulsion interaction--the 120 interactions are much less unfavorable. Sketch the one that you consider the preferable one (showing the lone pair), which should be the one called see saw. (Can you see why it has this name?! Remember this!) Draw the LDS for SF4, which has this atom geometry, and see how it corresponds to your sketch of "see saw". Sketch of See Saw (as others) Lewis Structure Asked For b. Rearrange the sticks as needed on the non-see-saw model from the prior exercise (don t dismantle the seesaw one!) to make it look identical to the see-saw model. Then remove an equatorial bonding cloud on this model and replace it with a lone pair (for the same reasons as above in (a)). This atom geometry is called Tshaped, and I hope you can see why!! Draw the LDS for ClF3, which has this atom geometry, and see how it corresponds to your sketch of "T-shaped". Sketch of T-Shaped (as others) Lewis Structure Asked For c. If you were going to replace three bonding clouds with lone pairs (using the third model of trigonal bipyramidal), which would you replace? This atom geometry is called linear (yep, this is a second way to get linear!). Sketch this (along with lone pairs) below, and draw the Lewis structure for XeF2, which has this atom geometry. Sketch of Linear (as others) Lewis Structure Asked For 5

6 4. In the ctahedral Family (6 charge clouds): "Square Pyramidal" and "Square Planar" To start, make two additional replicas of the octahedral geometry. a. Remove one bonding cloud from one of the replicas and replace with a lone pair. Sketch it, as well as the Lewis dot structure for BrF5, which has one lone pair, and see how they correspond. This atom geometry is called square pyramidal. Can you see why? Sketch of Square Pyramidal (as others) Lewis Structure Asked For b. Since all of the positions are identical in an octahedral geometry, it is best to place two lone pairs, if present, as far away from each other as possible (i.e., across from each other). Using the second model, replace two bonding clouds that lie across from one another with lone pairs. This geometry is called square planar (Can you see why?). Sketch this, as well as the Lewis structure for XeF4, which has two lone pairs, and see how they correspond. Sketch of Square Planar (as others) Lewis Structure Asked For PART B SUMMARY EXERCISE: G T BACK PAGE NW (then come back to do parts C & D) Part C. Small Molecule (one-center-only) Examples Determine the electron cloud geometry and atom geometry of the centers in the following, and then sketch the species using wedges and dashes to show proper geometry/angles. Include ATMS and lone pairs. 1. Cl, (Note: in ACIDS, the is nearly always bonded to an! This forces the Cl to be in the center in this case even though it is more electronegative than Cl!) Wedges/dashes sketch (with ATMS and lone pairs) F + 2. ClF 4+, Wedges/dashes sketch (with ATMS and lone pairs) F Cl F F 6

7 3. 3 (draw the Lewis structure for this one yourself! Make sure you double check your Lewis structure!) Lewis Structure: Wedges/dashes sketch (with ATMS and lone pairs) 4. KrF 4 (draw the Lewis structure) Lewis Structure: Wedges/dashes sketch (with ATMS and lone pairs) 5. 2 C (draw the Lewis structure) Lewis Structure: Wedges/dashes sketch (with ATMS and lone pairs) Part D. Larger molecules State the atom geometries of each center in the following molecules, and determine the bond angles indicated. BUILD TE MLECULE WIT MLECULAR MDELS IF INDICATED. 1. AG(C 1 ): C C N 1 2 AG(C 2 ): C bond angle: CC bond angle: CCN bond angle: 7

8 Br 2. CBr=CC 3. A structural formula is: C 1 C 2 C 3 Complete the sketch of this one (w/proper wedges and dashes to show geometry): C 2 C 3 AG s: C 1 : C 2 : C 3 : C 1 Br bond angle: BrC 1 C 2 bond angle: C 2 C 3 bond angle: C 3 bond angle: 3. Note: You must add lone pairs as needed in the structure below! Also, if time, BUILD TIS NE! F C 5 AG(C 5 ): F-C 5 - angle: C 4 Cl AG(C 2 ): AG(): -C 2 - angle: N C 3 C 3 -- angle: C 2 C 1 AG(C 4 ): C 5 -C 4 -C 3 angle: ANALYZE CLSELY TE SPACE-FILLING MDEL, IF PRVIDED, FR TIS MLECULE. Do you see why I don t like the term molecular geometry? You cannot describe the geometry of this molecule in one word or phrase, can you??? NTE: A final question relating to the above structure may be handed out to you on a separate page at some point later for hand-in. 8

9 Part B SUMMARY EXERCISE: Fill in the table below (with name and sketch of the appropriate atom geometry), and then physically arrange all your models on your desktop in the order indicated below. Take some time to study these as follows: Convince yourself that you can easily determine the atom geometry for a center in a Lewis dot structure as follows: 1) identify which family (column) you re in (ECG) by counting the number of total electron clouds around the center, and then 2) (in your mind) remove one atom/bond for each lone pair on the center in the Lewis structure (which in the table corresponds to moving down one row). D NT DISMANTLE YUR MDELS. Use them to help you do the questions in parts C and D. At some point in time before you leave lab, you must show your models (in this order) to your instructor along with the filled in table below. NTE: Each below is meant to represent an atom (not the symbol for the element oxygen). If I could have colored them in black, I would have. You may write solid spheres to represent atoms in your structures. # of e - clouds Trigonal Trigonal Tetrahedral ctahedral Planar Bipyramidal Family Family Family Family Trigonal Planar Tetrahedral Trigonal Bipyramidal ctahedral N lone pairs ( Parent Structure) Square Pyramidal NE lone pair Bent TW lone pairs (N/A) TREE lone pairs (N/A) (N/A) (N/A) 9