Metals

Metals, simple chemical substances (elements) possessing a complex of characteristic, more or less clearly expressed physical features that create favorable conditions for their practical application, such as: hardness, malleability, specific luster, high specific gravity, fusibility allowing for casting, high thermal conductivity and electrical conductivity, in some cases resistance to external chemical influences and strength under physical effects. Chemistry defines M. as elements whose oxides, when combined with water, yield hydroxides of a basic nature, capable of reacting with acids. In the salts formed in this process, M. enter as positively charged ions—cations. In this sense, M. are contrasted with metalloids, whose oxides form acids with water, which enter as anions. The two definitions given above do not coincide for different M., and therefore the class of M. combines very diverse and dissimilar representatives. The extreme members of the series of M., on one hand, are those most stable in a metallurgical sense—gold, platinum, silver—so-called noble M., unchangeable by most chemical agents (therefore they are found in nature predominantly in a native state), and, on the other hand—M. that enter into chemical compounds with extreme energy and therefore easily change in air; their preservation in the form of free M. requires special precautions, and therefore in nature they are found only in the form of compounds. Such are, for example, sodium, potassium, calcium. Between these extreme representatives lies a whole series of M. transitional in their properties, and if for comparison we take the relative affinity for oxygen, then all M. can be arranged in a sequential series (the so-called electromotive series), in which each member occupies, by its characteristics, an intermediate position between two adjacent ones. The most chemically active metal is cesium, the most "noble"—gold. Electromotive series: Cs, K, Na, Li, Ba, Ca, Mg, Al, Mn, Zn, Fe, Cd, Co, Ni, Sn, Pb, H, Cu, Bi, Sb, Hg, Ag, Pd, Pt, Au. In the periodic system, the most active M. stand at the beginning of both small and large periods (alkali and alkaline earth elements), while noble and transitional in their properties M. are located in the middle of large periods. By specific gravity, M. are divided into light and heavy. Alkali M. These include lithium, sodium, potassium, rubidium, and cesium. All of them are the most active monovalent cation-formers; they oxidize extremely easily and generally pass into various compounds; their salts even with weak acids are largely completely dissociated and almost all are soluble in water. Therefore, in nature they are more or less easily leached from rocks and are found predominantly in dissolved form in river and sea water. This determines their role in the chemistry of the earth's crust and biosphere as mobile cations and as carriers of anions participating in metabolism. There are much more sodium salts in nature than potassium. At present, alkali M. are prepared by electrolysis of concentrated solutions of chlorides at a mercury cathode, with which they form amalgams. Lithium belongs to the few elements capable of combining directly with gaseous nitrogen, forming nitride, Li₃N. Reacting energetically with water, alkali M. form hydroxides, releasing hydrogen. With halogens, alkali M. react energetically, forming haloid salts. Hydroxides of alkali M. have the properties of strong bases—caustic alkalis—and are used as such, for example NaOH, the so-called "caustic soda". Salts of alkali M. with weak acids possess buffer properties. In medicine, alkali M. play the role of cations in combination with various anions (for example, sodium salicylate, potassium iodide, potassium permanganate, potassium chlorate). Alkaline earth M. These include beryllium, magnesium, calcium, strontium, barium, and radium. Only beryllium and magnesium may have significance as free M.; magnesium has already acquired technical importance in alloys (for example, magnalite—an alloy of magnesium with aluminum); beryllium has recently also attracted attention in metallurgy as a very light M. All alkaline earth M. are characterized by their ability, when interacting with water, to pass into divalent bases. This ability is most strongly expressed in the heavy representatives of the barium and radium group, least strongly in beryllium. The same gradation is observed for the degree of dissociation of hydroxides, i.e., for their relative strength as bases. The corresponding salts are not all soluble in water, namely—sparingly soluble phosphates (2- and 3-metallic), normal carbonates, silicates, fluorides, and for Ba, Ra, Sr and partly Ca also sulfates. This constitutes the important role of these salts in the salt cycle of mineral nature and the biosphere. Salts of alkaline earth M. are relatively difficult to leach from rocks, mainly in the form of soluble bicarbonates [for example, Ca(HCO₃)₂]. The content of these salts determines the so-called hardness of water. When bicarbonates pass into carbonates or exchange with other salts, insoluble salts of alkaline earth M. are formed and precipitate. Such is the process of formation of powerful strata of sedimentary rocks from sea water, of stalactites in caves, of scale in steam boilers, of the deposition of lime and magnesium salts in the body under normal and pathological conditions (see Lime deposits). On the biological significance of alkaline earth M., see Ions, Calcium, Magnesium, Mineral metabolism. Noble metals. The noble metals include gold, platinum and its analogs, and silver. All of them are difficult to melt, enter into compounds with difficulty (with difficulty form cations), dissolve with difficulty in acids and then only under the condition of simultaneous action of an oxidizing agent, retain their natural luster in air, their oxides have weakly basic properties, and the highest oxide of gold in the form of hydroxide Au(OH)₃ has an amphoteric, i.e., both weakly basic and weakly acidic, character. Characteristic is the ease with which salts of noble M. are reduced to free M.; in this case, with careful action of reducing agents, colloidal solutions are often formed. Colloidal solutions of silver (collargol) are used in medicine. The reduction of haloid silver by light is the basis of photographic processes. Gold and silver, due to their softness, are used for decorative objects exclusively in alloys with so-called ligature, mostly with copper. Platinum is used for technical purposes in its pure form due to its resistance to chemical effects and high melting point (melting point 1,764°). Platinum metals in very fine division (so-called spongy metals) serve as excellent catalysts. Transition M. have significance both as free M. and in the form of compounds. In nature, although they may occur native (for example, copper), they are mostly found in compounds, especially often in the form of oxides, which are usually reduced at high temperatures by the action of carbon, sometimes on a very large scale, as for example iron in blast furnaces. Free M. are used for technical purposes, mainly in the form of alloys, which represent either solid solutions or stoichiometric compounds; the properties of alloys depend extremely strongly on their composition, and this is widely used in metallurgy. Sometimes due to the admixture of a foreign metal, the alloy acquires unusual and very valuable properties: for example, "invar", an alloy of iron with 36% Ni, 0.5% C and 0.5% Mn, does not expand when heated; tungsten steel does not lose its temper even at high temperatures, steels with chromium and nickel content are extremely resistant to chemical agents. Very valuable are alloys of tin with copper (various bronzes) and with zinc (brasses). No less important are alloys of aluminum, as the metal of the future, since the reserves of aluminum in the form of clay in nature are inexhaustible. Since transition M. are capable of oxidation, they must be protected from corrosion; for this they are coated with lacquer or better with a layer of a more noble M. (tinning, nickel plating, galvanizing, etc.). Some M. are capable of being covered with an oxide film that protects the entire thickness from further oxidation; this is the so-called passivity of the metal; it is characteristic, for example, for aluminum. As for the compounds of this group of M., they find application in electrochemistry, analytical and other branches of chemistry; in medicine they have significance mainly in the field of disinfection practice; in this respect, salts of mercury, silver, and bismuth are particularly important. Many M. in their compounds have a specific effect on the human body. On the biological significance of individual representatives of this group of M. and their effect on the body—see Bismuth, Iron, Manganese, Mercury, Lead, etc. Salts of transition M. (as well, however, as salts of noble M.) are capable of forming complex compounds. In higher oxides, some of the M. form acids.

They are part of anions; such are the acids of manganese (H₂MnO₄ and HMnO₄), chromic acid (H₂CrO₄); even iron is capable of forming an acid (H₂FeO₄), similar in composition to sulfuric acid. Metals were probably first known to man in their native state and were worked cold or by forging. The smelting of metals from ores, i.e., the beginnings of metallurgy, should be dated to about 5,000 years B.C. The first metals were alloys of copper with various other metals, the so-called prehistoric bronze, which gave its name to an era that lasted about 1,000 years. Around 4,000 years B.C., the smelting of iron from ores became known. For the pharmacological action of metals, see the respective articles; for their significance in histology, see Impregnation and other methods. In recent years, by the method of ashing sections, it has become possible to approach the question of the histotopography of metals in tissues, and their spectrophotometric determination in them.