Metal - Metal
They are called metal to chemical elements characterized by being good conductors of heat and electricity . They have high density and are solid at room temperature (except for mercury ); its salts form electropositive ions ( cations ) in solution. [ 1 ]
The materials science defines a metal as a material in which an overlap between the valence band and the conduction band in its electronic structure ( metallic bond ). [ 2 ] This gives it the ability to easily conduct heat and electricity (just like copper ) and generally the ability to reflect light, giving it its peculiar shine. In the absence of a known electronic structure , the term is used to describe the behavior of those materials in which, in certain ranges of pressure and temperature, electrical conductivity decreases with increasing temperature, in contrast to semiconductors . They react chemically with non-metals, they are not reactive with each other the great majority of the time (although some form alloys with each other).
Among the metals are alkaline (such as sodium) and alkaline earth (such as magnesium) which have low density, are good conductors of heat and electricity, as well as being very reactive. [ 3 ] Also included are transition metals (which make up most metals), those found in various groups, and lanthanides , actinides, and transactinides . [ 4 ] Theoretically, the rest of the elements that remain to be discovered and synthesized would be metals.
The concept of metal refers to both pure elements, as well as alloys with metallic characteristics, such as steel and bronze . Metals make up most of the periodic table of elements and are separated from nonmetals by a diagonal line between boron and polonium . [ 5 ] Compared to non-metals, they have low electronegativity and low ionization energy , making it easier for metals to give up electrons and more difficult for them to gain them.
They are obtained from the minerals that contain them, called metallic ores.
Metals such as gold , silver , iron and copper have been used since prehistoric times . At first, only those that were easily found in their pure state (in the form of native elements) were used, but gradually the technology necessary to obtain new metals from their ores was developed , heating them in a furnace using wood charcoal .
The first great advance occurred with the discovery of bronze , the result of the use of copper ore with incursions of tin , between 3500 a. C. and 2000 a. C. , in different regions of the planet, emerging the so-called Bronze Age , which happens to the Stone Age .
Another important fact in history was the use of iron , around 1400 BC. C. The Hittites were one of the first people to use it to develop weapons such as swords , and civilizations that were still in the Age of Bronze, as the Egyptians .
However, in ancient times it was not known to reach the necessary temperature to melt iron, so an impure metal was obtained that had to be molded with hammer blows. By the year 1400 they began to use ovens provided with bellows , [ 7 ] that achieve the melting temperature of the iron, about 1535 ° C .
Henry Bessemer discovered a way to produce steel in large quantities at a reasonable cost. After numerous unsuccessful attempts, he came up with a new furnace design (the Thomas-Bessemer converter ) and, from then on, improved the construction of structures in buildings and bridges, taking iron into the background.
The metallic elements, as well as the rest of the elements, are arranged in a system called the periodic table . Most of the elements in this table are metals.
Metals differ from other elements , mainly in the type of bond that their atoms make up . It is a metallic bond and in it the electrons form a "cloud" that moves, surrounding all the nuclei . This type of bond is what gives them the properties of electrical conduction , brightness , etc.
Properties of metals
Metals have characteristic physical properties, among them they are conductors of electricity. Most of them are grayish in color, but some have different colors; the bismuth (Bi) is pinkish, copper (Cu) reddish and gold (Au) yellow. In other metals more than one color appears; This phenomenon is called polychromism .
Other properties would be:
- Malleability : ability of metals to become sheets when subjected to compression forces.
- Ductility : property of metals to be molded into wire and threads when subjected to tensile stress.
- Tenacity : resistance that metals present when breaking or receiving sudden forces (blows, etc.)
- Mechanical resistance : ability to resist tensile, compression, torsion and bending stress without deforming or breaking.
Materials science defines a metal as a material in which there is an overlap between the valence band and the conduction band in its electronic structure ( metallic bond ). This gives it the ability to easily conduct heat and electricity, and generally the ability to reflect light, which gives it its peculiar shine.
Electronic gas theory
Metals have certain characteristic physical properties: (with the exception of mercury) they are solid under normal environmental conditions, they are usually opaque and shiny, they have high density, they are ductile and malleable, they have a high melting point, they are hard, and they are good conductors. of heat and electricity.
These properties are due to the fact that the outer electrons are bound only "slightly" to the atoms , forming a kind of gas (also called "electronic gas", "electronic cloud" or "sea of electrons"), which is known as metallic bond . Drude and Lorentz proposed this model 1900. [ 8 ]
Using the "electronic gas" theory, we can explain why metals are such good conductors of heat and electricity, but it is necessary to understand the nature of the bond between their atoms.
A first attempt to explain the metallic bond consisted in considering a model in which the valence electrons of each metal could move freely in the crystal lattice . In this way, the metallic lattice is considered to be made up of a set of positive ions (the nuclei surrounded by their electron shell) and electrons (the valence ones), instead of being made up of neutral atoms.
Ultimately, a metallic element is considered to be made up of regularly distributed metal cations immersed in a delocalized valence "electronic gas", acting as an electrostatic binder that holds the metal cations together.
The "electronic gas" model allows a simple qualitative explanation of the electrical and thermal conductivity of metals. Since electrons are mobile, they can move from the negative to the positive electrode when the metal is subjected to the effect of an electrical potential difference . Moving electrons can also conduct heat by transporting kinetic energy from one part of the crystal to another. The ductile and malleable character of metals is allowed by the fact that the delocalized bond extends in all directions; ie it is not limited to a particular orientation, as in the case of solid covalent networks.
When a metallic crystal is deformed, localized bonds are not broken; instead, the sea of electrons simply adapts to the new cation distribution, with the energy of the deformed structure being similar to the original. The energy required to deform a metal such as lithium is relatively low, being, of course, much higher than that required to deform a transition metal, because the latter has many more valence electrons that are the electrostatic binder of cations. .
Many properties of metals can be satisfactorily justified by the "electronic gas" theory, but it is not adequate to explain other aspects, such as the detailed description of the variation in conductivity between metallic elements.
Many metals are in the form of oxides . The oxygen , to the be present in large amounts in the atmosphere , it is easily combined with metals, which are reducing elements, forming compounds such as corundum (A the 2 O 3 ) and hematite (Fe 2 O 3 ).
The alkali metals besides the beryllium and magnesium , are usually extracted from the chlorides deposited due to the evaporation of seas and lakes , but is also extracted from the water sea . The most characteristic example is sodium chloride or common salt, NaCl.
Uses in industry
Yellow and orange of the pigments cadmium are highly sought for their great stability , as protection against corrosion , for welds and alloys corresponding and in the manufacture of batteries of nickel and cadmium , considered excellent for the safety of operation. It is also used as a stabilizer in plastic materials ( PVCsad ) and as an alloy to improve the mechanical characteristics of copper wire . [ 9 ] Its production takes place at the time ofrefining of zinc , with which is associated, is a dangerous pollutant.
The lithium , light metal, is used mainly in the ceramics and the crystals , as a catalyst for polymerization and as lubricant , and for obtaining the aluminum by electrolysis . It is also used for welding, in batteries and watch batteries , in medicine (treatment for manic-depressives) and in chemistry .
Dilation of metals
Metals are materials that have high expansion , in part due to their conductivity. The expansions are sometimes perceptible even with changes in ambient temperature. They are measured linearly and the unit of length is set for the 1 ° C temperature variation.
Malleability is the property of metals to be able to be modified in their shape and even be reduced to thin sheets at room temperature, by continuous pressure, hammered or drawn. Producing the modifications in the metal, a moment is reached when the elasticity limit is exceeded, making the metal hard and brittle; that is, it undergoes crystalline deformations that make it brittle. The malleability can be recovered by annealing , which consists of heating the metal to a high temperature after rolling or drawing, and allowing it to cool slowly. The malleability is appreciated by the subtlety of the laminate. Taking gold as a base, the following classification is usually made:
Definitions of terms used in foundries
- Alloy : An alloy is a homogeneous mixture of two or more elements, generally metals (although sometimes one of them can be a nonmetal).
- Pig iron : Liquid iron with less impurities than the initial iron.
- Slag : Impurities that react with limestone.
- Blast furnace : Furnace for making alloys and cast iron, reaching very high temperatures. It must be built with refractory materials, that is, very resistant to heat.
- Ultralight: Density in g / cm³ less than 2. The most common of this type are magnesium and beryllium .
- Light: Density in g / cm³ less than 4.5. The most common of this type are aluminum and titanium .
- Heavy: Density in g / cm³ greater than 4.5. They are the majority of metals.
See also the classification of metals in the periodic table .
Metallic materials fracture [ 12 ]
It usually occurs in a transgranular form, that is, through grains, in ductile metals and with good toughness.
The deformation occurs before the final fracture, a deformation can be observed, the visible modification that appears to be a neck, notch or stricture right in the part where the failure was caused. These fractures can be caused by simple overloads or by applying a very great stress to the material.
If a stress test is carried out in a simple test, the fracture process will be with the nucleation, growth and coalescence of micro-holes, formed when a great stress creates a separation at the grain boundaries, as the stress increases the separation of the grains create larger cavities with which the area of contact with the metal is very small and cannot support the load, eventually causing fracture.
It occurs in metals and high-strength alloys or may occur in those with poor ductility and toughness, regardless of whether metals have ductility within their properties when exposed to low temperatures, they may fail due to brittleness, likewise in thick sections or due to blemishes.
Brittle fractures are frequently observed when it is the impact and not the overload that causes the failure. The process begins by forming a small crack, imperfection, where the effort is concentrated. The crack can spread with a speed close to sound, which propagates more easily along specific crystallographic planes.
- [ https: //books.go s / books? Id = VmdvQfFbq9UC & pg = PA546 & dq = metals +++ salts ++ ions + electropositives & hl = es & sa = X & ved = 0ahUKEwj_p9jY5YfZAhWL7RQKHUjiCH0Q6AEINjAD% 20% 20 metals & q 20% onepage & v metal% 20sales = v% 20sales% 20% 20 sale 20iones% 20electropositivos & f = false [[Dictionary]] of chemistry ]. Editorial Complutense. 2003. ISBN 9788489784727 . Retrieved February 2, 2018 . Wiklink inside the URL title ( help )
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