Chapter 9 Outline - Chemistry 101 Vandan Desai Chapter 9:...

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Chemistry 101 Vandan Desai P a g e | 1 Chapter 9: Chemical Bonding—General Concepts (Lecture Outline) --------------------------------------------------------------------------------------------------------------------------- I. Electron transfer leads to formation of ionic compounds A. Ionic compounds form when metals and nonmetals react i. The attraction between positive and negative ions is called an ionic bond ii. Form because the potential energy of the system decreases (exothermic ) B. The energy change when NaCl forms can be calculated using the ionization energy (IE) of sodium , the electron affinity (EA) of chlorine , and the lattice energy of i. Lattice energy —the energy needed to produce separated gaseous ions from one mole of the solid ii. Starting from 1 mole of gas phase atoms: &a(g) b + (g) + e - +495.4 kJ (IE of sodium) Cl(g) + e - b Cl - (g) -348.8 kJ (EA of chlorine) + (g)+Cl - (g) b &aCl(s) -787.0 kJ (-lattice energy) &et: -640.4 kJ iii. It turns out that for any ionic compound, the chief stabilizing influence is the lattice energy —leads to lowering of potential energy C. The size of the lattice energy depends on ion size and charge i. Lattice energy increases with charge because the ions attract each other more strongly (Example: KCl = 709 kJ vs. CaO = 3401 kJ) ii. Smaller ions have larger lattice energies because they get closer together (Example: NaCl = 778 kJ vs. KCl = 709 kJ & LiF = 1033 kJ vs. LiCl = 845 kJ) D. The lattice energy can be calculated using the Born-Haber cycle i. A Born-Haber cycle for NaCl ii. The upper and lower paths must give the same energy change b/c enthalpy is a state function : both paths lead from the elements to solid NaCl E. Noble gas configurations are very stable and can be useful in predicting ion charges i. Formation of Na + is relatively inexpensive ii. Na 2+ doesn’t (ordinarily) form b/c breaking into the noble gas core costs too much energy F. All noble gases ( except He ) have 8 valence electrons —octet of electrons G. Most of the representative elements tend to gain or lose electrons until they have achieved the configuration of the nearest noble gas H. For example: Na and K lose electrons to achieve an octet of electrons while Cl and O gain electron to achieve octet of electrons I. The octet rule works best for ionic compounds of Group IA and IIA metals from Period 3 down and for the anions of the nonmetals
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Chemistry 101 Vandan Desai P a g e | 2 J. It fails for Li and Be b/c they achieve the He (1s 2 ) electron configuration K. Doesn’t work for H which can form H - (1s 2 ) when it reacts w/ very reactive metals L. The octet rule doesn’t work well for transition metals and post-transition metals M. For these cations: i. The first electrons lost
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Chapter 9 Outline - Chemistry 101 Vandan Desai Chapter 9:...

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