Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Bonding: General Concepts Chapter 8

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<ul><li> Slide 1 </li> <li> Slide 2 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 1 Bonding: General Concepts Chapter 8 </li> <li> Slide 3 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 2 Overview Types of chemical bonds, Electronegativity, Bond polarity and Dipole Moments. The Ions: electron configurations, size, formula, lattice energy calculations. Covalent bonds: model, bond energies, chemical reactions. Lewis structure, exceptions to the octet rule, resonance. Molecular structure models from Valence Shell Electron Pair Model VSEPR for single and multiple bonds. </li> <li> Slide 4 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 3 Bonds Forces that hold groups of atoms together and make them function as a unit. NaCl attraction is electrostatic since Na + and Cl - are the Stable forms for these elements. This is an example of Ionic Bonding </li> <li> Slide 5 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 4 Bond Energy 4 It is the energy required to break a bond. 4 It gives us information about the strength of a bonding interaction, as well as radius. </li> <li> Slide 6 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 5 Bond Length The distance where the system energy is a minimum. </li> <li> Slide 7 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 6 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. </li> <li> Slide 8 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 7 Change in electron density as two hydrogen atoms approach each other. </li> <li> Slide 9 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 8 Covalent Bond No electron transfer Electrons are shared between two atoms, positioned between the two nuclei Example: H 2, O 2, H 2 O, CO 2, etc. </li> <li> Slide 10 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 9 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. </li> <li> Slide 11 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 10 Li + F Li + F - The Ionic Bond 1s 2 2s 1 1s 2 2s 2 2p 5 1s 2 1s 2 2s 2 2p 6 [He][Ne] Li Li + + e - e - + FF - F - Li + + Li + F - </li> <li> Slide 12 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 11 Cation is always smaller than atom from which it is formed. Anion is always larger than atom from which it is formed. Ionic Compound </li> <li> Slide 13 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 12 Ionic Bonds and Coulombs Law Q 1 and Q 2 = numerical ion charges r = distance between ion centers (in nm) 4 4 Attractive forces are (-), repulsive are (+). </li> <li> Slide 14 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 13 Covalent or Ionic? Covalent and ionic are simply extreme cases. Most molecules share electrons but Unequally due to the difference in electronegativity and electron affinity. This will give rise to a dipole moment and the molecule becomes polar. </li> <li> Slide 15 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 14 Electronegativity The ability of an atom in a molecule to attract shared electrons to itself. Linus Pauling simple model = (H X) actual (H X) expected (H-H) + (X-X) 2 (H-X) experimental # (H-X) expected = If = 0 =&gt; no polarity </li> <li> Slide 16 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 15 </li> <li> Slide 17 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 16 Covalent share e - Polar Covalent partial transfer of e - Ionic transfer e - Increasing difference in electronegativity Classification of bonds by difference in electronegativity DifferenceBond Type 0 to 0.1Covalent 2 Ionic 0 &lt; and </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 42 Radii of Isoelectronic Ions O 2 &gt; F &gt; Na + &gt; Mg 2+ &gt; Al 3+ largest smallest 13 protons vs. 10 electrons </li> <li> Slide 44 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 43 Example Choose the largest ion in each of the following groups: 1.Li +, Na +, K +, Rb +, Cs + 2.Ba 2+, Cs +, I -, Te 2- </li> <li> Slide 45 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 44 To Reiterate Cation is smaller than parent molecule. Anion is larger than parent molecule. Size increase down in a group (+ve or ve). The larger the mass number the smaller the size for isoelectronic cations and anions. </li> <li> Slide 46 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 45 Electron Configurations of Cations of Transition Metals When a cation is formed from an atom of a transition metal, electrons are always removed first from the ns orbital and then from the (n 1)d orbitals. Fe: [Ar]4s 2 3d 6 Fe 2+ : [Ar]4s 0 3d 6 or [Ar]3d 6 Fe 3+ : [Ar]4s 0 3d 5 or [Ar]3d 5 Mn: [Ar]4s 2 3d 5 Mn 2+ : [Ar]4s 0 3d 5 or [Ar]3d 5 </li> <li> Slide 47 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 46 Why Compounds Exist? The driving force behind the naturally occurring compounds (such as NaCl, H 2 O, etc.) is to yield a stable lower energy form. A stable form is a an arrangement of atoms, held together by bonding that prevent decomposition. This bonding energy when ions condense from gas phase into ionic solid is called Lattice Energy. </li> <li> Slide 48 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 47 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. </li> <li> Slide 49 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 48 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 /2X 2 (g) X(g) [endothermic] 4.Formation of X ions in the gas phase: X(g) + e X (g) [exothermic] 5.Formation of the solid (LATTICE) MX M + (g) + X (g) MX(s) [quite exothermic] Lattice Energy </li> <li> Slide 50 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 49 The energy changes involved in the formation of solid lithium fluoride from its elements. From Gas to Solid LatticeEnergy </li> <li> Slide 51 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 50 The structure of lithium fluoride. Called also the NaCl structure where each ion is surrounded by 6 of the other ions Applicable for all alkali- metals/halogen except the Cesium salts. </li> <li> Slide 52 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 51 Q 1, Q 2 = charges on the ions (Na +, Cl -, Ca ++, O -- ) r = shortest distance between centers of the cations and anions (Ionic radii increases from top to bottom in a group) The magnitude of Qs and sizes of ions (ionic radii) will determine how strong is the lattice. </li> <li> Slide 53 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 52 Comparison of the energy changes involved in the formation of solid sodium fluoride and solid magnesium oxide. Note all ions are isoelectronic </li> <li> Slide 54 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 53 Lattice energy (E) increases as Q increases and/or as r decreases. Compd lattice energy MgF 2 MgO LiF LiCl 2957 3938 1036 853 Q= +2,-1 Q= +2,-2 r F &lt; r Cl Electrostatic (Lattice) Energy </li> <li> Slide 55 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 54 Partial Ionic Character of the Covalent Bond Introduce the concept of percent ionic character in a polar covalent bond % ionic character = Measured dipole of X-Y Calculated dipole of X + Y - x 100 </li> <li> Slide 56 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 55 Experiments showed that none of the studied systems have 100% ionic character despite large differences in electronegativity. The reason is Ion Pairing during motion, ions approach closely and for a short period of time their charges will be neutralized before of time their charges will be neutralized before moving away from each other. Ion Pairing </li> <li> Slide 57 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 56 Molten NaCl conducts an electric current, indicating the presence of mobile Na + and Cl - ions. The more mobile the ions the Stronger the current and the brighter the lamp! </li> <li> Slide 58 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 57 The relationship between the ionic character of a covalent bond and the electronegativity difference of the bonded atoms.&gt;50%Ionic </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 78 Exceptions to the Octet Rule Odd-Electron Molecules N 5e - O 6e - 11e - NO N O The Expanded Octet (central atom with principal quantum number n &gt; 2) SF 6 S 6e - 6F 42e - 48e - S F F F F F F 6 single bonds (6x2) = 12 18 lone pairs (18x2) = 36 Total = 48 </li> <li> Slide 80 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 79 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 (BF 3, BeH 2, etc.. ). 4 3rd row and heavier elements can exceed the octet rule using empty valence d orbitals (SF 6, PCl 5, etc.). 4 When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals. </li> <li> Slide 81 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 80 Note: For molecules that has several Third row (or higher) elements the extra electrons should be placed on the central atom. I3-I3-I3-I3- IIIIIIIIIIII.................... </li> <li> Slide 82 </li> <li> Copyright2000 by Houghton Mifflin Company. All rights reserved. 81 Resonance Occurs when more than one valid Lewis structure can be written for a particular molecule. C=C</li></ul>

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