Class Note for CHEM 111 at UMass(21)
Class Note for CHEM 111 at UMass(21)
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This 2 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Massachusetts taught by a professor in Fall. Since its upload, it has received 16 views.
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Date Created: 02/06/15
CHEMl 11 General Chemistry 1 RESONANCE AND RESONANCE STRUCTURES A single Lewis electron dot formula often adequately represents the locations of bonding electrons H T T in molecules For instance methane and ethyl alcohol are HCH HCC correctly represented by the Lewis formula at the right l I I quotH All electron pairs in these molecules are localized H H H 39 between the atoms that share them methane ethyl 31 c 01101 There are however molecules whose electronic distribution is inadequately represented by a single Lewis formula In these cases two or more Lewis formulas may be necessary to satisfactorily represent the molecule Resonance is a phenomenon invoked when more than one Lewis formula is needed to adequately describe the electronic distribution in a molecule Resonance is the use of contributing structures to represent electron delocalization dispersal in a molecule For example consider the sulfur dioxide S02 molecule Structural studies show that both SO bonds in S02 are of the same length and the same strength This electronic distribution is not explained by either single Lewis structure but is explained when both Lewis formulas are taken into consideration 0 3 O I OS e e c39 Both Lewis structures are called resonance structures or resonance contributors Resonance contributors are separated by double headed arrows as shown The actual molecule is NOT an equilibrium mixture of the imaginary resonance structures but rather a single unchanging composite structure called the s resonance hybrid Resonance hybrids are often represented by the quotpartial bondquot or combined notation The resonance hybrid for sulfur dioxide is shown 0 e p to the right Fundamentally the phenomenon of resonance is invoked because of the inability of single Lewis structures to show electron delocalization within the molecule In fact resonance is often called delocalization For instance in S02 two electron pairs are delocalized or spread over the entire OSO three atom bonding system and not localized in a single SO bond Thus the existence of electron delocalization in a molecule means that more than a single resonance structure must be drawn to fully describe the electronic distribution within the molecule Furthermore since electronic delocalization is stabilizing the existance of resonance implies molecular stabilization Generally the more resonance contributors the more stable the molecule Resonance amp Page 2 18 April 2007 Resonance Structures DRAWING RESONANCE STRUCTURES Resonance structures differ only in the location 0ftheir electrons The atomic nuclei must not move and the bond angles must not change Resonance structures must all contain equal numbers of electron pairs and or unpaired electrons One resonance structue can be converted to another by electronic movement denoted by curved arrows For instance the two resonance structures for S02 are interconverted by the electron movements shown 9 a 9333 lt 38 Of course no actual electronic movements O 39O 6 e 39 0 occur this process is an aid in generating additional resonance structures Another example of this relationship among resonance structures is shown with the three resonance structures of the nitrate ion NO3391 The implications of resonance are 1 the actual electron distribution is different than would be expected based on a single Lewis structure and 2 the energy of the actual molecule is lower than expected from a single Lewis structure This energy lowering is due to electron delocalization and is called resonance stabilization The degree of energy lowering or molecular stabilization is related to the number of resonance structures and the relative stabilities of the resonance structures D L Adams April 13 2007
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