Lewis Structures and Molecular Shapes

Transcription

1 Lewis Structures and Molecular Shapes Rules for Writing Lewis Structures 1. Determine the correct skeleton structure (connectivity of atoms). Usually, the most electronegative atoms go around the edges (terminal atoms). Hydrogen always goes to an outer edge. 2. As much as possible, use the 90% rules to make a reasonable estimate of the bonding scheme in the molecule. 3. Count up the total number of valence electrons in the structure. Add the number of valence electrons for each atom in the molecule, then add one electron if there is a negative charge on the molecule; subtract one electron if there is a positive charge on the molecule. 4. Subtract the number of electrons that have been used to make bonds (two electrons per bond) from the total number of valence electrons. 5. Use the remaining valence electrons to make lone pairs around the outer atoms (except H) to complete their octets. When making octets remember that for each atom, a bond counts as two electrons and a lone pair counts as two electrons. a) If electrons remain after this step, complete the octet on the central atom. b) If there aren't enough electrons to complete the central atom s octet, form double or triple bonds by converting lone pairs on the terminal atoms into bonds between the terminal atom and the central atom. c) Remember that the elements B, C, N, O, F and Ne can have no more than eight electrons around them. However, the elements after Ne can have expanded octets, containing from 8 up to 18 electrons. 6. Calculate the formal charge for each atom in your Lewis structure. The formal charge is a rough measure of the overall electric charge created by the positive nuclear charge and the negative electronic charges around an atom. To calculate formal charge on an atom, subtract the number of bonds on an atom (B) and the number of unbonded lone pair electrons (N) from the number of valence electrons expected for the atom (V). Formal Charge = V B N

2 Summary of Valence Shell Electron Pair Repulsion Model (VSEPR) 1. Electron pairs tend to arrange themselves to minimize repulsions. Ideal geometries (both bonds and lone pairs count toward the coordination number) are: coordination number 2, linear, 180 o coordination number 3, trigonal (planar), 120 o coordination number 4, tetrahedral, o coordination number 5, trigonal bipyramidal; equatorial, 120 o ; axial, 90 o coordination number 6, octahedral, 90 o 2. While lone pairs help determine shapes of molecules, they are considered to be invisible when molecular shapes are described. For example, the central atom in NH3 has four electron pairs (3 bond pairs and 1 lone pair). The four pairs of electrons form a tetrahedral arrangement around NH3, with o between each pair. However, the shape of this molecule is described by looking only at the arrangement of the atoms. In this case, NH3 is described as trigonal pyramidal, because that's how the atoms are arranged. 3. When lone pairs are present, the bond angles are somewhat less than predicted by rule 1. (This means that lone pairs appear to take up more space than bond pairs.) 4. The relative magnitudes of electron pair repulsions follow the order: lone pairlone pair > lone pairbond pair > bond pairbond pair 5. Lone pairs choose to occupy the equatorial sites in a trigonal bipyramidal arrangement of electron pairs. 6. If all sites are equivalent, lone pairs will arrange themselves in a trans geometry. 7. When applying rules 15, consider all double and triple bonds in the Lewis structure to be equivalent to single bonds. (This is an approximation that simplifies predictions of geometry using VSEPR.) 2

3 Lewis Structures Draw a good Lewis structure for each of the following molecules that includes formal charges and all lone pairs. Also draw a coordinate structure, a threedimensional representation that includes bonds, bond angles, and lone pairs on central atoms. Build a model of each molecule and name its molecular shape. H 2 O CO 2 NF 3 C 2 H 2 CCl 4 COH 2 H 2 O 2 N 2 NH 4 + BF 4 C 4 H 4 OH 3

4 NO 3 SF 6 XeO 2 F 4 PCl 5 Draw a good Lewis structure for each of the following molecules that includes formal charges and all lone pairs. Also draw a coordinate structure, a threedimensional representation that includes bonds, bond angles, and lone pairs on central atoms. Name each molecule s molecular shape. ClO 3 (chlorine is in the center) + ClO 3 (chlorine is in the center) N 2 O (one of the nitrogens is in the center) C 6 H 6 (Carbons in a circle) 4

5 OCN (carbon is in the center) ClO 4 (chlorine is in the center) NO 2 (nitrogen is in the center) NO 2 + (nitrogen is in the center) SO 4 2 (sulfur is in the center) 5

6 Molecular Modeling 1. Launch Spartan. 2. Let s build a molecule water. a. Click on the New icon (the leftmost one in the row). Note that the + icon (6 th from left on row of icons, labelled Add Fragment ) is illuminated. A palette of atoms with common bonding possibilities should appear at the right of the screen. b. Note: You will use the Entry palette today, but take a brief look at the other two palettes (Expert and Peptide). Return to the Entry palette before proceeding. c. Click on the oxygen atom with two single bonds (2 nd row, 3 rd column in the Entry palette). Note that your selection should appear in the large gray area just above the Entry palette. d. Now click anywhere in the open field and the oxygen atom with two single bonds should appear. e. Note that you can: a. rotate the oxygen atom by holding down the left mouse button and moving the mouse about. b. move the atom by holding down the right mouse button and moving the mouse about. c. make the atom larger by holding the Shift key and moving the mouse up (away from you) while holding the right mouse button. f. Now click on the hydrogen atom on the palette (1 st row, 4 th column) and then click once on each of the free ends of the single bonds extending from the oxygen atom. You ve made water! g. Note that if you were to have made a mistake in creating the water molecule, the Delete icon (7 th from left on row of icons) can be used to delete an atom. You also have the option to simply Close the file (under the File pulldown menu) and start over again. 3. Click on the Energy Minimization icon (an E with a down arrow above it). This performs a fairly straightforward molecular mechanics calculation to find the molecular structure with the lowest energy by optimizing a small set of molecular interactions. (You will do much more sophisticated energy calculations very shortly!) IMPORTANT NOTE: You should always use this Energy Minimization step before performing any further calculations! You will find out why later. 4. Click on the View icon (the V) and use the Model pulldown menu to look at the different ways (Wire, Ball and Wire, Tube, Ball and Spoke, Space Filling) of representing the structure of your water molecule. Try to imagine how each of these models might be useful. You will need to toggle back to the + icon to make structural additions or changes. 6

7 5. Click on the Geometry pulldown menu and the Measure Distance selection (or click on the icon labeled Distance). Clicking on any two atoms (whether they are bonded or not) in the structure will display the distance between the atoms in the lower right corner of the Spartan window. 6. Click on the Geometry pulldown menu and the Measure Angle selection (or click on the icon labeled Angle). Clicking on any three atoms in the structure will display the angle formed by the atoms in the lower right corner of the Spartan window. Caution: The second atom clicked will be the one at the vertex of the angle; thus, clicking on a hydrogen atom, then the oxygen atom, then the other hydrogen atom will give the HOH bond angle, but clicking on atoms in the order hydrogenhydrogenoxygen will give a different answer. 7. Measure and record the O H bond length and the H O H bond angle for H 2 O. Add this information to a data table in your notebook similar to the one below. 8. You now have a molecule with the approximate geometry of water The method used by Spartan for its default minimization is called Molecular Mechanics. The advantage of molecular mechanics methods is that they are extremely fast. The downside is that there is no guarantee that you will get an accurate answer. To use methods other than the default minimizer, you must use the Calculations option under the Setup menu. Bring up the Calculations dialog box menu and change the settings to calculate an Equilibrium Geometry using the HartreeFock method. 9. Hit the OK button when your setup is correct. Next, choose Submit from the Setup menu. After naming the file that will contain the output it will run and you will be notified when it has completed. Record the bond length and angle as before and add them to your table. 10. Close the water molecule file so that you will have a clean field in which to build new molecules. 11. Determine the bond lengths and bond angles for three more molecules from the first page of Lewis Structures that you drew, as you did for water. Complete the data table. Data Table Molecule Method Bond angle(s) Bond length(s) H 2 O HartreeFock HartreeFock HartreeFock HartreeFock 7