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scan0002 - ————1 304 Chapter8 Bonding Basics...

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Unformatted text preview: ————1, 304 Chapter8 ; Bonding Basics Visualization: Covalent Bonding ififl? Tutorial: Covalent and ionic Bonding ElGURE 8.1 The American chemist G. N. Lewis (1875—1946) developed a model of atomic bonding that is still used today. These notes, written by Lewis in 1902, illustrate his thinking on how electrons were arranged around an atom.The Lewis dot symbols that we use today are slightly modified from this original work. The molecular models that researchers use in their search for marketable drugs serve as representations of the chemical bonds, the forces that hold atoms together, within a compound. Chemical bonds arise between atoms when some of the out— ermost electrons 0n the bonding atoms interact. In some cases, the electrons tend to congregate on one of the atoms of the bond. In other cases, the electrons are shared more or less equally between the atoms. A vital point to keep in mind is that there are no ahsolutes. There are many degrees of electron sharing, from more or less complete ownership by one atom to about equal sharing by both atoms, and every possibility on the scale of sharing can occur. Still, we classically think of three types of chemical bonds: the covalent bond, the ionic bond, and the metallic bond. Sodium chloride, the stuff we sprinkle on our French—fries, is an example of a compound with an ionic bond. The ionic bond lies at one end of the spectrum of chemical bonds, Where the atoms are held together by the force of attraction of opposite charges. We say that one or more electrons have been removed from one atom (remember, there are no absolutesl) and congregate on the other atom in the compound. Aspirin is an example of a compound in which the atoms are held together by covalent bonds. The covalent bond lies at the other end of the spectrum of chem— ical bonds, Where electrons are shared between the atoms. The positively charged nuclei on either end of the bond attract the negatively charged electrons. It is this force that holds the atoms together. “Real” bonds Purely ionic Purely covalent Covalent and ionic bonds lie at opposite ends of the bonding spectrum. Real bonds between different elements are neither purely ionic nor purely covalent. The aluminum atoms in a soda can are held together with a metallic bond. The metallic bond is a special type of bond in which metal cations are spaced throughout a sea of mobile electrons. We’ll learn more about this bonding pattern in Chapter 13. Lewis Dot Symbols The first step in the construction of the molecular model of a compound, such as aspirin, sodium chloride, or aluminum, is drawing an atom itself. We recognize that the “business part” of the atom is its set of valence electrons. In 1916, G. N. Lewis (Figure 8.1) developed a useful shorthand representation employing ...
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