![]() Almost any substance can solidify in amorphous form if the liquid phase is cooled rapidly enough. An amorphous, translucent solid is called a glass. When cleaved or broken, they produce fragments with irregular, often curved surfaces and they have poorly defined patterns when exposed to x-rays because their components are not arranged in a regular array. Thus the intermolecular forces holding the solid together are uniform, and the same amount of thermal energy is needed to break every interaction simultaneously.Īmorphous solids have two characteristic properties. The resulting repulsive interactions between ions with like charges cause the layers to separate.Ĭrystals tend to have relatively sharp, well-defined melting points because all the component atoms, molecules, or ions are the same distance from the same number and type of neighbors that is, the regularity of the crystalline lattice creates local environments that are the same. Deformation of the ionic crystal causes one plane of atoms to slide along another. Figure 12.1: Cleaving a Crystal of an Ionic Compound along a Plane of Ions. In a covalent solid such as a cut diamond, the angles at which the faces meet are also not arbitrary but are determined by the arrangement of the carbon atoms in the crystal. When an ionic crystal is cleaved (Figure 12.1), for example, repulsive interactions cause it to break along fixed planes to produce new faces that intersect at the same angles as those in the original crystal. The characteristic angles do not depend on the size of the crystal they reflect the regular repeating arrangement of the component atoms, molecules, or ions in space. When exposed to x-rays, each structure also produces a distinctive pattern that can be used to identify the material. The faces intersect at angles that are characteristic of the substance. Obsidian, a volcanic glass with the same chemical composition as granite (typically KAlSi 3O 8), tends to have curved, irregular surfaces when cleaved.Ĭrystalline solids, or crystals, have distinctive internal structures that in turn lead to distinctive flat surfaces, or faces. The faces of crystals can intersect at right angles, as in galena (PbS) and pyrite (FeS 2), or at other angles, as in quartz.(Right) Cleavage surfaces of an amorphous solid. The constituents of a solid can be arranged in two general ways: they can form a regular repeating three-dimensional structure called a crystal lattice, thus producing a crystalline solid, or they can aggregate with no particular order, in which case they form an amorphous solid (from the Greek ámorphos, meaning “shapeless”). When we discuss solids, therefore, we consider the positions of the atoms, molecules, or ions, which are essentially fixed in space, rather than their motions (which are more important in liquids and gases). With few exceptions, the particles that compose a solid material, whether ionic, molecular, covalent, or metallic, are held in place by strong attractive forces between them. ![]()
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