How Are Crystals of a Substance Arranged?
Conceptual Overview Crystalline solids are defined by a regular arrangement of atomic or molecular collections called unit cells. Energetic tendencies due to electrical interactions serve to lower the total potential energy of a system by close-packing. Also, keep in mind that molecules or ions making up a substance are not points. Particle volumes affect close-packing geometry. Electrical Forces An example emphasizing electrical forces’ importance in crystal arrangement: sodium chloride, or common salt. Sodium exists as positive ions and chlorine exists as negative ions of same magnitude. There is energetic motive for oppositely charged ions to come as close together as possible. Energetic calculations confirm that if a charged spherical ion is surrounded on all sides by ions of the opposite charge, total potential energy of the system is lowered. In three dimensions, each ion is surrounded by six ions of opposite charge. If volume or shape of each ion allow, more or less than six ion
