Rules for Crystal Structure Formation

Because dipole forces are only secondary, the forces are very weak. Thus, the properties that they impart on the crystal are much weaker than in the covalent and ionic crystals. For this reason, van der Waals bonds do not significantly influence the crystal properties of phosphate ceramics. 

The hydrogen bond is not as strong as the ionic and covalent bonds, but is stronger than the van der Waals bond. 

Though the different bonds explain whether a particular crystal is hard or not, soluble or insoluble, thermally stable or unstable, the crystal structure itself may be visualized by considering the atoms and molecules as hard balls of different radii. Thus, the atomic and ionic radii are important factors in constructing crystal strucmres. Once these radii are known, one may consider hard balls of the sizes of these radii and construct physical models by stacking the balls in layers. While arranging the structures, however, the following rules apply. 

Generally, a crystal is electrically neutral. This implies that the crystal should have an equal number of positive and negative charges. Thus, when oppositely charged ions come together to form a neutral crystal structure, each ion coordinates with as many ions of opposite charge as the size permits. This coordination principle dictates both electrical neutrality of the crystal structure and compact packing of the atoms within the structure. 

The number of the nearest neighbors that an atom can accommodate in a given crystal is called the coordination number , and the shell formed by the nearest neighbors is called 

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