= Crystal chemistry
<Crystal chemistry> connects a crystalline material's composition, <chemical bonds> and atomic arrangement. It asks which <coordination numbers>, local geometries and substitutions are compatible with an observed <crystal structure>.
For example, the usual <sodium chloride> structure gives each ion six nearest oppositely charged neighbours, whereas the usual <caesium chloride structure> has eight. Charge balance constrains composition, while size, bonding and thermodynamic conditions influence which arrangement is stable. Ionic-size rules are useful guides rather than exact universal predictions. <X-ray diffraction> tests the arrangement experimentally, allowing composition-based expectations to be compared with measured periodic atomic structure.
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