Lead (Pb) is a soft silver-white or grayish metal of the 14th (IVa) group of the periodic table with atomic number 82. It is a very pliable, ductile and dense substance that does not conduct electricity well. The electronic formula for lead is [Xe] 4f 14 5d 10 6s 2 6p 2 . Known in antiquity and considered the oldest of all metals by alchemists, it is very durable and resistant to corrosion, as evidenced by the continued operation of water pipes installed by the ancient Romans. The symbol Pb in the chemical formula of lead is an abbreviation of the Latin word plumbum.
Prevalence in nature
Lead is often mentioned in early biblical texts. The Babylonians used metal to make writing plates. The Romans made water pipes, coins, and even kitchen utensils from it. The result of the latter was lead poisoning in the era of Emperor Augustus Caesar. The compound, known as white lead, was used as a decorative pigment back in 200 BC. e.
In terms of weight, the lead content in the earth's crust corresponds to tin. In space, 10 6 silicon atoms account for 0.47 lead atoms. This is comparable to the content of cesium, praseodymium, hafnium and tungsten, each of which is considered as a fairly deficient element.
Production
Although there is not much lead, natural concentration processes have led to significant deposits of commercial importance, especially in the United States, Canada, Australia, Spain, Germany, Africa and South America. Pure rare lead is present in several minerals, but all of them are of secondary importance, with the exception of PbS sulfide (galena), which is the main source of industrial production of this chemical element worldwide. The metal is also found in anglesite (PbSO 4 ) and cerussite (PbCO 3 ). To the beginning of the XXI century. The world's leading manufacturers of lead concentrate were countries such as China, Australia, the United States, Peru, Mexico and India.
Lead can be recovered by roasting the ore, followed by smelting in a blast furnace, or by direct smelting. Impurities are removed during additional cleaning. Almost half of all refined lead is recovered from recycled scrap.
Chemical properties
Elemental lead can be oxidized to Pb 2+ by hydrogen ions, but the insolubility of most of its salts makes this chemical element resistant to many acids. Oxidation in an alkaline environment is easier and favors the formation of soluble compounds with a lead oxidation state of +2. PbO 2 oxide with the Pb 4+ ion is one of the strongest oxidizing agents in an acidic solution, but it is relatively weak in an alkaline solution. The oxidation of lead is facilitated by the formation of complexes. Electrodeposition is best carried out from aqueous solutions containing lead hexafluorosilicate and hexafluorosilicate acid.
In air, the metal oxidizes rapidly, forming a dull gray coating, previously considered Pb 2 O suboxide. It is now generally accepted that this is a mixture of Pb and PbO oxide, which protects the metal from further corrosion. Although lead dissolves in dilute nitric acid, it is only superficially exposed to hydrochloric or sulfuric acids, because the resulting insoluble chlorides (PbCl 2 ) or sulfates (PbSO 4 ) prevent the reaction from continuing. The chemical properties of lead, which determine its overall resistance, make it possible to use metal for the manufacture of roofing materials, the sheath of electric cables placed in the ground or under water, and as a gasket for water pipes and structures used for transportation and processing of corrosive substances.
Lead application
Only one crystalline modification of this chemical element with a densely packed metal lattice is known. In a free state, a zero oxidation state of lead (like any other substance) is manifested. The widespread use of the elementary form of the element is due to its plasticity, ease of welding, low melting point, high density and the ability to absorb gamma and x-ray radiation. Molten lead is an excellent solvent and allows the concentration of free silver and gold. The structural use of lead is limited by its low tensile strength, fatigue and fluidity even at low load.
The element is used in the manufacture of batteries, in ammunition (shots and bullets), as part of solder, printing, bearing, light alloys and tin alloys. In heavy and industrial equipment, parts made of lead compounds can be used to reduce noise and vibration. Since metal effectively absorbs short-wave electromagnetic radiation, it is used for the protective shielding of nuclear reactors, particle accelerators, X-ray equipment and containers for transporting and storing radioactive materials. As part of the oxide (PbO 2 ) and the alloy with antimony or calcium, the element is used in conventional batteries.
Action on the body
The chemical element lead and its compounds are toxic and accumulate in the body over a long period of time (this phenomenon is known as cumulative poisoning) until a lethal dose is reached. Toxicity increases as the solubility of compounds increases. In children, lead accumulation can lead to cognitive impairment. In adults, it causes progressive kidney disease. Symptoms of poisoning include abdominal pain and diarrhea, followed by constipation, nausea, vomiting, dizziness, headache, and general weakness. Removing contact with a lead source is usually sufficient for treatment. Elimination of the chemical element from insecticides and pigment paints, as well as the use of respirators and other protective devices at the exposure sites, significantly reduced the number of cases of lead poisoning. The recognition that tetraethyl lead Pb (C 2 H 5 ) 4 as an anti-knock additive to gasoline pollutes air and water, led to its cessation in the 1980s.
Biological role
Lead does not play any biological role in the body. The toxicity of this chemical element is due to its ability to mimic metals such as calcium, iron and zinc. The interaction of lead with the same protein molecules as these metals leads to the cessation of their normal functioning.
Nuclear Properties
The chemical element lead is formed both as a result of neutron absorption processes and during the decay of radionuclides of heavier elements. There are 4 stable isotopes. The relative prevalence of 204 Pb is 1.48%, 206 Pb is 23.6%, 207 Pb is 22.6% and 208 Pb is 52.3%. Stable nuclides are the final products of the natural radioactive decay of uranium (up to 206 Pb), thorium (up to 208 Pb) and actinium (up to 207 Pb). More than 30 radioactive isotopes of lead are known. Of these, 212 Pb (thorium series), 214 Pb and 210 Pb (uranium series) and 211 Pb (actinium series) participate in the processes of natural decay. The atomic weight of natural lead varies from source to source, depending on its origin.
Monoxides
In compounds, the oxidation state of lead is generally +2 and +4. Among the most important of these are oxides. These are PbO, in which the chemical element is in the +2 state, PbO 2 dioxide, in which the highest oxidation state of lead (+4) is manifested, and tetraoxide, Pb 3 O 4 .
Monoxide exists in two versions - lithargue and gleta. Litarg (lead alpha oxide) is a red or reddish-yellow solid with a tetragonal crystalline structure, a stable form of which exists at temperatures below 488 ° C. Glet (lead beta-monoxide) is a yellow solid and has an orthorhombic crystalline structure. Its stable form exists at temperatures exceeding 488 ° C.
Both forms are insoluble in water, but dissolve in acids to form salts containing the Pb 2+ ion or in alkalis to form plumbites that have PbO 2 2 -ion. Litarg, which is formed by the reaction of lead with atmospheric oxygen, is the most important commercial compound of this chemical element. The substance is used in large quantities directly and as a starting material for the production of other lead compounds.
A significant amount of PbO is consumed in the manufacture of plates of lead-acid batteries. High-quality glassware (crystal) contains up to 30% of litarga. This increases the refractive index of the glass and makes it shiny, durable and sonorous. Literg also serves as a desiccant in varnishes and is used in the production of sodium lead acid, which is used to remove unpleasantly smelling thiols from gasoline (organic compounds containing sulfur).
Dioxide
In nature, PbO 2 exists as a brown-black mineral of platnerite, which is commercially produced from trialad tetraoxide by chlorine oxidation. It decomposes on heating and produces oxygen and oxides with a lower oxidation state of lead. PbO 2 is used as an oxidizing agent in the manufacture of dyes, chemicals, pyrotechnics and alcohols and as a hardener for polysulfide rubbers.
Lead tris lead tetraoxide Pb 3 O 4 (known as lead or minium) is obtained by further oxidation of PbO. It is a pigment from orange-red to brick-red, which is part of the corrosion-resistant paints used to protect iron and steel exposed to the environment. It also reacts with iron oxide to form ferrite, used in the manufacture of permanent magnets.
Acetate
Also an economically significant lead compound of oxidation state +2 is Pb acetate (C 2 H 3 O 2 ) 2 . This is a water-soluble salt, obtained by dissolving glute in concentrated acetic acid. The general form, trihydrate, Pb (C 2 H 3 O 2 ) 2 · 3H 2 O, called lead sugar, is used as a fixative in dyeing fabrics and as a drier in some paints. In addition, it is used in the production of other lead compounds and in gold cyanidation plants, where it, in the form of PbS, serves to precipitate soluble sulfides from a solution.
Other salts
Basic lead carbonate, sulfate and silicate were once widely used as pigments for white paints for external use. However, since the middle of the twentieth century. use of so-called white lead pigments decreased significantly due to concerns about their toxicity and associated dangers to human health. For the same reason, the use of lead arsenate in insecticides has virtually ceased.
In addition to the main oxidation states (+4 and +2), lead can have negative degrees of -4, -2, -1 in Zintl phases (for example, BaPb, Na 8 Ba 8 Pb 6 ), and +1 and +3 in organo lead compounds such as hexamethyldiplumban Pb 2 (CH 3 ) 6 .