Shapes and Polarity of Molecules

Using electronegativity values, classify each of the following bonds as nonpolar covalent, polar covalent, or ionic  

For each bond, we obtain the electronegativity values and calculate the difference in electronegativity. 

52 Describe the trend in electronegativity as increases or decreases for each of the following  

53 Using the periodic table, arrange the atoms in each of the following sets in order of increasing electronegativity  

---

We now turn from the use of quantum mechanics and its description of the atom to an elementary description of molecules. Although most of the discussion of bonding in this book uses the molecular orbital approach to chemical bonding, simpler methods that provide approximate pictures of the overall shapes and polarities of molecules are also very useful. This chapter provides an overview of Lewis dot structures, valence shell electron pair repulsion (VSEPR), and related topics. The molecular orbital descriptions of some of the same molecules are presented in Chapter 5 and later chapters, but the ideas of this chapter provide a starting point for that more modem treatment. General chemistry texts include discussions of most of these topics this chapter provides a review for those who have not used them recently. 

Electron-dot formulas can be used to diagram the sharing of valence electrons in molecules and polyatomic ions. The presence of multiple bonds can be identified, and possible resonance structures can be drawn. From the electron-dot formulas, we can predict the three-dimensional shapes and polarities of molecules. Then we examine how the different attractive forces between the particles of ions and molecules influence their physical properties, such as melting and boiling point. Finally, we discuss the physical states of solids, liquids, and gases and describe the energy involved in changes of state. 

Computer-generated electrostatic potential maps of methanol (CH3OH) and ethanol (CH3CH2OH). The surface encompassing each molecule shows the extent of electron charge density while the colors show the distribution of charge in the molecule. In this chapter, we study ideas that enable us to predict the geometric shapes and polarity of molecules. 

Fluorescence polarization measurements can thus provide useful information on molecular mobility, size, shape and flexibility of molecules, fluidity of a medium, and order parameters (e.g. in a lipid bi layer)2 . 

Insoluble polar molecules (e.g., long chain fatty acids) exhibit an extreme kind of adsorption at liquid surfaces. That is, they can be made to concentrate in one molecular layer at the surface. These interfacial films often provide the stabilizing influence in emulsions since they can both lower interfacial tension and increase the interfacial viscosity. The latter provides a mechanical resistance to coalescence. Such systems also lend themselves to the study of size, shape, and orientation of molecules at an interface. Having an adsorbed layer lowers the surface tension (to Ysolution) by the surface pressure jt= ysoivent - y solution as already noted. 

Whether you use gumdrops, electron dot diagrams, or supercomputers, the ability to model bonding between atoms is useful. By determining the shape and polarity of a molecule, you can predict its behavior and properties. In Chapter 10, you ll learn more about the forces between particles and the effects they have on the physical states of substances. 

Intramolecular forces are attractive forces within molecules. They are the chemical bonds that determine the shape and polarity of individual molecules. Intermolec-ular forces, on the other hand, are forces between molecules. 

However, for some specific compounds tested this was not the case. Specifically, liquid-assisted grinding experiments involving four steroids as target molecules and 24 small planar molecules as co-crystal formers showed surprising results (Figure 5.7). The shapes and polarities of the steroids... 

Most enzymes are specific the shape and polarity of the active site allow only the substrate molecule to bind reversibly to the enzyme. In the induced fit model, the shape of the substrate is a close, but not exact, match to the shape of the active site of the enzyme (Fig. 22.9). As the substrate binds to the enzyme, either or both molecules change shape slightly. It is believed that distortion of the shape of the substrate may... 

Altogether, the structural diversity of zeolites discussed above is responsible for a wide range of interesting zeolite properties such as ion-exchange capacity, specific adsorption behaviour, and catalytic activity due to acidity, shape selectivity caused by size and polarity of molecules, high thermal stability and resistance against solvents, and wide flexibility for adjustments by post synthesis modification. 

Predict the shape and polarity of each of the following molecules (6.7)... 

Adsorption Kinetics. In zeoHte adsorption processes the adsorbates migrate into the zeoHte crystals. First, transport must occur between crystals contained in a compact or peUet, and second, diffusion must occur within the crystals. Diffusion coefficients are measured by various methods, including the measurement of adsorption rates and the deterniination of jump times as derived from nmr results. Factors affecting kinetics and diffusion include channel geometry and dimensions molecular size, shape, and polarity zeoHte cation distribution and charge temperature adsorbate concentration impurity molecules and crystal-surface defects. 

We have to refine our atomic and molecular model of matter to see how bulk properties can be interpreted in terms of the properties of individual molecules, such as their size, shape, and polarity. We begin by exploring intermolecular forces, the forces between molecules, as distinct from the forces responsible for the formation of chemical bonds between atoms. Then we consider how intermolecular forces determine the physical properties of liquids and the structures and physical properties of solids. 

One important stracture in molecules are polar bonds and, as a result, polar molecules. The polarity of molecules had been first formulated by the Dutch physicist Peter Debye (1884-1966) in 1912, as he tried to build a microphysical model to explain dielectricity (the behaviour of an electric field in a substance). Later, he related the polarity of molecules to the interaction between molecules and ions. Together with Erich Hiickel he succeeded in formulating a complete theory about the behaviour of electrolytes (Hofimann, 2006). The discovery of the dipole moment caused high efforts in the research on physical chemistry. On the one hand, methods for determining the dipole momerrt were developed. On the other hand, the correlation between the shape of the molectrle and its dipole moment was investigated (Estermanrr, 1929 Errera Sherrill, 1929). 

---