1. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Bonds Forces that hold groups of atoms together and make them function as a unit. Bonds result from the tendency of a system to seek its lowest possible energy.

2. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Bond Energy 4 It is the energy required to break a bond. 4 It gives us information about the strength of a bonding interaction.

3. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Bond Length The distance where the system energy is a minimum.

4. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Figure 8.1: (a) The interaction of two hydrogen atoms. (b) Energy profile as a function of the distance between the nuclei of the hydrogen atoms.

5. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Ionic Bonds 4 Formed from electrostatic attractions of closely packed, oppositely charged ions. 4 Formed when an atom that easily loses electrons reacts with one that has a high electron affinity.

6. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Ionic Bonds Q 1 and Q 2 = numerical ion charges r = distance between ion centers (in nm)

7. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Figure 8.3: The Pauling electronegativity values. Electronegativity generally increases across a period and decreases down a group.

8. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Polarity A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment.

9. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Figure 8.4: (a) The charge distribution in the water molecule. (b) The water molecule in an electric field.

10. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Achieving Noble Gas Electron Configurations (NGEC) Two nonmetals react: They share electrons to achieve NGEC. A nonmetal and a representative group metal react (ionic compound): The valence orbitals of the metal are emptied to achieve NGEC. The valence electron configuration of the nonmetal achieves NGEC.

11. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Isoelectronic Ions Ions containing the the same number of electrons (O 2 , F , Na +, Mg 2+, Al 3+ ) O 2  > F  > Na + > Mg 2+ > Al 3+ largest smallest

12. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Lattice Energy The change in energy when separated gaseous ions are packed together to form an ionic solid. M + (g) + X  (g)  MX(s) Lattice energy is negative (exothermic) from the point of view of the system.

13. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Formation of an Ionic Solid 1.Sublimation of the solid metal M(s)  M(g) [endothermic] 2.Ionization of the metal atoms M(g)  M + (g) + e  [endothermic] 3.Dissociation of the nonmetal 1 / 2 X 2 (g)  X(g) [endothermic]

14. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Formation of an Ionic Solid (continued) 4.Formation of X  ions in the gas phase: X(g) + e   X  (g) [exothermic] 5.Formation of the solid MX M + (g) + X  (g)  MX(s) [quite exothermic]

15. Figure 8.8: The energy changes involved in the formation of solid lithium fluoride from its elements. Starthere

16. Figure 8.10: Comparison of the energy changes involved in the formation of solid sodium fluoride and solid magnesium oxide.

17. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Q 1, Q 2 = charges on the ions r = shortest distance between centers of the cations and anions

18. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Models Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world.

19. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Fundamental Properties of Models 4 A model does not equal reality. 4 Models are oversimplifications, and are therefore often wrong. 4 Models become more complicated as they age. 4 We must understand the underlying assumptions in a model so that we don’t misuse it.

20. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Bond Energies Bond breaking requires energy (endothermic). Bond formation releases energy (exothermic).  H =  D( bonds broken )   D( bonds formed ) energy requiredenergy released

21. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Localized Electron Model A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms.

22. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Localized Electron Model 1.Description of valence electron arrangement (Lewis structure). 2.Prediction of geometry (VSEPR model). 3.Description of atomic orbital types used to share electrons or hold long pairs.

23. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Lewis Structure 4 Shows how valence electrons are arranged among atoms in a molecule. 4 Reflects central idea that stability of a compound relates to noble gas electron configuration.

24. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Comments About the Octet Rule 4 2nd row elements C, N, O, F observe the octet rule. 4 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive. 4 3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals. 4 When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

25. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule. These are resonance structures. The actual structure is an average of the resonance structures.

26. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Formal Charge The difference between the number of valence electrons (VE) on the free atom and the number assigned to the atom in the molecule. We need: 1.# VE on free neutral atom 2.# VE “belonging” to the atom in the molecule

27. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Formal Charge Not as good Better

28. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 VSEPR Model The structure around a given atom is determined principally by minimizing electron pair repulsions.

29. Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Predicting a VSEPR Structure 1.Draw Lewis structure. 2.Put pairs as far apart as possible. 3.Determine positions of atoms from the way electron pairs are shared. 4.Determine the name of molecular structure from positions of the atoms.