Structural formulae of inorganic compounds

If the atoms (in a solid) are linked to form a 1-, 2-, or 3-dimensional system the term molecular weight has no meaning, and the structural formula must describe some characteristic set of atoms which on repetition reproduces the arrangement found in the crystal. The repeat unit in an infinite 1-dimensional system is readily found by noting the points at which the pattern repeats itself  

The complete description of the chain requires metrical information as in the case of a finite group. It should be noted that if the geometry of the chain is taken into account, that is, the actual spatial arrangement of the atoms in the crystal, then the (crystallographic) repeat unit may be larger than the simplest chemical repeat unit. The crystallographic repeat unit is that set of atoms which reproduces the observed 

The crystallographic repeat unit of a 3D pattern is a parallelepiped containing a representative collection of atoms which on repetition in the directions of its edges forms the (potentially infinite) crystal. As in the case of a 1- or 2-dimensional pattern this unit cell may, and usually does, contain more than one basic chemical unit (corresponding to the simplest chemical formula). 

The following remarks may be helpful at this point they are amplified in later chapters. There is no unique unit cell in a crystal structure, but if there are 12 

(a) Alternative unit cells of layer structure of AS2O3. (b) Projection of unit cell of a 

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