This article tells what the zone theory of solids is. It is shown what exactly causes such a representation of the structure of matter. Differences of metals from dielectrics and semiconductors are given.
Socket and button
How many times a day do we press a variety of buttons? It cannot even occur to anyone to consider this - this action has become so familiar. And a person does not think that all this is possible only due to how easily electric current flows in metals. To turn on the light, boil the kettle, start the washing machine, not to mention the actions on smartphones, means to close the circuit and allow the electrons in the conductors to work instead of people. There are many explanations for such a phenomenon as conductivity. Perhaps the most obvious is the band theory of solids.
Atom and Kettles
Everyone who studied at school has an idea about the structure of the atom. Recall that around small positively charged heavy nuclei (consisting of protons and neutrons) light small electrons revolve. The number of negative particles is exactly equal to the number of positive. In order not to bore readers, we explain in the style of "quantum mechanics for dummies." Each electron has a strictly limited orbit in which it can rotate around a nucleus in a given chemical element. In turn, each type of atom has a unique pattern of such orbits. This is how spectroscopic scientists distinguish boron from selenium and arsenic from sodium. However, in addition to pure substances, in nature there are innumerable various combinations. Quantum mechanics (for dummies, as the reader should remember) claims that in complex compounds, the orbits intersect, merge, transform, extend, creating bonds. Their quality depends on the type: covalent and ionic are stronger, hydrogen, for example, is weaker.
Crystal structure
In a solid, everything is more complicated. For the model that the band theory of solids uses, an ideal crystal is usually taken. This means that it is infinite and sinless - every atom in its allotted place, the total charge is zero. The nuclei oscillate around a specific equilibrium position, but the electrons, one might say, are common. Depending on how “simple” one atom gives away its negative particles to neighboring ones, a rigidly defined structure of dielectrics or an electron cloud of metals is obtained. It is worth adding that when considering the assumption is made that all the electrons occupy the minimum energy allocated to them, which means that the body is at zero Kelvin. At a higher temperature, the oscillation amplitude of both nuclei and electrons is stronger, which means that the latter are able to occupy higher energy levels. The distribution of negative particles becomes more "loose". In some problems, this is important, but temperature is not so important for describing this phenomenon as such.
Pauli principle and loader
The concept of the band theory of a solid can be found only by remembering carefully what the Pauli principle is. If you imagine that the electrons are bags of sugar, then if there are a lot of these bags, the conditional loader will lay them on top of each other. Each "bag" takes its place in space. For electrons, this means that in this particular state in a single system there can only be one. This is the Pauli principle. Note that we mean ideal conditions, that is, the temperature is Kelvin zero, and the crystal is infinite. The whole system is in the same conditions: temperature, mechanical stress, defectiveness are the same in all parts of the whole.
Electronic zones of crystals
There are many atoms of the same type in a crystal. One mole of substance contains ten to the twenty-third degree of elements. And how many moles per kilogram of, say, salt? So you could even say that even the smallest crystal contains an unimaginably many atoms. Each chemical element has its own pattern of electronic orbits, but what if there are several of them in one body? After all, according to the Pauli principle, they should all occupy different states. The zone theory of solids offers the following way out: electronic orbits acquire different energies. Moreover, the difference between them is so small that they are pressed, leaning against each other very tightly, and form a continuous zone. Thus, each level of an electron in one atom turns into a zone in a bulk crystal. Elements of the band theory of solids will help explain the difference between dielectrics and conductors.
Electron inside the zone
We have already discussed what happens to many electrons that occupy the same orbit in an atom when a crystal is formed. But their behavior inside the zone so far has remained unlit. It is important to talk about this because it determines the difference between metals and non-metals. As mentioned above, the band theory of solids suggests that within the band the energy levels of different orbits of individual atoms differ so little that they form an almost continuous spectrum. Thus, it is not difficult for an electron to overcome the potential barrier between them - it moves along them freely, even thermal energy is enough for this. However, each permitted zone has limits. There will always be an energy level that is above or below all others.
Valence, forbidden, conductivity
Between these zones is a region of energy in which there is not a single level at which the electron could be. On the graphs, it appears as a white gap. And it is called the forbidden zone. The electron can overcome this barrier only in a jerk. So, he must get the appropriate energy for this. The zone with the highest energy, in which the existence of electrons is allowed for a given type of atom, is called the valence, and the conduction that follows it.
Metal, dielectric
The band theory of the conductivity of solids states that the presence or absence of electrons in the conduction band shows how easily current flows in a given substance. Thus, metals and dielectrics are distinguished. In the first case, the conduction band already contains electrons, since it overlaps with the valence one. This means that negative particles can move freely under the influence of an electromagnetic field, without additional energy costs. Therefore, an electric current in metals arises as easily, in fact, instantly, as soon as a field appears. And for the same reason, the wires are made of steel, copper, aluminum.
Materials in which the conduction and valence bands are energetically separated are called dielectrics. Their electrons are locked in the lower allowed level. The forbidden zone separates the negative particles from the level at which they could move freely. And the energy that must be communicated to the electrons in order to overcome it will destroy the material. Or change its properties beyond recognition. Therefore, the plastic wrap of the wires melts and burns, but does not conduct electricity.
Semiconductors
But there is an intermediate class of materials that have a forbidden zone, but under certain conditions they are able to conduct electric current. They are called semiconductors. Like dielectrics, they have an energy gap between the conduction band and the valence band. However, it is smaller and can be overcome with some effort. The classic semiconductor is silicon (in Latin - silicium). The famous silicone valley is famous for technologies based on the use of crystals of this particular substance to create electronic equipment.