metal is a material conductor of heat and electricity, capable of reflecting light (thus giving rise to the so-called metallic luster), which can be attacked by acids (with the development of hydrogen) and bases, often with good mechanical strength characteristics; metals (especially those of the first and second group) can also be attacked by water, which rips their valence electrons giving hydrogen through an exothermic reaction; moreover metals are fusible if subjected to heat.

Metals are chemical elements, constituting one of the three categories in which these elements are subdivided, together with that of semimetals and that of non metals; with the expression metallic material we refer to a material that contains metals or alloys; according to the chemical properties, metals can give rise to basic oxides (for example: Na2O, CaO), or to anhydrides, that is oxides with an acid character (for example: V2O5, Mn2O7); there are various types of metals, discovered in ages distant in time, because very few metals are found in nature in their native state and because each metal has its own particular melting temperature that makes its extraction from the rocks containing it more or less easy; the first historically worked metals (copper and tin) naturally have a relatively low melting temperature, already obtainable with the ancient furnaces of about 10. 000 years ago (the time when, presumably, copper processing began).

Chemical elements with metallic behavior, that we will continue to indicate as metals, are solid at ordinary temperature except mercury, which is liquid and solidifies at -38,87 ºC, have low ionization energy and therefore easily give positive ions, low electronic affinity, emit electrons when hit by high frequency photons (photoelectric effect) or when heated (thermionic effect); most of them have negative electrochemical potential compared to that of hydrogen and consequently on Earth they exist only as compounds such as oxides, sulfates, sulfides, carbonates, etc.

Only metallic elements with positive electrochemical potential compared to hydrogen can exist free (noble metals: gold, platinum, etc..); in several elements many of these properties are attenuated or completely non-existent and so we pass, following the lines of the periodic table of chemical elements, gradually from metals to non-metals. The processes of extraction of metals from the minerals that contain them together with the gangue, their refining and processing constitute a well-defined science: metallurgy.

In the extraction processes the reduction of the element is carried out either by means of reducing substances such as coal (for example in the case of iron), carbon monoxide, hydrogen or electrolytically (for example in the case of aluminum, zinc, etc.). Concerning crystalline structure of metals, the most common lattices are face centered or body centered cubic (aluminum, cobalt, chromium, iron, manganese, molybdenum, nickel, lead, copper, tungsten, vanadium, etc.) and hexagonal (beryllium, magnesium, titanium, etc.) with individual atoms located at the vertices of geometric structures corresponding to crystallographic systems.

The crystals of metals are considered perfect only when all the sites provided by the three-dimensional geometry of the system are occupied by atoms, while when this arrangement is not realized we speak of crystalline defects. In this way we have the so called vacations that in the case of metallic elements explain many of the chemical-physical properties; an important role is played by the presence of foreign elements (impurities), dislocations, which are defects not in thermodynamic equilibrium, and interstitial atoms that can be considered as defects in thermodynamic equilibrium. Depending on temperature and pressure, many metals can crystallize in different crystallographic systems or classes, usually distinguished with letters of the Greek alphabet (form α, β), or with different adjectives, for example white tin, gray tin. Carbon, for example, in the hexagonal form constitutes the graphite, which has some of the properties of metals (good electrical conduction), while in the cubic form constitutes the diamond.

At the end of the eighties of the twentieth century has been highlighted a third allotropic form of C consisting of molecules with 60 carbon atoms arranged on the surface of a sphere and called fullerene. The chemical-physical properties of different allotropic forms of the same element are different and they are reflected in the structure of atoms and energy levels that electrons can occupy when single atoms are gathered in a crystalline building.

If, for example, we consider an element with metallic properties, such as lithium that has two electrons (1s and 2s) placed on two different energy levels, simple quantum mechanics considerations indicate that when two atoms are approaching to distances comparable to atomic size, each level is split, so there are four energy levels that can be occupied by electrons. In case of four approaching atoms, each level will quadruple for a total of eight levels, for N=200 there will be 400 new levels, etc… It follows that for a very high number the levels become very numerous and consequently the distance between the levels of different atomic orbitals become smaller and smaller until they are grouped in two bands that can be considered continuous for what concerns energy.

In general for all elements, if we consider the energy of electrons, we can always distinguish two bands: one at lower energy levels called valence band and one at higher levels which is the conduction band; it should be added that not all molecular orbitals are always full and therefore electrons can move from one orbital to another of the same band. The distance between valence band and conduction band is determinant for the mobility of electrons and therefore for the electrical and thermal conductivity of the metal. In the case where the orbitals of the conduction band are empty, if the distance between the two bands is small, or if the two bands overlap, some electrons can jump from the first to the second, while if the distance is large this jump is impossible (insulating dielectrics); in the intermediate case we have semiconductor elements.

Under the name transition metals are indicated the chemical elements characterized in their structure by internal orbitals of type d (transition elements of type d) or type f (transition elements of type f) not completed by electrons. Transition elements of d type have many similar properties and constitute three groups depending on the type of non-complete d orbitals (3d, 4d or 5d), while those of f type are called lanthanides and actinides (see element) respectively depending on the non-complete orbitals (4f or 5f). In nature the transition elements are found in elemental form or as compounds. Many of them are present in traces in living organisms while others are essential components of some enzymes; they are produced in large quantities both as pure metals and as alloys and have many important applications of which some are of strategic importance.

Transition elements have in common some properties: they all have a distinctly metallic character, they crystallize either in the hexagonal system or in the body or face centered cubic system: they have high hardness, ductility and malleability with high melting and boiling point; they are good conductors of electricity and heat. Some are easily soluble in dilute acids while others have very low oxidation-reduction potentials and consequently are not attacked by acids. Almost all of them exhibit different oxidation states and at least one of these states the compounds are colored; having almost all of their internal orbitals only partially filled, many of them form paramagnetic compounds. They also behave as Lewis acids with formation of complex ions.

Complex chemistry plays an important role for lighter transition elements and less so for heavier ones. Measurements of the magnetic susceptibility in the case of lighter transition elements allow the determination of the number of electron splits and thus the valence and configuration of the orbitals d. Transition elements form numerous compounds with two or more metal atoms bonded together: the bonded atoms can be the same or different. When more than two atoms interact, open or closed groupings called clusters can be formed.

In addition, the complexes of transition metals are particularly important in the study of optical activity of racemic compounds that can be studied spectroscopically in the visible due to the low speed of racemization of the same complexes. The structure of transition metal complexes can be rationalized by purely electrostatic models (crystal field theory) or by models with covalent bonds (molecular orbital theory). The results obtained by applying the two models are complementary and not contradictory, and are combined in the bonding field theory.

The presence of some metallic elements (in particular mercury, lead and cadmium) and their compounds in the air, water and soil can cause toxicity phenomena, because, through chemical, physical and biological transformation processes, they can give rise to harmful organic or inorganic complexes. Their toxicity comes from the fact that they are not biodegradable, and that they can be accumulated in certain organisms. Once they enter the food chain, they inevitably reach higher organisms, and eventually man. According to Italian regulations, the total amount of metals in wastewater must not exceed 3 mg/liter.

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