Why the analogy with sodium chloride (NaCl) crystals?
All the inorganic alkali halides are built of only two kinds of oppositely charged particles each with the unit electric charge of either the electron or positron. The binding is purely ionic (not covalent nor metallic, etc.) and the lattices are stable. Amongst the 20 alkali halide lattices, sodium chloride belongs to the most popular and most densely packed lattice type (viz. face centred cubic, fcc) Also, its ions each come from the same period of the periodic table of elements meaning that they have the same structure of inner electron orbitals and therefore are well matched. You could argue that potassium chloride (KCl) is also well matched but there is more information available about NaCl. Calculation of the dimensions of the unit cube of the epola, from a binding energy of 1.02MeV per epo pair, based on NaCl data give a value of 4.4fm whilst data from all the other alkali halides yield values ranging from 4.0 to 4.8 fm. Simhony points out that the analogy cannot be drawn too fa
