Bonding in Ionic Solids

What controls ionic bonding strength?

Why do some ionic compounds take one shape while others a different shape?

Ion-Pair EnergyComes 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+, 1s2 52 ex: F-, [He]2s22p6 73 ex: Ca2+, [Ne] 3s23p6 94 Br-, Sr2+ 105 I- 12

nrU

oo

11

ZZ 1389

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Lattice Energy = Ion-pair energy x Madelung constant

nrU

oo

11

MZZ 1389

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Crystal Structure Madelung constantNaCl 1.748CsCl 1.763Zinc blend 1.638Wurtzite 1.641Fluorite 2.519Rutile 2.408

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 theanions 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 Hdissolution?

Entropy EffectsPredictions on Z and r effects in solid state and in solution?

Combining Enthalpy and Entropy

Acid Hydrolysis of Metal Cations