Chemical Bonding © 2009, Prentice-Hall, Inc. Intramolecular Bonds  Ionic Electrostatic attraction between ions  Polar Covalent Unequal sharing of electrons  NonPolar Covalent Equal sharing of electrons

Chemical Bonding © 2009, Prentice-Hall, Inc. Ionic Bonding It takes 495 kJ/mol to remove electrons from sodium. (ionization energy) We get 349 kJ/mol back by giving electrons to chlorine. (electron affinity)

Chemical Bonding © 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding But these numbers don’t explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!

Chemical Bonding © 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding There must be a third piece to the puzzle. What is unaccounted for so far is the electrostatic attraction between the newly-formed sodium cation and chloride anion.

Chemical Bonding © 2009, Prentice-Hall, Inc. Lattice Energy This third piece of the puzzle is the lattice energy:  The change in energy that takes place when separated gaseous ions are packed together to form an ionic solid. The energy associated with electrostatic interactions is governed by Coulomb’s law: E el =  Q1Q2dQ1Q2d

Chemical Bonding © 2009, Prentice-Hall, Inc. Lattice Energy Lattice energy, then, increases with the charge on the ions. It also increases with decreasing size of ions.

Chemical Bonding Predicting charges Position on periodic table can be used to predict charges on ions. Representative elements tend to gain/lose enough electrons to attain noble gas configurations. Transition metals rarely do. Transition metals lose outermost s electron before outermost d electrons. Mg: [Ne]3s 2 Mg + : [Ne]3s 1 not stableMg 2+ : [Ne]stable Cl: [Ne]3s 2 3p 5 Cl  : [Ne]3s 2 3p 6 = [Ar] stable © 2009, Prentice-Hall, Inc.

Chemical Bonding Energetics of Ionic Bonding Li (s) + ½ F 2(g)  LiF (s) 1.Li (s)  Li (g) sublimation +161kJ 2.Li (g)  Li + (g) + e - I.E. +520kJ 3.½ F 2(g)  F (g) Dissociation +77kJ 4.F (g) + e -  F - (g) E.Affinity -328kJ 5.Li + (g) + F - (g)  LiF (s) Lattice Energy -1047kJ -617 kJ © 2009, Prentice-Hall, Inc.

Chemical Bonding © 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding By accounting for all three energies (ionization energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process.

Chemical Bonding © 2009, Prentice-Hall, Inc. Energetics of Ionic Bonding These phenomena also helps explain the “octet rule.” Metals, for instance, tend to stop losing electrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.

Chemical Bonding Practice with Lattice Energy 1. Arrange the following ionic compounds in order of increasing lattice energy: NaF, CsI, and CaO. 2. Which substance would you expect to have the greatest lattice energy, MgF2, CaF2, or ZrO2? 3. Predict the ion generally formed by (a) Sr, (b) S, (c) Al © 2009, Prentice-Hall, Inc.