Can all elements become solids?
Solid refers to matter in the natural sciences solid state of aggregation. This is a special case of condensed matter. In the narrower sense, this also means a substance which has a solid state of aggregation at a temperature of 20 ° C., the term solid in this case being substance-specific, but not temperature-specific.
The properties of solids differ considerably from the properties of free particles or solutions due to the mutual interaction of the building blocks of matter. A special characteristic of solids is the stability of the order (amorphous or crystalline) of their building blocks.
Structure of solids
In technical parlance, solids have a certain Minimum expansionwhich, however, is not sharply defined. They are therefore macroscopic bodies - in contrast to microscopic bodies. For example, a macromolecule on its own is not usually considered a solid. Matter in the transition area is called a cluster.
All solids can be imagined as being made up of building blocks. A building block can be a single atom or molecule, but also a group of them. If all components are of the same type, one speaks of monostructures, otherwise of heterostructures.
A distinction is made between amorphous (in the smallest "shapeless") and crystalline solids. Solid-state physics mainly deals with the properties of crystalline solids. As in geology, this is understood to mean a fine structure, as can be seen, for example, in marble by the sparkle of the smallest grains. The word crystalline means consisting of crystals.
In contrast to rocks, however, this structure is much finer in most of the solids observed or used in solid-state or geophysics, industry and technology and hardly recognizable even under the microscope at the highest magnification. An exception to this are, for example, the Weiss regions of metals, which are responsible for ferromagnetism.
- Single crystals
- A distinctive feature of crystals is the regular arrangement of their building blocks. The nature of the underlying structure is responsible for many properties of a solid. For example, carbon has two different crystal structures - graphite and diamond - which have completely different electrical conductivities (graphite conducts electricity, diamond is an insulator).
- Amorphous solids
- The physics of amorphous solids is multi-layered, because it includes all solids that do not have a regular structure. Most glasses or some solidified liquids are only a few representatives of this genus. With the loss of a macroscopic structure, many typical properties of a crystal are also lost. For example, most amorphous solids are poor electrical conductors. Nevertheless, this area is an interesting topic in research, since a lack of the crystal structure also means a lack of anisotropy effects.
- Polycrystalline solids
- Crystalline and amorphous are not the only possible manifestations of solids. In between there is an area that is, so to speak, a hybrid: the polycrystalline solids. These consist of a collection of small single crystals that are disordered and built into a large whole.
The cohesion of a solid is based on an attractive (attractive) interaction between atoms or molecules over long distances and a repulsive one over short ones. The energetically most favorable distance is called Equilibrium distance. If the thermal energy of the atoms is too low to escape this potential trap, rigid arrangements form - the atoms are bound to one another.
There are essentially four types of bond that significantly influence the structure and properties of a solid:
- Ionic bond
- This type of bond always occurs - at least in part - when the solid is made up of different elements that have different electronegativity. One element gives the other an electron, i.e. one becomes an anion and the other becomes a cation. The different charges cause an electrostatic attraction. Salts are a typical representative of this type of bond.
- Atomic bond
- This bond, also known as covalent, is based on a lowering of the electronic energy. The principle is the same as for the formation of molecules such as e.g. O2. Elements of the fourth main group (carbon, silicon, germanium) are bound in this way. The state of the electrons is then also referred to as sp3Hybridization.
- Metal bond
- The metal bond is an extreme case of the atomic bond. This bond is also due to a reduction in the electronic energy. Only here the overlap of the orbitals of the atoms is so great that they also interact with those of its next-but-one (or even more) neighbors. One can imagine that the ion cores of the atoms are embedded in a lake of electrons. As the name suggests, metals form this bond.
- Van der Waals bond
- The Van der Waals bond always occurs, but it is so weak that it is only noticeable in the absence of another type of bond. The attractive force is an electrostatic one, but here it is caused by induced dipole moments. Noble gas and molecular crystals are only held together by these.
These types of attachment are by no means isolated cases that only occur either-or. The transition from ionic to covalent to metallic bonds is fluid. In addition, different bonds can occur side by side in solids. Graphite, for example, consists of layers of covalently bonded carbon atoms, while the layers as a whole are held together by van der Waals bonds. Since the latter bond is so weak, graphite is used as a pencil lead - the bonds tear when rubbing over paper.
With the surface one means the final 1–3 atomic layers on the border to the vacuum. The lack of binding partners to one side usually results in relaxation or recombination for atoms in these layers. The atoms try to adopt an energetically more favorable state by changing their bond length to deeper layers (relaxation) or by rearranging their positions and saturating open bonds (recombination). The result is new surface structures that can have a different periodicity than the substrate (deeper layers).
Another special feature is the appearance of surface conditions. This means that in the otherwise forbidden energetically forbidden areas - the band gaps - permitted energy states for electrons can arise. In the case of semiconductors, these new states cause the ribbons to bend and thus change the electrical conductivity. This can lead to conduction channels, which is used, for example, for field effect transistors.
Properties of solids
All solid bodies can be assigned to conductors, semiconductors or non-conductors according to their ability to conduct electrical current. This classification was historically determined. However, an explanation for the differences in conductivity could only be provided by the ribbon model. Nowadays, therefore, the group assignment is determined by the size of the band gap.
- Almost all metals are electrically good conductors. The conduction electrons behave as if they can move freely in the solid. However, as the temperature rises, the conductivity decreases, which can be explained by increased collisions between the electrons and at defects in the crystal structure.
- The most striking feature of the conductivity of semiconductors is their strong dependence on internal (degree of purity) as well as external parameters (temperature). In the case of pure (intrinsic) semiconductors, the conductivity increases sharply with increasing temperature - often by an order of magnitude with a difference of approx. 20 K. In addition to electrons, so-called defect electrons, also called holes, also contribute to the conductivity. The charge carrier densities of holes and electrons are the same in intrinsic semiconductors, but the ratio can be changed on one side by targeted contamination (doping).
- Insulators conduct virtually no electrical current under normal circumstances.
The electrical conductivity is one of the most variable quantities in physics, the possible values extend over more than 30 orders of magnitude. Most non-magnetic solids show another amazing effect at very low temperatures: below a critical temperature, the electrical resistance disappears completely; this state is called the superconducting phase.
In contrast to liquids and gases, the particles in the solid state of aggregation are only minimally mutually displaceable - according to their crystal-like fine structure. Such deformations are difficult to model in the smallest detail, but they follow clear laws over millions or trillions of particles. They are related to the elasticity and its modules, as well as to the shape and dimension of the body to be deformed.
An idealized solid that is used as a model of a solid in classical mechanics is the rigid body. It is not subject to any deformations, but does not occur in nature. In most cases it is a good model for the real objects in our environment. The real solid body, on the other hand, usually does not have a simple, but a direction-dependent deformability. This covers, for example, solid state physics and the theory of matter waves.
Compared to reactions in solution, solid-state reactions are usually characterized by very high activation barriers. The reason is the reaction mechanism according to which solid-state reactions take place: The gaps in the crystal lattice move like in a "sliding puzzle". For this, the crystal structure has to be deformed, which causes a great deal of energy.
Other typical attributes of solids are their conductivity for heat or electrical current. These two properties are usually closely linked.
Categories: Solid State Physics | Physical chemistry | Materials science
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