Solid state chemistry studies the structure, properties, application, and synthesis of crystals. This branch of science calls crystalline substances, consisting of particles arranged in an orderly way relative to each other. Such a structure is described in a special way - a crystal lattice, the types of which are determined by its constituent elements.
Solids by their properties and structure are usually classified into crystalline and amorphous. This separation is due to the different positions of ions, atoms or molecules in them. Particles of amorphous bodies are distributed chaotically. This structure makes them isotropic and makes it impossible to have a fixed melting point. Amorphous materials include waxes, polymers and plastics, and hardened resins.
In a crystalline substance, the particles are arranged in a strictly defined order and form structures that periodically repeat throughout its volume. This gives the material an anisotropic match. The spatial representation of such a structure is a crystal lattice. It is not static, its molecules or atoms constantly vibrate around equilibrium positions. Consists of nodes in which particles of matter are located, and imaginary lines connecting them. Nodes are formed by molecules, atoms, or ions. The lines represent the chemical bonds between these particles.
The existence of lattices is due to the fact that only under the condition of periodicity in three-dimensional space is the minimum value of the potential energy of the system achieved by balancing the forces of repulsion and attraction between atoms. They are ionic, metallic, covalent polar and non-polar.
The smallest lattice fragment is considered an elementary cell. The complete structure of a material can be constructed in the following way - to transfer a unit cell in parallel in any direction. The compactness and fullness of such a cell is an important characteristic that determines the chemical and physical properties of a substance. It is described by such indicators: the number of atoms in it, the packing density and the parameter, i.e., the distance between the nearest particles. The parameter is measured in nanometers and is calculated using X-ray diffraction analysis.
The simplest unit cell consists of eight particles located at the vertices of a cube. Other possible geometric constructions of structures describe the Bravais models.
The chemical structure of such solids is determined by the type of bond between its particles, which are metallic or ionic. A structure consisting of positively charged ions of a single chemical element, between which there are electrons of an external energy level, which easily lose their connection with the nucleus and move freely throughout the metal body (electron gas), is called a metal crystal lattice. Ions and atoms are located in the nodes. Such a lattice consists of elementary cells, which are of the following types:
Body-centered cube. Includes two atoms. Metal ions with a positive charge sign are located at the nodes of the cube, and one more atom is at its center. Such a lattice has very hard and refractory metals - tungsten, molybdenum, chromium, vanadium.
Face-centered cube. Contains four atoms, positive ions are found not only in the nodes, but also in the center of each face of the cube. This lattice is typical for lead, copper, nickel, iron, aluminum and many other metals.
Hexagonal tightly packed. It consists of six atoms and is a hexagonal prism. Ions are located at the nodes, in the center of the bases and in the middle of the prism. All elements of group II of the periodic table have such a structure - beryllium, calcium, magnesium, strontium, barium, radium.
The metallic type of structure determines the presence of properties typical of metals and their alloys - luster, ductility and ductility, high electrical and thermal conductivity.
If a crystal contains chemical elements consisting of atoms with different electronegativity (the ability of an atom to attract a common electron pair to itself), then electrons from some atoms can completely transfer to others. The result is positively and negatively charged ions, called cations and anions, respectively. The bond between them is called ionic, and the structure, in the nodes of which there are differently charged ions, is called the ionic crystal lattice.
The geometric views of the elementary cells of ionic structures are the same as those of metallic ones, only the nodes contain ions of not one, but different chemical elements, and equally charged ones are as far away from each other as possible, and differently charged ones are as close as possible.
This structure is usually found in compounds formed by a typical metal and non-metal, while the metal should be less active than the non-metal. The ionic structure corresponds to oxides, hydroxides, alkalis and salts, and their constituents are simple or complex. So, a crystal of sodium chloride (sodium chloride) is formed from simple ions: potassium cation K and chlorine anion Cl, and potassium sulfate contains a simple potassium cation K and a complex negative sulfate ion SO4.
The bonds between ions in the structures are very stable, therefore, substances with such a structure have strength, hardness and refractoriness and have little volatility.
The structures formed by polar molecules, between which there are forces of interaction, are called molecular crystal lattices. Intermolecular bonds in them are of an electrical nature; they do not require a lot of energy to break them. At the same time, the atoms of these molecules are covalent.
Substances with a molecular structure include:
white phosphorus, monoclinic and rhombic sulfur;
halogens, hydrogen halides and other liquids and gases in a solid state of aggregation (iodine, fluorine, chlorine, nitrogen, oxygen, ozone, water, carbon monoxide - the so-called dry ice);
inert gases with monoatomic molecules (neon, argon, xenon).
This structure corresponds to most organic compounds.
Weak interaction between molecules determines the properties of substances with such a structure. They are soft and fragile, and have low melting and boiling points. These crystals are volatile and dissolve well in water and other liquids.
Due to the variety of spatial patterns of crystals, materials that are significantly different from each other can be obtained from the same chemical elements.