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Dipole Moments and Intermolecular Interactions Although the forces that hold molecules and solids together dominate the study of matter, there are other forces that affect chemical and physical properties. These are forces that arise as a result of the interactions between complete molecular units. Matter is composed of electrically charged particles, so it is reasonable to expect that there exists some force between any two molecules in close proximity. Forces between molecules are of several types. Some compounds consist of polar molecules that attract each other as a result of the electrical charges. Other compounds consist of nonpolar molecules, but the electrons in one molecule are weakly attracted to the nuclei in another as a result of instantaneous electron distributions that are not symmetrical. Still other molecules contain hydrogen atoms that are attached to other atoms having high electronegativity, which leaves the hydrogen with a residual positive charge. As a result, the hydrogen atom can become attracted to an unshared pair of electrons on an atom in the same or another molecule. This type of interaction is known as hydrogen bonding. Although the forces that exist between molecules may amount to only 10 to 20 kJmol -1 , they have a great influence on physical properties and in some cases chemical behavior. It is essential to have an understanding of these types of forces (sometimes called nonchemical or non-valence forces in order to predict and interpret the properties and behavior of inorganic compounds. This chapter is devoted to the subject of intermolecular interactions. 6.1 DIPOLE MOMENTS Because atoms have different electronegativities, pairs of electrons that are shared in covalent bonds are not necessarily shared equally. The result is that the bond has a polarity with the center of negative charge generally residing on the atom having the higher electronegativity. For a covalent bond between two atoms, the dipole moment, μ, is expressed as µ = q x r Where q is the quantity of charge separated and r is the distance of separation. Several properties of molecules are related to their polarity, and it is a useful parameter for understanding molecular structure, so it is appropriate to
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explore this topic in greater detail. Before doing so, a comment on units is appropriate. The charge on an electron is 1.6022 x 10 -19 coulomb, and internuclear distances can be expressed in meters. As a result, the units on dipole moments are coulomb-meter (C m). A unit of polarity is defined as the debye, which is named after Peter Debye , who did pioneering work on polar molecules. The relationship in SI units is 1 debye = 1 D = 3.33564 x 10 -30 Cm Historically (as well as currently by many chemists), the quantity of charge separated is expressed in electrostatic units, esu, which is g 1/2 cm 3/2 sec -1 . The charge on the electron is 4.80 x 10 -10
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