Solids: Properties, Structure, Density and Examples

Solid substances are those that are capable of forming bodies and have volume. From liquids and gases they differ in their form. Solids retain the shape of the body due to the fact that their particles are not able to move freely. They differ in their density, plasticity, electrical conductivity and color. They also have other properties. For example, most of these substances melt during heating, acquiring a liquid aggregate state. Some of them, when heated, immediately turn into gas (sublime). But there are also those that decompose into other substances.

Types of solids

All solids are divided into two groups.

1. Amorphous, in which individual particles are located chaotically. In other words: they do not have a clear (definite) structure. These solids are able to melt in some established temperature range. The most common of them can be attributed glass and resin.
2. Crystalline, which, in turn, is divided into 4 types: atomic, molecular, ionic, metallic. In them, the particles are located only according to a certain scheme, namely at the nodes of the crystal lattice. Its geometry in different substances can vary greatly.

Solid crystalline substances predominate over amorphous in number.

Types of crystalline solids

In the solid state, almost all substances have a crystalline structure. They differ in their structure. Crystalline lattices in their nodes contain different particles and chemical elements. It is in accordance with them that they got their names. Each type has its characteristic properties:

• In an atomic crystal lattice, solid particles are bound by a covalent bond. It is distinguished by its strength. Due to this, such substances are characterized by a high melting point and boiling point. Quartz and diamond are of this type.
• In a molecular crystal lattice, the bond between particles is notable for its weakness. Substances of this type are characterized by the ease of boiling and melting. They differ in volatility due to which they have a certain smell. Such solid bodies include ice, sugar. Motions of molecules in solids of this type are distinguished by their activity.
• In the ionic crystal lattice , the nodes alternate with the corresponding particles charged positively and negatively. They are held by electrostatic attraction. This type of lattice exists in alkalis, salts, basic oxides. Many substances of this type are readily soluble in water. Due to a fairly strong connection between the ions, they are refractory. Almost all of them are odorless, since they are characterized by non-volatility. Ion-lattice substances are unable to conduct an electric current, since they do not contain free electrons. A typical example of an ionic solid is sodium chloride. Such a crystal lattice makes it fragile. This is due to the fact that any of its shifts can lead to the appearance of ion repulsive forces.
• In a metal crystal lattice, only ions of chemical substances are present in the nodes, which are positively charged. Between them there are free electrons, through which the thermal and electrical energy passes perfectly. That is why any metals differ in such a feature as conductivity.

General concepts of a solid

Solid bodies and substances are practically the same thing. These terms are one of the 4 aggregate states. Solid bodies have a stable shape and character of the thermal motion of atoms. And the latter make small fluctuations next to the equilibrium positions. The section of science dealing with the study of composition and internal structure is called solid state physics. There are other important fields of knowledge dealing with such substances. The change in shape under external influences and motion is called the mechanics of the deformed body.

Due to the different properties of solids, they have found application in various technical devices created by man. Most often, the basis for their use lay such properties as hardness, volume, mass, elasticity, ductility, brittleness. Modern science allows the use of other qualities of solids, which can be detected exclusively in the laboratory.

What are crystals?

Crystals are solid bodies with particles arranged in a certain order. Each chemical substance has its own structure. Its atoms form a three-dimensional-periodic laying, called a crystal lattice. Solids have a different symmetry of structure. The crystalline state of a solid is considered stable, since it has a minimal amount of potential energy.

The vast majority of solid materials (natural) consists of a huge number of randomly oriented individual grains (crystallites). Such substances are called polycrystalline. These include technical alloys and metals, as well as many rocks. Single crystals are called single natural or synthetic crystals.

Most often, such solids are formed from the state of the liquid phase represented by the melt or solution. Sometimes they are obtained from the gaseous state. This process is called crystallization. Thanks to scientific and technical progress, the procedure of growing (synthesizing) various substances has become an industrial scale. Most crystals have a natural shape in the form of regular polyhedra. Their sizes are very different. So, natural quartz (rock crystal) can weigh up to hundreds of kilograms, and diamonds - up to several grams.

In amorphous solids, atoms are in constant oscillation around chaotically located points. They retain a certain short-range order, but there is no long-range order. This is due to the fact that their molecules are located at a distance that can be compared with their size. The most common example in our life of such a solid is the glassy state. Amorphous substances are often regarded as a fluid with an infinitely high viscosity. The time of their crystallization is sometimes so great that it does not appear at all.

It is the above properties of these substances that make them unique. Amorphous solids are considered unstable, since they can eventually go over into a crystalline state.

The molecules and atoms that make up the solid are packed with high density. They practically retain their mutual position relative to other particles and stay together due to intermolecular interaction. The distance between molecules of solid matter in different directions is called the parameter of the crystal lattice. The structure of the substance and its symmetry determine a variety of properties, such as the electronic zone, cleavage and optics. When exposed to a solid substance of sufficiently high strength, these qualities can be to some extent violated. In this case, the solid is susceptible to permanent deformation.

Atoms of solids perform vibrational motions, which are due to their possession of thermal energy. Since they are negligible, they can be observed only under laboratory conditions. The molecular structure of a solid substance greatly influences its properties.

Study of solids

Features, properties of these substances, their quality and particle motion are studied in various subsections of solid state physics.

For the study used: radio spectroscopy, structural analysis using X-rays and other methods. So we study the mechanical, physical and thermal properties of solids. Hardness, resistance to loads, ultimate strength, phase transformations, studies material science. It largely resonates with the physics of solids. There is another important modern science. Investigation of existing and synthesizing of new substances is carried out by solid state chemistry.

Features of solids

The nature of the motion of the outer electrons of solid atoms determines many of its properties, for example, electrical properties. There are 5 classes of such bodies. They are established depending on the type of bond of atoms:

• Ionic, the main characteristic of which is the force of electrostatic attraction. Its features: reflection and absorption of light in the infrared region. At low temperatures, the ionic bond is characterized by low electrical conductivity. An example of such a substance is the sodium salt of hydrochloric acid (NaCl).
• Covalent, carried out due to an electron pair, which belongs to both atoms. This connection is divided into: single (simple), double and triple. These names indicate the presence of electron pairs (1, 2, 3). Double and triple bonds are called multiple. There is one more division of this group. Thus, depending on the distribution of the electron density, a polar and nonpolar bond is isolated. The first is formed by different atoms, and the second is the same. Such a solid state of matter, examples of which are diamond (C) and silicon (Si), differs in its density. The hardest crystals belong to the covalent bond.
• Metal, formed by combining the valence electrons of atoms. As a result, there is a common electronic cloud, which shifts under the influence of electrical voltage. A metallic bond is formed when the bound atoms are large. They are the ones who are capable of giving away electrons. For many metals and complex compounds, a solid state of matter is formed by this bond. Examples: sodium, barium, aluminum, copper, gold. Of the nonmetallic compounds, the following can be noted: AlCr ₂ , Ca ₂ Cu, Cu ₅ Zn ₈ . Substances with a metal bond (metals) are diverse in physical properties. They can be liquid (Hg), soft (Na, K), very hard (W, Nb).
• Molecular, arising in crystals, which are formed by individual molecules of matter. It is characterized by gaps between molecules with zero electron density. The forces connecting the atoms in such crystals are significant. In this case, the molecules are attracted to each other only by a weak intermolecular attraction. That is why the connections between them are easily destroyed when heated. The connections between atoms are much more complicated. The molecular bond is divided into orientation, dispersion and induction. An example of such a substance is solid methane.
• Hydrogen, which occurs between positively polarized atoms of a molecule or a part of it and a negatively polarized smallest particle of another molecule or another part. These links include ice.

Properties of solids

What do we know today? Scientists have long studied the properties of the solid state of matter. When the temperature is affected, the temperature also changes. The transition of such a body into a liquid is called fusion. The transformation of a solid into a gaseous state is called sublimation. As the temperature is lowered, solidification takes place. Some substances under the influence of cold pass into the amorphous phase. This process is called vitrification by scientists.

In the case of phase transitions, the internal structure of solids changes. The greatest orderliness it acquires when the temperature is lowered. At atmospheric pressure and temperature T> 0 K any substances existing in nature harden. Only helium, for the crystallization of which a pressure of 24 atm is needed, constitutes an exception to this rule.

The solid state of the substance gives it different physical properties. They characterize the specific behavior of bodies under the influence of certain fields and forces. These properties are divided into groups. There are 3 modes of exposure corresponding to 3 types of energy (mechanical, thermal, electromagnetic). Accordingly, there are 3 groups of physical properties of solids:

• Mechanical properties associated with stress and deformation of bodies. According to these criteria, solids are divided into elastic, rheological, strength and technological. In rest, such a body retains its form, but it can change under the influence of an external force. In this case, its deformation can be plastic (the initial form does not return), elastic (it returns to its original form) or destructive (when a certain threshold is reached, decay / break occurs). The response to the applied force is described by the elastic moduli. The solid body resists not only compression, stretching, but also shifts, twisting and bending. Strength of a solid body is called its property to resist destruction.
• Thermal, manifested when exposed to thermal fields. One of the most important properties is the melting point at which the body goes into a liquid state. It is noted for crystalline solids. Amorphous bodies have a latent heat of fusion, since their transition to a liquid state with increasing temperature occurs gradually. Upon reaching a certain heat, the amorphous body loses its elasticity and acquires plasticity. This state means that it reaches the glass transition temperature. When heated, deformation of a solid takes place. And it often expands. Quantitatively, this state is characterized by a certain coefficient. Body temperature affects such mechanical characteristics as fluidity, ductility, hardness and strength.
• Electromagnetic, associated with the impact on the solid matter of microparticles and electromagnetic waves of high rigidity. The radiation properties are also conventionally referred to.

Zone structure

Solids are also classified according to the so-called band structure. So, among them are distinguished:

• Conductors, which differ in that their conduction and valence bands overlap. In this case, electrons can move between them, receiving the slightest energy. Conductors include all metals. When a potential difference is applied to such a body, an electric current is formed (due to the free movement of electrons between the points with the smallest and largest potential).
• Dielectrics whose zones do not overlap. The interval between them exceeds 4 eV. To carry electrons from the valence to the conductive zone, a large amount of energy is needed. Due to these properties of dielectrics, practically no current is flowing.
• Semiconductors characterized by the absence of conduction and valence bands. The interval between them is less than 4 eV. To transfer electrons from the valence band to the conducting zone, we need an energy smaller than for dielectrics. Pure (undoped and proprietary) semiconductors poorly pass current.

The motion of molecules in solids causes their electromagnetic properties.

Other properties

Solids are also subdivided according to their magnetic properties. There are three groups:

• Diamagnets whose properties depend little on temperature or the state of aggregation.
• Paramagnetics, which are a consequence of the orientation of the conduction electrons and the magnetic moments of the atoms. According to the Curie law, their susceptibility decreases in proportion to temperature. Thus, at 300 K, it is 10 ^-5 .
• Bodies with an ordered magnetic structure possessing a long-range order of atoms. At the nodes of their lattice, particles with magnetic moments are periodically located. Such solids and substances are often used in various spheres of human activity.

The most solid substances in nature

What are they? The density of solids largely determines their hardness. In recent years, scientists have discovered several materials that claim to be the "most enduring body". The hardest substance is fullerite (a crystal with fullerene molecules), which is about 1.5 times harder than a diamond. Unfortunately, it is still only available in very small quantities.

To date, the most solid substance, which may later be used in industry, is lonsdaleite (hexagonal diamond). He is 58% harder than a diamond. Lonsdale is an allotropic modification of carbon. Its crystal lattice is very similar to the diamond lattice. The cell of Lonsdaleit contains 4 atoms, and the diamond - 8. Of the widely used crystals, today the hardest remains the diamond.