Chemical Bonding I Basic Concepts Chapter 9 Copyright The McGraw Hill Companies Inc Permission required for reproduction or display Valence electrons are the outer shell electrons of an atom The valence electrons are the electrons that participate in chemical bonding Group e configuration of valence e 1A ns1 1 2A ns2 2 3A ns2np1 3 4A ns2np2 4 5A ns2np3 5 6A ns2np4 6 7A ns2np5 7 9 1 Lewis Dot Symbols for the Representative Elements Noble Gases 9 1 The Ionic Bond Li F 1s22s11s22s22p5 e Li Li F He 1s 1s2 2Ne 2s22p6 Li Li e F F F Li F 9 2 Electrostatic Lattice Energy Lattice energy E is the energy required to completely separate one mole of a solid ionic compound into gaseous ions E k Q Qr Q is the charge on the cation Q is the charge on the anion r is the distance between the ions Lattice energy E increases as Q increases and or as r decreases cmpd MgF2 MgO LiF LiCl lattice energy 2957 Q 2 1 3938 Q 2 2 1036 r F r Cl853 9 3 Born Haber Cycle for Determining Lattice Energy o Hoverall H1o H2o H3o H4o H5 o 9 3 Born Haber Cycle for Determining Lattice Energy IE1 Li EA 1 F H0association LE H0dissociation H0sublimation H0formation LiF o o o Hf Hsubl Hdissoc IE1 Li EA 1 F LE LiF 9 3 Given the following enthalpy data pertaining to AlCl 3 Enthalpy of formation of AlCl3 704 2 kJ mol for the reaction Al s 3 2 Cl2 g AlCl3 s Enthalpy of sublimation of Al 326 kJ mol Enthalpy of dissociation of Cl2 242 8 kJ mol First ionization energy of Al Al0 Al 580 kJ mol Second ionization energy of Al Al Al2 1815 kJ mol Third ionization energy of Al Al2 Al3 2740 kJ mol Electron affinity of Cl Cl0 Cl 348 7 kJ mol Calculate the lattice energy of AlCl3 Al3 g 3 Cl g H0assoc AlCl3 LE IE3 Al 2740 kJ mole H0e gain 1 EA1 Cl 3 348 7 kJ mole 3 1046 1 kJ mole Al2 g IE2 Al 1815 kJ mole 3 Cl g Problem 1 Al g IE1 Al 580 kJ mole H0dissoc Cl2 242 8 kJ mole 3 2 364 2 kJ mole Al g H0subl Al 326 kJ mole Al s 3 2 Cl2 g H0f AlCl3 704 2 kJ mole AlCl 3 s H0assoc AlCl3 LE Problem 1 Use Hess Law to determine the lattice energy H0f AlCl3 H0subl Al H0dissoc Cl2 IE1 Al IE2 Al IE3 Al H0e gain 1 H0assoc AlCl3 704 2 kJ mol 330 0 kJ mol 242 8 kJ mol 3 2 577 9 kJ mol 1820 kJ mol 2750 kJ mol 348 7 kJ mol 3 H0assoc AlCl3 Solve for H0assoc then for LE H0assoc AlCl3 LE 5153 kJ mol LE lattice energy 5153 kJ mol 9 3 Chemistry In Action Sodium Chloride Mining Salt Solar Evaporation for Salt A covalent bond is a chemical bond in which two or more electrons are shared by two atoms Why should two atoms share electrons F 7e F F F 7e 8e 8e Lewis structure of F2 single covalent bond lone pairs F F lone pairs lone pairs F F lone pairs single covalent bond 9 4 Lewis structure of water H O H single covalent bonds H O H or H O H 2e 8e 2eDouble bond two atoms share two pairs of electrons O C O or O O C double bonds 8e 8ebonds 8edouble Triple bond two atoms share three pairs of electrons N N triple bond 8e 8e or N N triple bond 9 4 Lengths of Covalent Bonds Bond Lengths Triple bond Double Bond Single Bond 9 4 9 4 Polar covalent bond or polar bond is a covalent bond with greater electron density around one of the two atoms electron poor region H electron rich region F e poor H e rich F 9 5 Electronegativity is the ability of an atom to attract toward itself the electrons in a chemical bond Electron Affinity measurable Cl is highest X g e X g Electronegativity relative F is highest 9 5 Electronegativity related to ionization energy IE and electron affinity EA in electron volts eV another energy unit IE EA 5 6 Arbitraryscale factor 1 eV 1 602 x 10 19 J Electronegativity related to ionization energy IE and electron affinity EA F HIGH ionization energy takes a hell of a lot of energy to pry its e s loose HIGH electron affinity gladly takes on edensity HIGH electronegativity Na LOW ionization energy Its 1 valence e is easy prey LOW electron affinity has no interest in acquiring more e density LOW electronegativity The Electronegativities of Common Elements 9 5 Variation of Electronegativity with Atomic Number 9 5 Classification of bonds by difference in electronegativity Difference Bond Type 0 Covalent 2 0 and 2 Ionic Polar Covalent Increasing difference in electronegativity Covalent Polar Covalent share e partial transfer of e Ionic transfer e9 5 Classify the following bonds as ionic polar covalent or covalent The bond in CsCl the bond in H2S and the NN bond in H2NNH2 Cs 0 7 Cl 3 0 3 0 0 7 2 3 Ionic H 2 1 S 2 5 2 5 2 1 0 4 Polar Covalent N 3 0 N 3 0 3 0 3 0 0 Covalent 9 5 Writing Lewis Structures 1 Draw skeletal structure of compound showing what atoms are bonded to each other Put least electronegative element in the center 2 Count total number of valence e Add 1 for each negative charge Subtract 1 for each positive charge 3 Complete an octet for all atoms except hydrogen 4 If structure contains too many electrons form double and triple bonds on central atom as needed 9 6 Write the Lewis structure of nitrogen trifluoride NF3 Step 1 N is less electronegative than F put N in center Step 2 Count valence electrons N 5 2s22p3 and F 7 2s22p5 5 3 x 7 26 valence electrons Step 3 Draw single bonds between N and F atoms and complete octets on N and F atoms Step 4 Check are of e in structure equal to number of valence e 3 single bonds 3x2 10 lone pairs 10x2 26 valence electrons F N F F 9 6 Write the Lewis structure of the carbonate ion CO 32 Step 1 C is less electronegative than O put C in center Step 2 Count valence electrons C 4 2s22p2 and O 6 2s22p4 2 charge 2e4 3 x 6 2 24 valence electrons Step 3 Draw single bonds between C and O atoms and complete octet on C and O atoms Step 4 Check are of e in structure equal to number of valence e 3 single bonds 3x2 10 lone pairs 10x2 26 valence electrons Step 5 Too many electrons form double bond and re check of e O C O O 2 single bonds 2x2 4 1 double bond 4 8 lone pairs 8x2 16 Total 24 9 6 Two possible skeletal structures …