Ideal solids consist of an arrangement of atoms repeated indefinitely in space. The term extended solid refers to the case where the bonding between the atoms is reasonably uniform in strength throughout the structure (e.g. diamond, metallic Cu).
An ionic material is an extended solid in which substantial charge transfer has occurred (e.g. NaCl). A molecular (or atomic) solid consists of small discrete molecules (or atoms) weakly bound together (e.g. solid CH4 or Xe, white P). The latter category should be characterised by low melting points (or low sublimation temperatures).
The forces binding the fundamental units (atoms, ions or molecules) together may be essentially non-directional or directional. Non-directional forces result in structures based on close packing, while directional forces lead to more open structures, often based on linked polyhedra or groups of polyhedra. The two categories are now considered separately.