Bonding in Ionic Solids What controls ionic bonding strength? Why do some ionic compounds take one shape while others a different shape?

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Presentation transcript:

Bonding in Ionic Solids What controls ionic bonding strength? Why do some ionic compounds take one shape while others a different shape?

Ion-Pair Energy Comes from sum of unlike charges attracting + electron clouds repelling (Born repulsion). Ionic bond distance occurs at the minimum of this energy. NaCl LiF

Ion-Pair Energy Math n = Born exponent, which depends on atom size Principal Quantum Born n # of outer electron 1 ex: Li +, 1s ex: F -, [He]2s 2 2p ex: Ca 2+, [Ne] 3s 2 3p Br -, Sr I - 12

Lattice Energy = Ion-pair energy x Madelung constant Crystal Structure Madelung constant NaCl CsCl1.763 Zinc blend1.638 Wurtzite1.641 Fluorite2.519 Rutile2.408

Origin of the Madelung Constant?

Concentric Cube Calculation Does Converge: Madelung Series does not converge:

Ionic Solids are thought of as anion structures with cations filling cavities (holes) between the anions. KEY: bonding is strongest with the most cation-anion interactions that do not crowd the anions into each other (which is the same as leaving gaps between anions and the cation. At the “stability limit” the cation is touching all the anions and the anions are just touching at their edges. Beyond this stability limit the compound will be stable. 3d View What decides which shape an ionic compound takes?

Ionic Compounds in Water

Dissolution vs. Precipitation: Enthalpy Trends with charge (Z) and ion radius (r) Ionic bond strength (lattice energy) increases as:

Hydration Enthalpy Overall effect of Z and r on  H dissolution ?

Entropy Effects Predictions on Z and r effects in solid state and in solution?

Combining Enthalpy and Entropy

Acid Hydrolysis of Metal Cations