Investigating Crystal Structures

When we teach ionic bonding at GCSE we usually show students the structure of sodium chloride. We tell them that oppositely charged ions attract one another, and that the ions in the crystal lattice are, accordingly, ordered so that the immediate neighbours around a given ion have the opposite charge. At A-level we build on this model and explain that an ion in sodium chloride has six neighbours and that the ions are arranged in a cubic lattice. We can challenge our students further by introducing them to the Madelung constant as a way of calculating the lattice energy of an ionic structure, given the atom positions in the crystal from X-ray diffraction studies. By comparing the values of lattice energy (directly or indirectly) through experiment provides a good test of the accuracy of the ionic bond model and thus a better insight into the nature of chemical bonding.