Ionic crystals lattice energy

LATTICE THEORY OF IONIC CRYSTALS Lattice energy... 

Energy Changes in the Formation of Ionic Crystals— Lattice energies of ionic crystals can be related to certain atomic and thermodynamic properties by means of the Born-Fajans-Haber cycle (Fig. 12-51). 

An important property of an ionic crystal is the energy required to break the crystal apart into individual ions, this is the crystal lattice energy. It can be measured by a thermodynamic cycle, called the Born-Haber cycle. 

The crystal lattice energy can be estimated from a simple electrostatic model When this model is applied to an ionic crystal only the electrostatic charges and the shortest anion-cation intermiclear distance need be considered. The summation of all the geometrical interactions be/Kveeti the ions is called the Madelung constant. From this model an equatitWjor the crystal lattice energy is derived ... 

Fig. 2 Conceptual illustration of a energy band scheme and b an ionic crystal lattice consisting of M2+ and O2- ions...

Apart from the difference in the bond energy, other criteria, both direct and indirect, exist between the two different types of bond. Ionic molecules in the solid state form an ionic crystal lattice which r.har ir,teristic lo he particular molecule In such a lattice, ea ion is surrounded by ions of opj site char at equal distances from the central ion and individual molecules cease to exist. The number of ions packed round the central ion is limited by the size of the ions, the attraction and repulsion energies between the ions and other factors. The links of the central ion with its nearest neighbours are all of the same strength and the number of such... 

The fact that the ionic model works rather well for crystal lattice energy calculations of compounds such as MgO and Li20 cannot be interpreted as establishing that 0 " actually exists in the solid. Crystal lattice energy calculations of Mg3N2,... 

AI2O3, and AIN are in reasonable accord with experiment, but certainly all of these compounds have significant covalent character. Crystal lattice energy calculations based on the ionic model are found to be relatively insensitive to a significant covalent contribution to the bonding. Ahlrichs makes a similar comment using MgO as an example in his discussion of the Hartree-Fock theory for negative ions. 

The energy associated with the attraction of separated gaseous positive and negative ions to form an ionic solid is the crystal lattice energy of the solid. For NaCl, this energy... 

Nonpolar solids such as naphthalene, CjQHg, do not dissolve appreciably in polar solvents such as water because the two substances do not attract each other significantly. This is true despite the fact that crystal lattice energies of solids consisting of nonpolar molecules are much less negative (smaller in magnitude) than those of ionic solids. Naphthalene dissolves readily in nonpolar solvents such as benzene because there are no strong attractive forces between solute molecules or between solvent molecules. In such cases, the increase in disorder controls the process. These facts help explain the observation that like dissolves like. ... 

Choose the ionic compound from each pair for which the crystal lattice energy should be the most negative. Justify your choice, (a) LiE or LiBr (b) KF or CaF2 (c) FeF2 or FeFj (d) NaF or KE... 

The melting point is the temperature at which a solid is converted to a liquid and the boiling point is the temperature at which a liquid is converted to a gas at a specified pressure. Considerable energy is required to break apart an ionic crystal lattice with uncountable numbers of ionic interactions and convert the ionic substance to a liquid or a gas. As a result, the melting and boiling temperatures for ionic compounds are generally higher than those of covalent compounds, whose... 

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