Lewis Model Bonding models and ionic bonds By John Fleming

Lewis Model Named after GN Lewis - shows electrons as dots around the symbol Predicts how elements will bond Draw dots on the four compass points of the chemical symbol, put one on each side before going back to put a second on each side For cations you put the charge at the top left side For anions you enclose the symbol and electrons in brackets before putting the charge

Types of chemical bonds - review Ionic bond - between metal and non-metal Covalent bond - between two nonmetals Metallic bond - between two metals

Valence Electrons - Octet Rule Every elements wants to be stable - needs full valence shell Elements ionize to get to closest full valence shell Metals lose electrons to get to Noble Gas config from previous period Non-metals (generally) gain electrons to reach the noble gas config farther down their group Noble gases are uninterested in bonding It’s called the octet rule because atoms generally are looking to have 8 electrons in their outermost shell, unless they’re small enough that their outermost shell would have 2 electrons

Lattice Energy & the Born-Haber Cycle Lattice energy - energy associated with the formation of a crystalline lattice of alternating cations and anions. lattice energy can be calculated with the Born-Haber cycle. The Born-Haber cycle is a “hypothetical series of steps that represents the formation of an ionic compound from its constituent elements.” Born Haber

Finding Lattice Energy of NaCl - from the Textbook Finding the lattice energy requires using known ΔHo values. Na(s) Na(g) ΔHostep 1 = 108 kJ 1/2 Cl(g) Cl2(g) ΔHostep 2 = 122 kJ Na(g) Na+(g) ΔHostep 3 = 496 kJ Cl(g) + e- Cl-(g) ΔHostep 4 = -349 kJ Na+(g)+ Cl-(g) NaCl(s) ΔHostep 5 = ΔHolattice = ?

Finding Lattice Energy (cont.) Using Hess’s law, we know that the overall enthalpy of formation is equal to the sum of the steps in the Born-Haber cycle. ΔHof = Hostep 1 + ΔHostep 2 +ΔHostep 3 +ΔHostep 4 +ΔHostep 5 ΔHof = Hostep 1 + ΔHostep 2 +ΔHostep 3 +ΔHostep 4 +ΔHolattice ΔHolattice = -411 kJ -(108 kJ + 122 kJ + 496 kJ - 349 kJ) ΔHolattice = -788 kJ

Practice Problem Draw a Lewis structure for Sodium, Phosphorus and Oxygen.

Chemistry Joke What did Na+ say to Cl-? I’ve got my ion you.