Shapes, of molecules

On page 432, we reasoned that because of resonance, oxygen-ojgrgen bonds in O3 were halfway between single and double bonds, that is, 1.5 bonds. Do you expect the sulfur-oxygen bonds in SO2 to be single, double, or 1.5 bonds Explain your answer, bearing in mind the ability of sulfur to expand its valence shell. 

The Lewis structure for water gives the impression that the constituent atoms are arranged in a straight line. 

What we seek in this section is a simple model for predicting the approximate shape of a molecule. Unfortunafely, Lewis theory tells us nothing about the shapes of molecules, but it is an excellent place to begin. The next step is to use an idea based on repulsions between valence-shell electron pairs. We will discuss this idea after defining a few ferms. 

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory 

The shape of a molecule is esfablished by experiment or by a quantum mechanical calculation confirmed by experiment. The results of fhese experiments and calculations are generally in good agreement with the valence-shell electron-pair repulsion theory (VSEPR theory). In VSEPR theory, we focus on pairs of elecfrons in fhe valence elecfron shell of a central atom in a structure. 

So far, we have only considered the electronic properties of the atoms within a molecule. We will now turn our attention to the spatial arrangement of those atoms within the molecule. 

There are two main types of bonding, ionic and covalent. Ionic bonding is characterised by the non-directional nature of the Coulombic attractions between ions, i.e. the electrostatic force radiating from the central ion is felt equally in all directions. The main factor that influences the structure of the crystal lattice is the relative sizes of the cations and anions, because this affects how the ions will pack together within the lattice. 

In simple ionic compounds, each ion only occupies one type of environment, with all the ions of the same type having exactly the same geometric relationship to all the other ions in the crystal lattice. In more complicated ionic compounds, it is possible for ions of one species to occupy one of a limited number of environments, but this is the exception rather than the rule at this level. 

Covalent bonds are different from ionic bonds, in that they are directional in nature. Furthermore, each covalent bond has a particular length. The consequence of this is that when one atom is covalently bonded to another atom, the relative position in space of these two atoms is fixed. This means that in a molecule held together by covalent bonds, each and every atom has a defined, and predictable, geometric relationship to every other atom within the molecule. Furthermore, in the case of covalent compounds, it is now the norm for every atom to be considered individually in respect of its geometric relationship to every other constituent atom. 

We will now look more closely at certain types of covalent bonding before examining some of the more common geometric possibilities that occur in organic molecules. 

Polyethylene (Section 14.4) is the most widely used polymer. Examples of plastics made from polyethylene include milk bottles, sandwich bags, garbage bags, toys, and molded objects. 

Lewis dot structures can only be drawn in two dimensions. Since the shapes of very few molecules can be adequately represented using two dimensions, it is necessary to go beyond the Lewis structure to infer the shapes of molecules. 

Unsaturated hydrocarbons Hydrocarbons that contain fewer than the maximum number of hydrogen atoms alkenes and alkynes 

FIGURE 5.6 Balloon models of electron-pair geometries for two to six electron pairs. Balloons of similar size and shape, when tied together, naturally assume the arrangements shown. 

Molecule Bonds Lone Pairs Bond Angle (DEGREES) Molecular Geometry Electron Pair Geometry 

Lewis structures can be used to explain and predict the shapes of molecules. The basic assumption is that, if the core of an atom is effectively spherical (as for most atoms it is), groups of electrons in the Lewis shell (single bond pairs, double-bond quartets, fractional bond pairs, etc.) get as far apart as possible. Thus, two groups take up a linear arrangement, three a trigonal-planar one, four tetrahedral, five trigonal-bip5Tamidal, six octahedral, and so on. 

For main-group atoms, lone pairs are included. For example, in an H2O molecule, there are two bond pairs and two lone pairs round the oxygen atom. These take up a tetrahedral arrangement, making the molecule angular, as observed. For transition-metal atoms, lone pairs are excluded (Chap. 13). 

Interpretation of main-group vaiencies in terns of a simpie modei of main-group atoms 

This theory is called valence-shell electron-pair repulsion (VSEPR) theory or electron-domain (ED) theory . It is described in detail in most textbooks. 

A fundamental principle of chemistry An understanding of the structure of matter at the atomic and molecular level is the key to understanding the properties of materials at the everyday, macroscopic level. We have already considered the structure of atoms (Chapters 15 and 16) and the nature of the chemical bonds that hold atoms together within molecules (Chapter 19). In this chapter we will look at the structure of molecules. 

So far, we are able to predict the dipoles of individual bonds. The overall dipole moment of a molecule is the vector sum of these individual bond dipoles. Before the bond dipoles can be used to predict the overall dipole moment of a molecule, however, the three-dimensional orientation of the bonds must be known. That is, we need to know the shapes of molecules. 

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 

When we attempt to show the shapes of molecules, we are faced with the problem of how to represent such three-dimensional objects on paper. Some bonds extend in 

Click Molecular Models to view the molecules in this book as interactive three-dimensional models. 

Ball-and-stick model of methane Space-filling model of methane 

Although Lewis structures allow us to account for the number of bonds in a molecule and show us the bonding pattern (which atoms are joined together and by how many bonds), they give us an incomplete, two-dimensional picture of the molecular structure. If we extend our analysis of the molecular structure to three dimensions, we need to examine the order in which the atoms are joined to one another and also how they are arranged in space—in other words, the molecular shape. In addition to being of theoretical interest, the shapes of molecules are 

If the central atom is surrounded only by single bonds, the process of determining the molecular geometry is fairly simple. We count the number of single bonds emanating from the central atom and use the information in Table 7.3 to correlate that number of electron pairs with the geometry predicted by VSEPR, and this describes the shape of the molecule. 

If you don t have access to molecular models, you can build your own by usingsmall balloons and holding them together with a clip of some sort. The balloons must stay out of each other s way physically, analogous to the electronic repulsions of electrons. 

Each of the geometrical arrangements shown in the table minimizes the electron pair repulsions for the indicated number of electron pairs. To visualize the shapes of molecules, it is essential that you have a sound mental picture of each of these geometries. 

Electron Pairs Geometric Name Bond Angles 

---

Table 1 3 lists the dipole moments of various bond types For H—F H—Cl H—Br and H—I these bond dipoles are really molecular dipole moments A polar molecule has a dipole moment a nonpolar one does not Thus all of the hydrogen halides are polar molecules To be polar a molecule must have polar bonds but can t have a shape that causes all the individual bond dipoles to cancel We will have more to say about this m Section 1 11 after we have developed a feeling for the three dimensional shapes of molecules... 

Section 1 10 The shapes of molecules can often be predicted on the basis of valence shell electron pair repulsions A tetrahedral arrangement gives the max imum separation of four electron pairs (left) a trigonal planar arrange ment is best for three electron pairs (center) and a linear arrangement for two electron pairs (right)... 

The geometries obtained from optimizations with semi-empirical calculations describe the shapes of molecules. The calculations have varying degrees of accuracy and take more time than molecular mechanics methods. The accuracy of the results depends on the molecule. 

On the assumption that the pairs of electrons in the valency shell of a bonded atom in a molecule are arranged in a definite way which depends on the number of electron pairs (coordination number), the geometrical arrangement or shape of molecules may be predicted. A multiple bond is regarded as equivalent to a single bond as far as molecular shape is concerned. 

The bright colors of flowers and the varied hues of autumn leaves have always been a cause for delight, but it was nor until the twentieth century that chemists understood how these colors arise from the presence of organic compounds with common structural features. They discovered how small differences in the structures of the molecules of these compounds can enhance photosynthesis, produce important vitamins, and attract pollinating bees. They now know how the shapes of molecules and the orbitals occupied by their electrons explain the properties of these compounds and even the processes taking place in our eyes that allow us to see them. 

To help us predict the shapes of molecules, we use the generic VSEPR formula ... 

Example the n = 2 shell of Period 2 atoms, valence-shell electron-pair repulsion model (VSEPR model) A model for predicting the shapes of molecules, using the fact that electron pairs repel one another. 

Fig. 6. The two generic shapes of molecules which exhibit flexoelectric polarisation under distortion of the equilibrium director distribution...

In this chapter, we will see how to predict the 3D shape of molecules. This is important because it limits much of the reactivity that you will see in the second half of this course. For molecules to react with each other, the reactive parts of the molecules must be able to get close in space. If the geometry of the molecules prevents them from getting close, then there cannot be a reaction. This concept is called sterics. 

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. 

Elaving developed ideas about Lewis structures and shapes of molecules, we are now in a position to explore some of the important properties of covalent bonds. These properties provide revealing evidence about molecular shapes. 

C09-0038. Draw as many different pictures as you can that illustrate geometric shapes of molecules that have no dipole moments. 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

Figure 4.20 Predicted shapes of molecules containing multiple bonds.

In Chap. 3 the elementary structure of the atom was introduced. The facts that protons, neutrons, and electrons are present in the atom and that electrons are arranged in shells allowed us to explain isotopes (Chap. 3), the octet rule for main group elements (Chap. 5), ionic and covalent bonding (Chap. 5), and much more. However, we still have not been able to deduce why the transition metal groups and inner transition metal groups arise, why many of the transition metals have ions of different charges, how the shapes of molecules are determined, and much more. In this chapter we introduce a more detailed description of the electronic structure of the atom which begins to answer some of these more difficult questions. 

Table 1.4 Shapes of Molecules and Ions from VSEPR Theory...

Number of Electron Pairs at Central Atom Hybridization State of Central Shape of Molecule or Iona Examples... 

The shapes of molecules and ions are explained by the valence shell... 

Shapes of Molecules with More Than One Central Atom... 

The recent book Conformational Analysis of Molecules in Excited States, by Jacek Waluk, Wiley, New York, 2000, describes the way we can experimentally determine the shapes of molecules in the ground and excited states. It can be a little high brow at... 

Nakatsuji, H. 1974b. Electron-cloud following and preceding and the shapes of molecules. 

---