Chemistry, Julia Burdge, 2st Ed McGraw Hill Chapter Chemical Bonding I: Lewis Theory Mr Truong Minh Chien ; losedtales@yahoo.com http://tailieu.vn/losedtales http://mba-programming.blogspot.com 2011, NKMB Co., Ltd Bonding Theories • explain how and why atoms attach together • explain why some combinations of atoms are stable and others are not  why is water H2O, not HO or H5O • one of the simplest bonding theories was developed by • • G.N Lewis and is called Lewis Theory Lewis Theory emphasizes valence electrons to explain bonding using Lewis Theory, we can draw models – called Lewis structures – that allow us to predict many properties of molecules  aka Electron Dot Structures  such as molecular shape, size, polarity Chemistry, Julia Burdge, 2nd e., McGraw Hill Why Do Atoms Bond? • processes are spontaneous if they result in a system with • • • lower potential energy chemical bonds form because they lower the potential energy between the charged particles that compose atoms the potential energy between charged particles is directly proportional to the product of the charges the potential energy between charged particles is inversely proportional to the distance between the charges Chemistry, Julia Burdge, 2nd e., McGraw Hill Potential Energy Between Charged Particles E potential ∀ ∈0 is a constant  q1 • q2  =   4π ∈0  r   = 8.85 x 10-12 C2/J∙m • for charges with the same sign, Epotential is + and the • • magnitude gets less positive as the particles get farther apart for charges with the opposite signs, Epotential is − and the magnitude gets more negative as the particles get closer together remember: the more negative the potential energy, the more stable the system becomes Chemistry, Julia Burdge, 2nd e., McGraw Hill Potential Energy Between Charged Particles The repulsion between attraction opposite-charged like-charged particles particles as the increasesincreases as the particles closer particles get get closer together To bring Bringing them closer requires the lowers the potential energy addition of more of the system energy Chemistry, Julia Burdge, 2nd e., McGraw Hill Bonding • a chemical bond forms when the potential energy of the bonded atoms is less than the potential energy of the separate atoms • have to consider following interactions: nucleus-to-nucleus repulsion electron-to-electron repulsion nucleus-to-electron attraction Chemistry, Julia Burdge, 2nd e., McGraw Hill Types of Bonds Types of Atoms metals to nonmetals nonmetals to nonmetals metal to metal Tro, Chemistry: A Molecular Approach Type of Bond Ionic Bond Characteristic electrons transferred Covalent electrons shared Metallic electrons pooled Types of Bonding Ionic Bonds • when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atoms metals have low ionization energy, relatively easy to remove an electron from nonmetals have high electron affinities, relatively good to add electrons to Tro, Chemistry: A Molecular Approach Covalent Bonds • nonmetals have relatively high ionization energies, so it • is difficult to remove electrons from them when nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons  potential energy lowest when the electrons are between the nuclei • shared electrons hold the atoms together by attracting nuclei of both atoms Tro, Chemistry: A Molecular Approach 10 Trends in Bond Energies • the more electrons two atoms share, the stronger the covalent bond C≡C (837 kJ) > C=C (611 kJ) > C−C (347 kJ) C≡N (891 kJ) > C=N (615 kJ) > C−N (305 kJ) • the shorter the covalent bond, the stronger the bond Br−F (237 kJ) > Br−Cl (218 kJ) > Br−Br (193 kJ) bonds get weaker down the column Tro, Chemistry: A Molecular Approach 79 Using Bond Energies to Estimate ∆H°rxn • the actual bond energy depends on the surrounding atoms • and other factors we often use average bond energies to estimate the ∆Hrxn  works best when all reactants and products in gas state • bond breaking is endothermic, ∆H(breaking) = + • bond making is exothermic, ∆H(making) = − ∆Hrxn = ∑ (∆H(bonds broken)) + ∑ (∆H(bonds formed)) Tro, Chemistry: A Molecular Approach 80 81 Using Bond Energies to Estimate ∆Η°ρξν ∆Η°ρξν ΝαΧλ Tro, Chemistry: A Molecular Approach 82 Estimate the Enthalpy of the Following Reaction H H + O O H O O H 83 Estimate the Enthalpy of the Following Reaction H2(g) + O2(g) → H2O2(g) reaction involves breaking 1mol H-H and mol O=O and making mol H-O and mol O-O bonds broken (energy cost) (+436 kJ) + (+498 kJ) = +934 kJ bonds made (energy release) 2(464 kJ) + (142 kJ) = -1070 ∆Hrxn = (+934 kJ) + (-1070 kJ) = -136 kJ (Appendix ∆H°f = -136.3 kJ/mol) Tro, Chemistry: A Molecular Approach 84 Bond Lengths • the distance between the nuclei of bonded atoms is called the bond length • because the actual bond length depends on the other atoms around the bond we often use the average bond length averaged for similar bonds from many compounds Tro, Chemistry: A Molecular Approach 85 Trends in Bond Lengths • the more electrons two atoms share, the shorter the covalent bond  C≡C (120 pm) < C=C (134 pm) < C−C (154 pm)  C≡N (116 pm) < C=N (128 pm) < C−N (147 pm) • decreases from left to right across period  C−C (154 pm) > C−N (147 pm) > C−O (143 pm) • increases down the column  F−F (144 pm) > Cl−Cl (198 pm) > Br−Br (228 pm) • in general, as bonds get longer, they also get weaker Tro, Chemistry: A Molecular Approach 86 Bond Lengths Tro, Chemistry: A Molecular Approach 87 Metallic Bonds • low ionization energy of metals allows them to lose electrons easily • the simplest theory of metallic bonding involves the metals atoms releasing their valence electrons to be shared by all to atoms/ions in the metal an organization of metal cation islands in a sea of electrons electrons delocalized throughout the metal structure • bonding results from attraction of cation for the delocalized electrons Tro, Chemistry: A Molecular Approach 88 Metallic Bonding Tro, Chemistry: A Molecular Approach 89 Metallic Bonding Model vs Reality • metallic solids conduct electricity • because the free electrons are mobile, it allows the electrons to move through the metallic crystal and conduct electricity • as temperature increases, electrical conductivity decreases • heating causes the metal ions to vibrate faster, making it harder for electrons to make their way through the crystal Tro, Chemistry: A Molecular Approach 90 Metallic Bonding Model vs Reality • metallic solids conduct heat • the movement of the small, light electrons through the solid can transfer kinetic energy quicker than larger particles • metallic solids reflect light • the mobile electrons on the surface absorb the outside light and then emit it at the same frequency Tro, Chemistry: A Molecular Approach 91 Metallic Bonding Model vs Reality • metallic solids are malleable and ductile • because the free electrons are mobile, the direction of the attractive force between the metal cation and free electrons is adjustable • this allows the position of the metal cation islands to move around in the sea of electrons without breaking the attractions and the crystal structure Tro, Chemistry: A Molecular Approach 92 Metallic Bonding Model vs Reality • metals generally have high melting points and boiling points  all but Hg are solids at room temperature • the attractions of the metal cations for the free electrons • • • • is strong and hard to overcome melting points generally increase to right across period the charge on the metal cation increases across the period, causing stronger attractions melting points generally decrease down column the cations get larger down the column, resulting in a larger distance from the nucleus to the free electrons Tro, Chemistry: A Molecular Approach 93 ... Molecular Approach •• •N • • • •• •N • N· · 39 Covalent Bonding Predictions from Lewis Theory • Lewis theory allows us to predict the formulas of • molecules Lewis theory predicts that some combinations... theories was developed by • • G.N Lewis and is called Lewis Theory Lewis Theory emphasizes valence electrons to explain bonding using Lewis Theory, we can draw models – called Lewis structures – that... Approach in NaCl(aq), the ions are separated and allowed to move to the charged rods 19 Lewis Theory and Ionic Bonding • Lewis symbols can be used to represent the transfer of electrons from metal