Crystal Structures, Compact Packing, Coordination

Abstract

AbstractUnderlying any structural criteria based on chemical bonding, there is one fact: the main condition for a crystalline structure to be s is that its free energy is minimal; therefore, the structures most likely to be energetically stable are those whose atoms or molecules are arranged in space in the most compact way possible. Despite the chemical diversity observed in the large number of resolved structures, all these structures can refer to a rather reduced number of fixed structural types which, in turn, are characterized by the fact that their atoms or molecules are arranged in space in the most compact way possible. The structure depends on the type of bond, the number and diversity of atoms and the repetition associated with the network. However, although the types of bonds are included in the program, they are not explained, since the student has knowledge of them through chemistry studied in pre-university courses and even in this first course of Geology. It is simply to remind them of this dependence on the structure. Covalent, metallic, and ionic crystals will be briefly explained. Depending on the different nature of the chemical bonding forces, crystalline structures can be grouped into two large categories: structures in whose space molecules are energetically individualized, and structures that can be considered constituted by a single molecule extending to the entire crystalline space. The mentioned crystals belong to this category. Therefore, the types of packing will be explained based on the hypothesis that atoms can be represented by hard and impenetrable spheres. In this explanation, it is necessary to introduce the concepts of coordination polyhedron and coordination number and, since this depends on the relative sizes of the coordinated ions, we will talk about the ratio of radii. Finally, the Pauling rules are presented, which fundamentally consider the conditions of maximum compactness and markedly ionic character of the bond presented by many crystalline structures.