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Unformatted text preview: 9 Sizes and Shapes of Molecules ,,;," l edgeof molecular sh a pes and sizes is important to an understanding of ': :a i reactions. The shape of a molecule ( and the bond types it possesses) po rtant implications for the manner in which it enters into chemical s. The shape and size of molecules also influence their packing in the es t a te. a toms combine to produce molecules , they often do so in a ccord w ith ( ru l e . Your text undoubtedly contains a fairly detailed discu ss ion of the : u l e . In essence, it may be de s cribed as the tendenc y for an atom to lo s e , s h a re electrons in order to achieve an S 2 p 6 configur a tion in the outer- s h e l l. The simplest atom s (H , Li, Be, and so on) tend to achieve a l s 2- ~ a t ion , according to wh a t might be called the duet rule . a pter 8 , we emphasize the los s a nd gain of electron s, le a ding to the : ;;' o n of electrically charged ions , such as Na + and Cl-. When electrons are _ ~ . a molecule is formed , and the atom s are connected by a co v a l e nt bond . '- ha pter we emphasize the approximate s h a p es , i nteratomic di s tances, n d energies o f m olec ul es and molec ul ar ions tha t are held toget h e r b y- :- : = T I t bon d s. e n gths of the bonds that hold the atoms together in a mol e cule can be ~ . ed in a variety of w ay s : for example , by direct c a lorimetric measure- ment, by dissociation equilibrium measurements, by absorption spectrum mea- surements, or by mass spectrometry. We define bond energy as the energy change (I:::, . H) for the chemical process in which one mole of a given bond is broken, when both the reactants and the products are in the hypothetical ideal-gas state of 1 atm and 25 C . For a diatomic molecule, the bond energy is identical to the energy required to dis- sociate the gaseous molecule into its respective gaseous atoms. For the dissoci- ation of Cl z gas this corresponds to the reaction for which the CI-CI bond energy is 58.0 kcal . We say that I:::, . H C 1- 1 = 58.0 kcal. For a polyatomic molecule of the type AB n , which possesses n A-B bonds, our definition of bond energy implies that each bond is the same, and that it corresponds to 1 / n of the total energy required to dissociate the gaseous AB n molecule into A + n B gaseous atoms . This is a useful definition except when studying the detailed steps of a chemical reaction. For example, the total bind- ing energy in a CH 4 molecule is 397 kcal l mole and, by our definition of bond energy, the C-H bond energy = 397 / 4 = 99.3 kcal . Extensive, complicated, and detailed studies have shown, however , that each H atom is not equally easily removed from carbon in this molecule; it is estimated that the individual bond energies are 104kcal for CH 3- H, 106kcal for CHrH, 106 kcal for CH- H, and 81 kcal for C-H, with a total of 397 kcal . In most cases, such detailed informa- tion is not available; neither is it normally needed except in discussion of the individual steps involved in chemical reactions.individual steps involved in chemical reactions....
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