l01 - CH 203 O R G A N I C C H E M I S T R Y I Lewis...

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Lewis structures of organic compounds © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Lewis structures of organic compounds Theories of bonding There are two theories of bonding: the classical and the molecular orbital theory. In the classical theory, a chemical bond is viewed as the result of the transfer of electrons from one atom to another or as the result of the sharing of electrons between atoms. In the molecular orbital theory, a chemi- cal bond is viewed as the result of the overlap of atomic orbitals to form molecular orbitals. In this lecture we consider the Lewis structure, which is the most useful expression of the classical theory of bonding. Experimental basis of the classical theory of bonding Ironically, the classical theory of bonding was developed to explain why the Group VIIIA gases (He, Ne, Ar, Kr, Xe, Rn) fail to form bonds with other ele- ments. The two experiments that shed the most light on the low reactivity of the Group VIIIA gases are the measurement (1) of an element’s first ioniza- tion potential IP and (2) of an element’s electron affinity EA. First ionization potential The first ionization potential IP 1 measures the energy required to rip an electron (e ) away from an atom X to produce a monocation X + : X ! X + + e E = IP 1 This process is accomplished by bombarding the atom with high-energy X-rays. Ionization potentials are always positive numbers signifying that energy (furnished by the X-rays) must be added to the atom to remove an electron. The plot below shows IP 1 as a function of atomic number of the first 19 ele- ments.
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Lewis structures of organic compounds © Bruno I. Rubio 2 The Group VIIIA elements represent local maxima on the plot: more energy is needed to remove an electron from these elements than is required to remove an electron from their near neighbors on the periodic table, that is, the Group VIIIA elements resist giving up an electron. In contrast, the Group IA elements Li, Na and K represent local minima on the plot: it takes the least amount of energy to remove an electron from these elements. Note that, when a Group IA element loses an electron, it acquires the same number of electrons as its nearest neighbor inert gas. Electron affinity The electron affinity EA measures the energy required or released when an atom X gains an electron to produce a monoanion X : X + e ! X E = EA The electron affinity may be positive or negative. A negative EA means that the atom releases energy when it absorbs an electron, that is, the atom is more stable (lower energy) after it gains an electron. A positive EA means that the atom requires energy to force it to accept an electron, that is, the atom is less stable (higher energy) after it gains an electron.
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This note was uploaded on 02/27/2012 for the course CH 203 taught by Professor Rubio during the Fall '07 term at BU.

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l01 - CH 203 O R G A N I C C H E M I S T R Y I Lewis...

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