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Contents 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 Lewis Theory: An Overview Covalent Bonding: An Introduction Polar Covalent Bonds Writing Lewis Structures Resonance Exceptions to the Octet Rule The Shapes of Molecules Bond Order and Bond Lengths Bond Energies Focus On Molecules in Space: Measuring Bond Lengths Slide 1 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 10-1 Lewis Theory: An Overview Valence e- play a fundamental role in chemical bonding. e- transfer leads to ionic bonds. Sharing of e- leads to covalent bonds. e- are transferred or shared to give each atom a noble gas configuration the octet. Slide 2 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Lewis Symbols A chemical symbol represents the nucleus and the core e-. Dots around the symbol represent valence e-. • • Si • • • Al • • Slide 3 of 50 • As • • P• • •• • Se • • •• •• • Bi • • • Sb • • •• I •• • General Chemistry: Chapter 10 •• Ar •• • •• •• •N• • •• •• •• •• Prentice-Hall © 2007 EXAMPLE 10-2 Writing Lewis Structures of Ionic Compounds. Write Lewis structures for the following compounds: (a) BaO; (b) MgCl2 ; (c) aluminum oxide. • O• •• 2+ •• Ba O •• 2- •• Ba• •• •• BaO • Note the use of the “fishhook” arrow to denote a single electron movement. A “double headed” arrow means that two electrons move. Slide 4 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 EXAMPLE 10-2 •• •• 2+ Mg •• 2 Cl •• - •• • Cl •• •• •• Mg • •• MgCl2 • • Cl •• Slide 5 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 10-2 Covalent Bonding: An Introduction Slide 6 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Coordinate Covalent Bonds + H Cl H N H H •• Cl •• - •• •• H N H •• H H Note the “double headed” arrow showing that two electrons move. Slide 7 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Multiple Covalent Bonds Often, more than one pair of electrons must be shared for an atom to attain an octet. •O •• • C• • •• • • • •• • •• Slide 8 of 50 •• •• O C O • • • •• • •• O C O •• •• O• • •• •• •• •• •• • •• • O C O General Chemistry: Chapter 10 Prentice-Hall © 2007 Multiple Covalent Bonds • • Slide 9 of 50 •• •• N N • • • • •• N N N N General Chemistry: Chapter 10 •• •• • •• •N • N• •• • •• • Prentice-Hall © 2007 Polar Covalent Bonds Most chemical bonds fall between 100% ionic and 100% covalent. In a polar covalent bond, electrons are not shared equally between two atom. In such a bond, electrons are displaced toward the more nonmetallic element. Slide 10 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Electronegativity: an atom’s ability to compete for electrons with other atoms to which it is bonded. Slide 11 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Percent Ionic Character of a bond based on ∆EN Slide 12 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Electrostatic potential map There are methods that allow us to display the electron density distribution within molecules. The Electrostatic Potential map is obtained by hypothetically probing an electron density surface with a positive point charge. Slide 13 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 10-3 Polar Covalent Bonds and Electrostatic Potential Maps We can map the electron density throughout a molecule. The electrostatic potential is the work done in moving the unit of +ve charge from one region of a molecule to another. Slide 14 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Polar Molecules Slide 15 of 50 Learn how to apply Valence Shell Electron Pair Repulsion, VSEPR, Theory to predict the shapes of molecules. The three-dimensional shape of a molecule is important in determining its chemical behavior, particularly for biologically important molecules. Applying VSEPR Theory Draw a plausible Lewis structure. Determine the number of electron groups around the central atom and identify them as bond or lone pairs. Establish the electron group geometry. Determine the molecular geometry. Multiple bonds count as one group of electrons. 17 Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal 18 Octahedral Dipole Moments and Polarity: In molecules containing atoms with different electronegativities, there is an unequal sharing of electron density. In other words, a polar covalent bond is formed. Dipole Moments and Polarity: Dipoles are vector quantities that are additive to produce an overall molecular dipole moment. For Diatomic molecules from atoms of different electronegativity there is always a dipole moment. For polyatomic molecules the resultant dipole moment is a vector sum of the individual bond dipoles. As a result not all molecules with polar covalent bonds have a net molecular dipole. Slide 22 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 23 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Dipole Moments Slide 25 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Dipole Moments Slide 26 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Electrostatic potential map There are methods that allow us to display the electron density distribution within molecules. The Electrostatic Potential map is obtained by hypothetically probing an electron density surface with a positive point charge. Slide 27 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 10-3 Polar Covalent Bonds and Electrostatic Potential Maps We can map the electron density throughout a molecule. The electrostatic potential is the work done in moving the unit of +ve charge from one region of a molecule to another. Slide 28 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Polar Molecules Slide 29 of 50 Bond Order and Bond Length Bond Order Single bond, order = 1 Double bond, order = 2 Bond Length Distance between two nuclei Higher bond order Shorter bond Stronger bond Slide 30 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 31 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Bond Energies Slide 32 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 33 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 34 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 A phase is a homogenous part of the system in contact with other parts of the system but separated From them by a well-defined boundary Slide 35 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Intermolecular Forces: are attractive forces between molecules. These are the forces primarily responsible for the bulk physical properties of matter, such as boiling point. Intramolecular Forces: are forces that hold molecules together (bonds). These stabilize individual molecules. Types of Intermolecular Slide 36 of 50 Prentice-Hall © 2007 General Chemistry: Chapter Forces: 10 Intermolecular Forces: are attractive forces between molecules. These are the forces primarily responsible for the bulk physical properties of matter, such as boiling point. Intramolecular Forces: are forces that hold molecules together (bonds). These stabilize individual molecules. Types of Intermolecular Slide 37 of 50 Prentice-Hall © 2007 General Chemistry: Chapter Forces: 10 Ion-Ion interaction: The force of attraction between ions is described by Coulomb’s law. Slide 38 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 EXAMPLE 10-15 Calculating an Enthalpy of Reaction from Bond Energies. The reaction of methane (CH4) and chlorine produces a mixture of products called chloromethanes. One of these is monochloromethane, CH3Cl, used in the preparation of silicones. Calculate H for the reaction. ΔHrxn = ΔH(product bonds) - ΔH(reactant bonds) = ΔH bonds formed - ΔH bonds broken = -770 kJ/mol – (657 kJ/mol) = -113 kJ/mol Slide 39 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 The Lewis Structure is a representation of a molecule that shows how the valence electrons are arranged among the atoms in the molecule. Slide 42 of 50 1. Skeletal structure; determine central and terminal atoms; low EN atoms central, O and H usually terminal. 2. Determine # of valence electrons. 3.Determine # of electrons needed for each atom to achieve octet. 4. # of electrons in step 3 minus # of electrons in step 2 will give you # of bonding electrons. 5. Place bonding electrons among atoms in the structure (connect atoms and use all of the bonding electrons; you may get more than one plausible structure). Prentice-Hall © 2007 6. Did you use all of the valence electrons from step 2? If yes, you are done, go to step 8. if no, go to step 7. 7. Place the left over valence electrons ( from step 2) as non-bond pairs on the atoms for each atom to achieve octet ( exception, H must achieve duet). 8. Check if structure(s) are valid by determining the formal charges on the atoms. Slide 44 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Writing Lewis Structures All the valence e- of atoms must appear. Usually, the e- are paired. Usually, each atom requires an octet. H only requires 2 e-. Multiple bonds may be needed. Readily formed by C, N, O, S, and P. Slide 45 of 50 General Chemistry: Chapter 10 Formal Charge: The formal charge on an atom in a Lewis structure is the number of valence e- in the free atom minus the number of e- assigned to that atom in the Lewis structure. FC = #valence e- - #lone pair e- - 1 2 #bond pair e- Formal charges is a measure of the extent to which an atom has gained or lost an electron in the process of forming a covalent bond. Slide 46 of 50 Sum of FC is the overall charge. FC should be as small as possible. Negative FC usually on most electronegative elements. FC of same sign on adjacent atoms is unlikely. + + •• - Slide 47 of 50 •• •• O≡N—O •• Exceptions to the Octet Rule Odd e- species. •• •• • N=O •• H •• • Slide 48 of 50 O—H •• • H—C—H Prentice-Hall © 2007 Exceptions to the Octet Rule Incomplete octets. •• •• •• F + F •• •• •• B B Slide 49 of 50 F F F F •• •• •• + - F F - B F Exceptions to the Octet Rule Expanded octets. P Cl F S P Cl Cl Cl F F F •• Cl •• •• Cl F •• Cl •• •• Cl •• •• •• •• •• •• F Learn how to apply Valence Shell Electron Pair Repulsion, VSEPR, Theory to predict the shapes of molecules. The three-dimensional shape of a molecule is important in determining its chemical behavior, particularly for biologically important molecules. Valence Shell Electron Pair Repulsion VSEPR Theory • Electron Pairs in the Valence Shell of the central atom Repel each other whether they are in chemical bonds (bond pairs) or unshared (lone pairs). • Electron pairs assume orientations about an atom to minimize repulsions. .. .. .. .. .. 52 Balloon Analogy Slide 53 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Applying VSEPR Theory Draw a plausible Lewis structure. Determine the number of electron groups around the central atom and identify them as bond or lone pairs. Establish the electron group geometry. Determine the molecular geometry. Multiple bonds count as one group of electrons. 54 Linear Trigonal Planar Tetrahedral Trigonal Bipyramidal 55 Octahedral Methane, Ammonia and Water Slide 56 of 50 Table 10.1 Molecular Geometry as a Function of Electron Group Geometry Slide 57 of 50 Slide 58 of 50 Slide 59 of 50 Slide 60 of 50 Slide 61 of 50 Slide 62 of 50 Applying VSEPR Theory Draw a plausible Lewis structure. Determine the number of e- groups and identify them as bond or lone pairs. Establish the e- group geometry. Determine the molecular geometry. Multiple bonds count as one group of electrons. More than one central atom can be handled individually. Slide 63 of 50 Dipole Moments Slide 64 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Dipole Moments Slide 65 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 The difference in electronegativity values between two atoms in a bond (ΔEN) may be used to predict if a bond is: a) non-polar, covalent b) polar, covalent c) ionic For the following compounds predict whether the bond type is a), b) or c) and justify your answer.(a bond is considered to be ionic if ΔEN > 1.7). Find the melting point of these compounds from the library ('The Handbook of Chemistry and Physics') and confirm whether the melting point agrees with the predicted bond ty NCl3 AlCl3 SO3 KI CH4 Slide 66 of 50 CF4 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 67 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Of the following compounds of sulfur, which, if any, do not possess a permanent dipole. Take the Pauling electronegativity value for a lone pair = 3.7. SF2 SF3+ Slide 68 of 50 SF3− SF4 SF5+ General Chemistry: Chapter 10 SF5− SF6 Prentice-Hall © 2007 Bond Order and Bond Length Bond Order Single bond, order = 1 Double bond, order = 2 Bond Length Distance between two nuclei Higher bond order Shorter bond Stronger bond Slide 69 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 70 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Bond Energies Slide 71 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Slide 72 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 EXAMPLE 10-15 Calculating an Enthalpy of Reaction from Bond Energies. The reaction of methane (CH4) and chlorine produces a mixture of products called chloromethanes. One of these is monochloromethane, CH3Cl, used in the preparation of silicones. Calculate H for the reaction. ΔHrxn = ΔH(product bonds) - ΔH(reactant bonds) = ΔH bonds formed - ΔH bonds broken = -770 kJ/mol – (657 kJ/mol) = -113 kJ/mol Slide 73 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 Focus on Molecules in Space: Measuring Bond Lengths Slide 74 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007 End of Chapter Questions •Testing your decisions: –If you get an error or a nonsense result, then climb back to an intersection where you KNOW you were correct, and take another route. a b c d? e?!# e f Answer Slide 75 of 50 General Chemistry: Chapter 10 Prentice-Hall © 2007