Structure and Bonding in Crystalline Alkali Metal Halides

2 Structure and Bonding in Crystalline Alkali Metal Halides 

The first chemical compound to have its crystal structure determined by X-ray diffraction was NaCl (rock salt). The author of the study, which was published in 1913, was a young man of 23 years, William L. Bragg [7]. Two years later, he and his father, W. H. Bragg, shared the Nobel Prize in physics. 

The crystal stracture of NaCl is face-centered cubic. Each ion, Na or CE, is surrounded by six ions of opposite charge at the comers of an octahedron (Fig. 2). The Na-Cl distance was found to be 280 pm, which is about 2 pm shorter than the presently accepted value. 

It has since been established that of the 20 alkali metal halides, MX, seventeen have face-centered cubic structures. The exceptions, CsCl, CsBr, and Csl, aU have body-centered cubic structures where each ion is surrounded by eight counterions at the comers of a cube. 

The lattice energy A /l of a crystalline alkali metal halide may be defined as the energy difference between the separated and X ions and the crystal in its equilibrium stracture at zero K. The lattice energy cannot be measured directly, but may be computed from standard enthalpies of formation at zero K  

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