Contents 10-1 Lewis Theory: An Overview 10-2 Covalent Bonding: An Introduction 10-3 Polar Covalent Bonds 10-4 Writing Lewis Structures 10-5 Resonance 10-6 Exceptions to the Octet Rule 10-7 The Shapes of Molecules 10-8 Bond Order and Bond Lengths 10-9 Bond Energies Focus On Molecules in Space: Measuring Bond Lengths General Chemistry: Chapter 10 Prentice-Hall © 2007

10-1 Lewis Theory: An Overview Chemistry 140 Fall 2002 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. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Lewis Symbols A chemical symbol represents the nucleus and the core e-. Dots around the symbol represent valence e-. • Si • • • N •• • P As Sb Bi •• Al • Se Ar I General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 EXAMPLE 10-2 Writing Lewis Structures of Ionic Compounds. Write Lewis structures for the following compounds: (a) BaO; (b) MgCl2 ; (c) aluminum oxide. Ba • O •• •• O Ba 2+ 2- BaO Note the use of the “fishhook” arrow to denote a single electron movement. A “double headed” arrow means that two electrons move. Binary ionic compounds. Note the types of arrows used to move electrons – fishhooks for single e-. Write the Lewis symbol for each atom Determine how many e- each atom must gain or lose. Use multiples of one or both ions to balance the number of electrons. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 EXAMPLE 10-2 Mg • Cl •• •• Cl Mg 2+ - 2 MgCl2 General Chemistry: Chapter 10 Prentice-Hall © 2007

10-2 Covalent Bonding: An Introduction General Chemistry: Chapter 10 Prentice-Hall © 2007

Coordinate Covalent Bonds Chemistry 140 Fall 2002 Coordinate Covalent Bonds N H + N •• H •• Cl - H Cl Note double headed arrow for two electron movement Note the “double headed” arrow showing that two electrons move. 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. • • • C O • •• • • O • C O • • • • • • • • • • • C O •• C O •• General Chemistry: Chapter 10 Prentice-Hall © 2007

Multiple Covalent Bonds • • N • •• •• N • • N •• • • N • •• N •• General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 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. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Electronegativity: an atom’s ability to compete for electrons with other atoms to which it is bonded. General Chemistry: Chapter 10 Prentice-Hall © 2007

Percent Ionic Character of a bond based on ∆EN 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. 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. General Chemistry: Chapter 10 Prentice-Hall © 2007

Polar Molecules

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.

Trigonal Planar Linear Octahedral Tetrahedral Trigonal Bipyramidal

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.

General Chemistry: Chapter 10 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. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 Dipole Moments Ana electrical condenser (or capacitor) consists of a pair of electrodes separated by a medium that does not conduct electricity. When the field is off the molecules orient randomly. When the filed is on the molecules align with the field. The alignment can be detected. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 Dipole Moments HCl is a polar molecule 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. 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. General Chemistry: Chapter 10 Prentice-Hall © 2007

Polar Molecules

Bond Order and Bond Length Single bond, order = 1 Double bond, order = 2 Bond Length Distance between two nuclei Higher bond order Shorter bond Stronger bond General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Bond Energies General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 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 General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 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 Forces: • Ion-Ion interaction • Ion-Dipole interaction • Dipole-Dipole interaction • Hydrogen-Bonding • Ion-Induced dipole interaction • Dipole-Induced dipole interaction • Dispersion (induced dipole-induced dipole or van der Waals forces) General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 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 Forces: • Ion-Ion interaction • Ion-Dipole interaction • Dipole-Dipole interaction • Hydrogen-Bonding • Ion-Induced dipole interaction • Dipole-Induced dipole interaction • Dispersion (induced dipole-induced dipole or van der Waals forces) General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Ion-Ion interaction: The force of attraction between ions is described by Coulomb’s law. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 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 You can use bond energies in exactly the same way you can use enthalpies of formation. Enthalpy of formation is more accurately known and bond energy is usually an average, but it can be used effectively if formation data is unavailable. 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.

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

General Chemistry: Chapter 10 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. 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. General Chemistry: Chapter 10

FC = #valence e- - #lone pair e- - #bond pair e- Chemistry 140 Fall 2002 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 #bond pair e- 2 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. 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.

O≡N—O 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. + •• O≡N—O -

Exceptions to the Octet Rule Chemistry 140 Fall 2002 Exceptions to the Octet Rule Odd e- species. N=O • •• •• •• H •• • O—H H—C—H H • •• Prentice-Hall © 2007

Exceptions to the Octet Rule Incomplete octets. •• B F - + •• B F - •• + •• F •• B •• F F •• •• •• •• ••

Exceptions to the Octet Rule Expanded octets. S F •• •• P Cl •• •• Cl •• P •• •• Cl Cl •• •• •• •• ••

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 Chemistry 140 Fall 2002 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. .. .. .. .. .. ..

General Chemistry: Chapter 10 Balloon Analogy 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.

Trigonal Planar Linear Octahedral Tetrahedral Trigonal Bipyramidal

Methane, Ammonia and Water

Table 10.1 Molecular Geometry as a Function of Electron Group Geometry

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.

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 Dipole Moments Ana electrical condenser (or capacitor) consists of a pair of electrodes separated by a medium that does not conduct electricity. When the field is off the molecules orient randomly. When the filed is on the molecules align with the field. The alignment can be detected. General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 Dipole Moments HCl is a polar molecule General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 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 type. NCl3 AlCl3 SO3 KI CH4 CF4 General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 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+ SF3− SF4 SF5+ SF5− SF6 General Chemistry: Chapter 10 Prentice-Hall © 2007

Bond Order and Bond Length Single bond, order = 1 Double bond, order = 2 Bond Length Distance between two nuclei Higher bond order Shorter bond Stronger bond General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Bond Energies General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Prentice-Hall © 2007

General Chemistry: Chapter 10 Chemistry 140 Fall 2002 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 You can use bond energies in exactly the same way you can use enthalpies of formation. Enthalpy of formation is more accurately known and bond energy is usually an average, but it can be used effectively if formation data is unavailable. General Chemistry: Chapter 10 Prentice-Hall © 2007

Focus on Molecules in Space: Measuring Bond Lengths 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 c d? e e?!# f Answer General Chemistry: Chapter 10 Prentice-Hall © 2007