Lecture 9 Notes The Covalent Bond and Lewis Structures

Lecture 9 Notes The Covalent Bond and Lewis Structures -...

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The Covalent Bond and Lewis Structures B. A. Rowland 53750/53760 The Covalent Bond A covalent bond is one that is formed through the sharing of a pair of electrons in a molecule. Just as in our discussion of ionic bonds, there will be two parameters we will be interested in: the covalent bond length (i.e. how far apart are the individual atomic nuclei in the molecule) and the covalent bond strength (i.e. how much energy must be expended to break the covalent bond). There will be a new parameter that we didn’t discuss for ionic bonds—that of a bond angle (exception: diatomic molecules have no bond angles). Molecules are 3-dimensional structures (not 2-dimensional as the Lewis structures would imply). Hence, there will be angles between the bonds. We will learn more about this when we begin assigning molecular geometries to our Lewis structures. The way we can determine both bond length and bond strength for covalent bonds is through the use of spectroscopy (i.e. using EMR to probe the structure of matter—we will study this at the end of this unit) or X-ray diffraction. Covalent Bond Lengths As discussed above, covalent bond lengths are the distance between the nuclei in a chemical bond. There is a general trend you should be aware of concerning covalent bond lengths, and that is as the atomic number increases, the bond length will increase. As an example, for the hydrides of the halogens, the order of increasing bond length will be (HF < HCl < HBr < HI). This trend can be rationalized using the concept of the atomic radius: as the atomic number increases down a column, we expect the atomic radii to increase, meaning that there will be more distance between the nuclei and hence a longer bond. You should keep in mind that this trend is only qualitative—bond lengths really aren’t just sums of atomic radii (very complicated things can happen to the electron cloud when bonding occurs that precludes us from making too specific a trend). A significant experimental result is that covalent bonds tend to be about the same size in
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Lecture 9 Notes The Covalent Bond and Lewis Structures -...

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