Lewis structures determining shape

C09-0079. Silicon forms a tetramethyl compound with the formula (0113)4 Si Determine the Lewis structure, determine the shape about each inner atom, and draw a ball-and-stick model of the compound. 

These three frameworks and the framework for glycine in Figure 9 illustrate an important point about Lewis structures. Although Lewis structures show how atoms are connected to one another, a Lewis structure is not intended to show the actual shape of a molecule. Silicon tetrachloride is not flat and square, SO2 is not linear, and the fluorine atoms in CIF3 are not all equivalent. We describe how to use Lewis structures to determine the shapes of molecules later in this chapter. 

The Lewis stmcture of a molecule shows how the valence electrons are distributed among the atoms. This gives a useful qualitative picture, but a more thorough understanding of chemistry requires more detailed descriptions of molecular bonding and molecular shapes. In particular, the three-dimensional structure of a molecule, which plays an essential role in determining chemical reactivity, is not shown directly by a Lewis structure. 

Our approach to these molecules illustrates the general strategy for determining the electron group geometry and the molecular shape of each inner atom in a molecule. The process has four steps, beginning with the Lewis structure and ending with the molecular shape. 

Follow the four-step process described in the flowchart. Begin with the Lewis structure. Use this stracture to determine the steric number, which indicates the electron group geometry. Then take into account any lone pairs to deduce the molecular shape. 

C09-0020. Tin tetrachloride (S11CI4) is an important starting material for the preparation of a variety of tin compounds. Build the Lewis structure for SnCl4 and determine its shape. 

Use the Lewis structure of CIF3 to determine the steric number of the chlorine atom. Obtain the molecular shape from the orbital geometry after placing lone pairs in appropriate positions. 

C09-0114. In the lower atmosphere, NO2 participates in a series of reactions in air that is also contaminated with unbumed hydrocarbons. One product of these reactions is peroxyacetyl nitrate (PAN). The skeletal arrangement of the atoms in PAN appears at the right, (a) Complete the Lewis structure of this compound, (b) Determine the shape around each atom marked with an asterisk, (c) Give the approximate values of the bond angles indicated with arrows. 

Determine the Lewis structure and the molecular shapes of the carbon atoms of this molecule. Suggest a reason for the reactivity of C3 Hg. ... 

Creating the Lewis structures of molecules is a method for determining the sequence of bonding within a molecule and its three-dimensional shape. This works best for covalently bonded molecules, but can also work for ionic compounds. For example, this method can be used to explain why the sequence of bonding in water is H-O-H, rather than H-H-O, and why it has a bent structure, rather than linear. 

Keep in mind that the need for an expanded valence level for the central atom may not always be as obvious as in the previous Sample Problem. For example, what if you were asked to predict the molecular shape of the polyatomic ion, BrCU" Drawing the Lewis structure enables you to determine that the central atom has an expanded valence level. 

Trisodium phosphate (TSP), Na3P04, is used as a paint stripper and grease remover. Disodium hydrogen phosphate, Na2HP04, can be found in some laxatives. Draw a Lewis structure for each molecule, and determine the shape of the phosphate ion,... 

To determine the shape of a molecule, write the Lewis structure, then identify the arrangement of electron pairs and bonds in which the lone pairs are farthest from each other and from bonds name the molecular shape by considering only the locations of the atoms. Lone pairs distort the shape of a molecule to reduce lone pair-bonding pair repulsions. 

Determine the shape of the water molecule (H20). Rewrite its Lewis structure using the correct shape. 

Determine the shape of the PF3 molecule. Step 1. Write the Lewis structure ... 

For each of the following, write the Lewis structure, identifying the number of electron groups and the number of lone pairs. Determine the electron group geometry and the molecular geometry. Rewrite the Lewis structure in the correct shape. 

The shapes of molecules are determined by actual experiments, not by theoretical considerations. But we do not want to have to memorize the shape of each molecule. Instead, we would like to be able to look at a Lewis structure and predict the shape of the molecule. Several models enable us to do this. One of the easiest to use is valence shell electron pair repulsion theory, which is often referred to by its acronym VSEPR (pronounced vesper ). As the name implies, the theory states that pairs of electrons in the valence shell repel each other and try to stay as far apart as possible. You probably remember this theory from your general chemistry class. The parts of VSEPR theory that... 

Drawing the Lewis structure of a molecule can help you determine the molecule s shape. In Figure 3.30, you can see the shape of the ammonia, NH3, molecule. The ammonia molecule has three bonding electron pairs and one lone pair on its central atom, all arranged in a nearly tetrahedral shape. Because there is one lone pair, the molecule s shape is pyramidal. The molecule methane, CH4, is shown in Figure 3.31. This molecule has four bonding pairs on its central atom and no lone pairs. 

We can now use Lewis structures to determine the shape around a particular atom in a molecule. Consider the H2O molecule. The Lewis structure tells us only which atoms are connected to each other, but it implies nothing about the geometry. What does the overall molecule look like Is H2O a bent or linear molecule ... 

Once the Lewis structure has been determined, it is possible to know the shape of the molecule or ion. The most important piece of information needed to determine the shape is the total number of groups around the central atom, where a group could be another atom or a lone pair. A central atom connected to one or two other atoms is linear. When the central atom is connected to three atoms, the shape is trigonal planar. When the central atom is connected to four atoms, the shape is tetrahedral. When the central atom is connected to five atoms the shape is trigonal bipyramidal (two triangular-based pyramids joined at the base). When the central atom is connected to six atoms, the shape is octahedral. Other shapes are possible when atoms are replaced with lone pairs. 

Lewis structures are two-dimensional and do not show the three-dimensional shape of a molecule. However, the three-dimensional shape of a molecule is important in determining the molecule s physical and chemical properties. Sugar, or sucrose, is an example. Sucrose has a shape that fits certain nerve receptors on the tongue. Once stimulated, the nerves send signals to the brain, and the brain interprets these signals as sweetness. Inside body cells, sucrose is processed for energy. 

According to the VSEPR theory, the shape of a molecule is determined by the valence electrons surrounding the central atom. For example, examine the Lewis structure for CO2. 

Name the following compounds, draw their Lewis structures, and determine their shapes. 

One of the most useful molecular models is the structural formula, which uses letter symbols and bonds to show relative positions of atoms. The structural formula can be predicted for many molecules by drawing the Lewis structure. You have already seen some simple examples of Lewis structures, but more involved structures are needed to help you determine the shapes of molecules. 

You now know how to draw Lewis structures for molecules and polyatomic ions. You can use them to determine the number of bonding pairs between atoms and the number of lone pairs present. Next, you will learn to describe molecular structure and predict the angles in a molecule, both of which determine the three-dimensional molecular shape. 

Once a Lewis structure is drawn, you can determine the molecular geometry, or shape, of the molecule. The model used to determine the molecular shape is referred to as the Valence Shell Electron Pair Repulsion model, or VSEPR model. This model is based on an arrangement that minimizes the repulsion of shared and unshared pairs of electrons around the central atom. 

In this chapter we have explored the structure of organic compounds. This is important since structure determines reactivity. We have seen that weak bonds are a source of reactivity. Strong bonds are made by good overlap of similar-sized orbitals (same row on periodic table). Bends or twists that decrease orbital overlap weaken bonds. Lewis structures and resonance forms along with electron flow arrows allow us to keep track of electrons and explain the changes that occur in reactions. VSEPR will help us predict the shape of molecules. Next we must review how bonds are made and broken, and what makes reactions favorable. Critical concepts and skills from this chapter are ... 

---