Solid State

Solid State In solids the kinetic energy is low and the cohesion forces are high. Solids are rigid and have a well-defined shape and contours. The arrangement of particles is disorderly in amorphous solids (e.g. glass) and very neat and repetitive in crystalline solids (e.g. salts). The latter are provided with a rigid and orderly three-dimensional crystalline lattice, which generally gives high density, hardness and heat resistance. Solids are characterized by properties that can be summarized in hardness (property to nick or scratch another body), malleability (property to reduce into small thin sheets), ductility (property to reduce into threads), toughness (resistance to breakage), plasticity (property to shape under the action of a force acting for more or less long times) and elasticity (property to resume the initial shape and volume after being subjected to a deformation). Crystalline solids are characterized by having an ordered and periodic crystalline structure, called the crystalline lattice, consisting of the atoms or molecules making up the solid arranged in space. How can you recognize a crystal lattice? - using X-ray diffraction; - measuring the physical properties of a single solid crystal; - a well-defined melting point. The crystalline lattice is formed by nodes, i.e. atoms or molecules that form the solid under examination, aligned with each other to form a defined and ordered geometric figure. The nodes may consist of equal atoms/molecules or different atoms, such as anions and cations of an ionic salt. Several knots arranged at the same distance and in the same direction can be called "rows". By regularly placing several nodes along a surface we can construct an elementary parallelogram, identified by two sides and the angle between them. Several "elementary parallelograms" (or "elementary cells") repeated on a plane generate the entire crystalline structure. So a monocrystal consists of several elementary cells arranged neatly in space. There are 14 different types of crystalline grids. A substance may possess as many types of crystalline reticles as possible, because of its solid state, depending on the temperature and pressure conditions to which the solid itself is subjected. This phenomenon is known as polymorphism, in case of molecules, or allotropy, in case of pure elements. The different crystalline forms of the same substance have similar chemical properties but different physical characteristics (e.g. carbon has at least two crystalline allotropic forms: graphite, which is opaque/black, excellent electrical conductor and has the highest melting temperature, and diamond, which is very hard, transparent, colourless and a very bad electrical conductor). The crystal-shaped transition is a real state changeover (therefore it takes place under isothermal conditions). If the transition to crystalline structure results in both directions and under the same conditions of temperature and pressure, we will obtain a solid composed at the same time of two types of crystalline forms: enantiotropy. If under certain conditions of temperature and pressure we have the presence of one or the other crystalline form, we would have the phenomenon of monotropy. Substances with only two possible crystalline forms are called dimorphic. Two different substances can generate very similar crystalline reticles: isomorphism. If we caused the crystallization of a solution containing, as solutes, two isomorphic solids between them, we would obtain a "mixed crystal". To obtain an isomorphism the two substances must be composed of atoms with similar atomic rays and must have a similar chemical formula, as in the case of KCr(SO4)2 and KAI(SO4)2. Usually a substance that passes from the liquid or aeriform state to the solid state shows a contraction in volume but this is not always valid, e.g. water and ice. Solids can be classified according to the type of bond that unites the various particles of the solid itself, i.e. the various nodes of the crystalline lattice: molecular solids, hydrogen bonded solids, ionic solids, covalent solids.