GENERAL CHEMISTRY CHEM 152
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This 21 page Class Notes was uploaded by Carmela Kilback on Wednesday September 9, 2015. The Class Notes belongs to CHEM 152 at University of Washington taught by Philip Reid in Fall. Since its upload, it has received 18 views. For similar materials see /class/192566/chem-152-university-of-washington in Chemistry at University of Washington.
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Date Created: 09/09/15
Lecture 22 Lewis Dot Structures 0 Reading Zumdahl 139 1312 0 Outline Lewis Dot Structure Basics Resonance Those annoying exceptions Localized Bond Models Consider our energy diagram for H2 bonding 39 Hiatom atom Sufficiently far apart to have no interaction 9 o E gt 9 3 0 E H atom H atom The atoms begin to interact as they move closer together 458 3 39 39 0 0074 Internuclear distance run H2 molecule Optimum distance to achieve a lowest overall energy of system b H H bond length Localized Model Limitations 0 It is important to keep in mind that the models we are discussing are just that models 0 We are operating under the assumption that When forming bonds atoms share electrons using atomic orbitals 0 Electrons involved in bonding bonding pairs Electrons not involved in bonding lone pairs Lewis Dot Structures cont 0 Developed by G N Lewis to serve as a way to describe bonding in polyatomic systems Central idea the most stable arrangement of electrons is one in which all atoms have a noble gas configuration 0 Example NaCl versus Nat Cl Na Ne3s1 Cl Ne3sz3p5 Nat Ne Cl39 Ne3sz3p6 Ar LDS Mechanics 0 Atoms are represented by atomic symbols surrounded by valence on TI 0 0 on o l t Lone Pair 6 X 6 CC mns o o g 0 Electron pairs between F F atoms indicate bond 39 39 lt39 formation Bonding Pair LDS Mechanics cont 0 Three steps for basic Lewis structures 1 Sum the valence electrons for all atoms to determine total number of electrons 2 Use pairs of electrons to form a bond between each pair of atoms bonding pairs 3 Arrange remaining electrons around atoms lone pairs to satisfy the octet rule duet rule for hydrogen LDS Mechanics cont 0 An example C120 o I o 20 equot o CI39O39CI o o 16e39left LDS Mechanics cont 0 An example CH4 HoHo HOHO ago 8639 H HCH Oe39left H Done LDS Mechanics cont 0 An example CO2 00 16 equot O C O 12 equot left Octet ViolatioM 0 equot left 00 00 CO double bond 0 CO I O LDS Mechanics cont 0 An example NO N O 8 equot left Resonance Structures 0 We have assumed up to this point that there is one correct Lewis structure 0 There are systems for Which more than one Lewis structure is possible Different atomic linkages Structural Isomers Same atomic linkages different bonding Resonance Both structures are correct O quoton O quoton O 0 The classic example 03 Resonance Structures cont Resonance Structures cont 0 In this example 03 has two resonance structures 0006 06 bO OZOO 0 Conceptually we think of the bonding being an average of these two structures 0 Electrons are delocalized between the oxygens such that on average the bond strength is equivalent to 15 0 0 bonds Structural Isomers 0 What if different sets of atomic linkages can be used to construct correct LDSs IHI Ii cjz o xiiiII 5133335 0 Both are correct but Which is more correct Formal Charge 0 Formal Charge Compare the nuclear charge 2 to the number of electrons dividing bonding electron pairs by 2 Difference is known as the formal charge 00 O O O Q Q 0 g g 0 O 101 1CIO e 7 6 7 7 6 7 Z 7 6 7 7 7 6 Formal C 0 0 0 0 1 1 0 Structure With less F C is more correct Formal Charge 0 Example CO2 quot o co O C 9 O C C 6 4 6 6 4 6 7 4 5 2 6 4 6 6 6 4 6 6 4 FC 0 0 0 0 2 2 1 2 1 More Correct Beyond the Octet Rule 0 There are numerous exceptions to the octet rule 0 We ll deal With three classes of Violation here Sub octet systems Valence shell expansion Odd electron systems Beyond the Octet Rule cont 0 Some atoms Be and B in particular undergo bonding but Will form stable molecules that do not fulfill the octet rule if F F B F F39 39F39 0 Experiments demonstrate that the B F bond strength is consistent With single bonds only Beyond the Octet Rule cont 0 For third row elements Period 3 the energetic proximity of the d orbitals allows for the participation of these orbitals in bonding 0 When this occurs more than 8 electrons can surround a third row element 0 Example ClF3 a 28 equot system F F obey octet rule Cl has 106 Jr Beyond the Octet Rule cont 0 Finally one can encounter odd electron systems Where full pairs Will not exist 0 Example Chlorine Dioxide O O o o o O I O Unpa1red electron O 0 g Summary 0 Remember the following C N O and F almost always obey the octet rule B and Be are often sub octet Second row Period 2 elements never exceed the octet rule Third Row elements and beyond can use valence shell expansion to exceed the octet rule 0 In the end you have to practice a lot
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