Weekly Web Work #8:
aShapes of Molecules
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The purpose of this week's assignment is to examine some molecules and explore the geometry and bond angles around the atoms. Remember, that the first step is to draw the Lewis dot structure of the molecule making sure that all the valence electrons present in the molecule are shown in the drawing. Once the Lewis dot structure has been correctly drawn, find the atom whose geometry you want to describe. Determine the number of electron charge clouds (atoms and/or lone pairs) around this atom to predict the bond angle. Now look to see how the atoms are arranged to predict the shape. See chapter 5 section 7 (pg 112 - 116) in your text for more information.
Let's look at the structure of aspartic acid, one of the amino acids, as an example. The correct Lewis structure has already been drawn for you.
aspartic acid
Look at the carbon indicated by arrow I. There are 3 electron charge clouds around it: a single bond to oxygen, a double bond to oxygen, and a single bond to carbon. The farthest apart 3 "things" can be from each other is a trigonal planar arrangement with the charge clouds 120oapart. Thus, the O-C-O bond angle is 120o. As all of the charge clouds are connected to atoms, the geometry around this carbon atom is trigonal planar.
Now look at the oxygen indicated by arrow II. There are 4 electron charge clouds around it: a single bond to carbon, a single bond to hydrogen, and two lone pairs of electrons. The farthest apart 4 "things" can be is in a tetrahedral arrangement where the charge clouds are approximately 109o apart. Thus, the C-O-H bond angle here is about 109o. The geometry around this oxygen is bent. (We have to think tetrahedral for the arrangement of the electron charge clouds to determine the bond angles. To describe the shape around this oxygen, we look at the atoms. Since only two of the charge clouds are atoms, the shape around this oxygen atom is described as bent.)
Now it is your turn. Look at the Lewis dot structure of caffeine. Determine the bond angles and geometry around atoms indicated by the arrows.
caffeine
Here is a ball and stick picture of PFCl4. Phosphorus is shown in blue and is the central atom. The fluorine atom is shown in green and the chlorine atoms are shown in red. Notice that three of the chlorine atoms are in the same plane and the plane also includes the phosphorus atom.
PFCl4
Isopropyl alcohol, C3H8O, is shown below. All three pictures are of the same molecule but in different orientations. Hydrogen atoms are shown as gray, oxygen as red, and carbon as black.
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