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UMNO your UADOUR with regards to Lewis Structures and VSEPR. You may also review the attached powerpoint slides that we will cover in lab this week. b. Complete a brief 1-2 page introduction in your lab notebook that gives an example of how a molecular formula relates to its Lewis structure and geometry.

Answer 1

Let's look at how molecular formulas, electron dot diagrams and structural formulas are related to one another for several simple compounds. The molecular formulas show the correct number of each type of atom in the molecule. The electron dot diagrams (Lewis structures) show the arrangement of those atoms and all of the valence electrons. The structural formulas show the arrangement of the atoms and the covalent bonds between them. The following diagrams are also shown in Example 2 in you workbook. Note that the structures shown here are slightly different from what is in your workbook. This is because all of these diagrams are left-right-up-down representations of the actual three-dimensional molecules and as such can be shown with different orientations. Not only is there random variation in how these formulas are drawn, the medium (chalkboard, word processing, HTML) dictates which orientations are easiest to format. Molecular Formula Electron Dot Diagram Structrual Formula CH4 H ·· H : C : H ·· H H | H - C - H | H First we have the molecular formula CH4. In the electron dot diagram you can see how the four valence electrons of carbon match up with the one valence electron of each of the four hydrogen atoms to give eight electrons around carbon. The electrons are arranged in four pairs representing four covalent bonds. You should remember that the arrangement of those four pairs of electrons is called a tetrahedral arrangement. The structural formula emphasizes the bonds rather than the electrons and shows one line for each of the bonds that connect the atoms together. Each line represents one shared pair of electrons. Molecular Formula Electron Dot Diagram Structrual Formula CH2O : O : :: H : C : H O || H - C - H Next you can see the same things for CH2O. However, in this case, the resulting structural formula is different in two ways. First, since there are two pairs of electrons shared between the oxygen atom and the carbon atom, a double line is drawn. Second, since the other two pairs of electrons around the oxygen atom do not form a bond to another atom, they are not shown in the structural formula. Sometimes, when trying to emphasize the effect of unbonded electron pairs on structure or bonding or Lewis base properties, the unbonded electron pairs will be shown in structural formula. Usually, however, they will be left out. You should also remember that the arrangement of three groups of electrons around this carbon atom is called trigonal planar or flat triangular arrangement. Sometimes to emphasize that, the structural formula is drawn this way. H   \       C = O / H    Molecular Formula Electron Dot Diagram Structural Formula HCN H : C ::: N : H - C º N With HCN we have a triple bond between the carbon and nitrogen. The electrons around the central carbon atom are clustered into two groups, the single bond to hydrogen and the triple bond to nitrogen. You should remember that this is called a linear arrangement and results in a linear molecul. Molecular geometry, also known as the molecular structure, is the three-dimensional structure or arrangement of atoms in a molecule. Understanding the molecular structure of a compound can help determine the polarity, reactivity, phase of matter, color, magnetism, as well as the biological activity. Introduction To determine the shapes of molecules, we must become acquainted with the Lewis electron dot structure. Although the Lewis theory does not determine the shapes of molecules, it is the first step in predicting shapes of molecules. The Lewis structure helps us identify the bond pairs and the lone pairs. Then, with the Lewis structure, we apply the valence-shell electron-pair repulsion (VSPER) theory to determine the molecular geometry and the electron-group geometry. To identify and have a complete description of the three-dimensional shape of a molecule, we need to know also learn about state the bond angle as well. Lewis Electron Dot Structures play crucial role in determining the geometry of molecules because it helps us identify the valence electrons. valence-shell electron-pair repulsion (VSEPR) theory states that electron pairs repel each other whether or not they are in bond pairs or in lone pairs. Thus, electron pairs will spread themselves as far from each other as possible to minimize repulsion. VSEPR focuses not only on electron pairs, but it also focus on electron groups as a whole. An electron group can be an electron pair, a lone pair, a single unpaired electron, a double bond or a triple bond on the center atom. Using the VSEPR theory, the electron bond pairs and lone pairs on the center atom will help us predict the shape of a molecule. The shape of a molecule is determined by the location of the nuclei and its electrons. The electrons and the nuclei settle into positions that minimize repulsion and maximize attraction. Thus, the molecule's shape reflects its equilibrium state in which it has the lowest possible energy in the system. Although VSEPR theory predicts the distribution of the electrons, we have to take in consideration of the actual determinant of the molecular shape. We separate this into two categories, the electron-group geometry and the molecular geometry. Electron-group geometry is determined by the number of electron groups. 2 linear 3 trigonal-planar 4 tetrahedral 5 trigonal-bipyramidal 6 octahedral Molecular geometry, on the other hand, depends on not only on the number of electron groups, but also on the number of lone pairs. When the electron groups are all bond pairs, they are named exactly like the electron-group geometry. See the chart below for more information on how they are named depending on the number of lone pairs the molecule has. VSEPR Notation As stated above, molecular geometry and electron-group geometry are the same when there are no lone pairs. The VSEPR notation for these molecules are AXn. "A" represents the central atom and n represents the number of bonds with the central atom. When lone pairs are present, the letter Ex is added. The x represents the number of lone pairs present in the molecule. For example, a molecule with two bond pairs and two lone pairs would have this notation: AX2E2. Bond Angles Bond angles also contribute to the shape of a molecule. Bond angles are the angles between adjacent lines representing bonds. The bond angle can help differentiate between linear, trigonal planar, tetraheral, trigonal-bipyramidal, and octahedral. The ideal bond angles are the angles that demonstrate the maximum angle where it would minimize repulsion, thus verifying the VSEPR theory. Essentially, bond angles is telling us that electrons don't like to be near each other. Electrons are negative. Two negatives don't attract. Let's create an analogy. Generally, a negative person is seen as bad or mean and you don't want to talk to a negative person. One negative person is bad enough, but if you have two put together...that's just horrible. The two negative people will be mean towards each other and they won't like each other. So, they will be far away from each other. We can apply this idea to electrons. Electrons are alike in charge and will repel each other. The farthest way they can get away from each other is through angles. Now, let's refer back to tetrahedrals. Why is it that 90 degrees does not work? Well, if we draw out a tetrahedral on a 2-D plane, then we get 90 degrees. However, we live in a 3-D world. To visualize this, think about movies. Movies in 3D pop out at us. Before, we see movies that are just on the screen and that's good. What's better? 3D or 2D? For bond angles, 3D is better. Therefore, tetrahedrals have a bond angle of 109.5 degrees. How scientists got that number was through experiments, but we don't need to know too much detail because that is not described in the textbook or lecture. Using the example above, we would add that H2O has a bond angle of 109.5° and CO2 would have a bond angle of 180°. Steps Used to Find the Shape of the Molecule To sum up there are four simple steps to apply the VSEPR theory. Draw the Lewis Structure. Count the number of electron groups and identify them as bond pairs of electron groups or lone pairs of electrons. Remember electron groups include not only bonds, but also lone pairs! Name the electron-group geometry. (State whether it is linear, trigonal-planar, tetrahedral, trigonal-bipyramidal, or octahedral.) Looking at the positions of other atomic nuclei around the central determine the molecular geometry. (See how many lone pairs there are Rate it positive