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Lead, atomic number 82 in the periodic table, is a metallic element with the chemical symbol Pb, which stands for plumbum, the Latin name for the element. It is a soft, pliable metal that is silvery-white in color when freshly cut, but on exposure to air quickly acquires a dull gray appearance due to the formation of a layer of oxide. Although occasionally found in its elemental state, the main lead ore is galena, or lead sulphide (PbS); other lead ores include cerussite — lead carbonate (PbCO3) — and anglesite — lead sulfate (PbSO4). Historically, the chemical and physical properties of lead have made it a very useful element, but since the late 20th century, its use has diminished due to its toxicity. Lead, however, still has a number of important applications — for example in lead-acid batteries, for radiation shielding, and as a flexible, resilient roofing material.
The metal melts at 622.4 °F (328 °C) and boils at 3,164 °F (1,740 °C). The four stable isotopes of lead are the end products of the decay of various naturally occurring radioactive elements, such as uranium and thorium, through a number of steps. Lead is the heaviest stable element, a distinction that used to belong to bismuth — element number 83 — until it was found to be very slightly radioactive. One of the most important physical properties of lead is its ability to absorb high frequency electromagnetic radiation, such as x-rays and gamma rays. This is due to its high density and the large number of electrons in the lead atom.
Lead belongs to same group as carbon, silicon, germanium and tin. These elements become more metallic in character with increasing atomic weight, and while the chemical properties of lead bear some resemblance to those of the other members of the group, it is chemically most similar to the metal, tin. In its compounds, lead usually has an oxidation state of +2, which means that it donates two electrons to other atoms or molecules. Less commonly, it can have an oxidation state of +4.
The metal combines with oxygen to form several oxides. “Red lead,” formed by heating lead in air, has the formula Pb3O4, but is thought to be a compound of lead oxide (PbO) and lead dioxide (PbO2). Lead oxide, also known as litharge, is formed when the metal is heated strongly in air and can take the form of a yellow powder or a red crystalline material.
“White lead” is basic lead carbonate (2PbCO3·Pb(OH)2). It was formerly widely used in paints due to its strong white color before being largely replaced by non-toxic titanium dioxide. Aside from its toxicity, a problem with white lead was that it tended to slowly react with traces of hydrogen sulfide (H2S) in the air to form black lead sulfide. This is a good test for H2S, but it meant that old paintings would tend to darken over time.
Lead is resistant to corrosion by most acids, due to the fact that the majority of lead salts have little or no solubility in water and form a layer that protects the lead from further action. It will, however, react with acetic and nitric acids, as the salts formed by these reactions — lead acetate and lead nitrate, respectively — are very soluble. Lead reacts with “hard” water to form insoluble basic lead carbonate, but forms soluble compounds with soft water, which means that lead water pipes pose more of a risk of lead poisoning in soft water areas.
Probably the best known of the properties of lead is its toxicity. Cases of acute lead poisoning are rare, but it is a cumulative poison, and chronic exposure to low levels of lead can lead to a variety of serious symptoms. It deactivates the enzymes that manufacture hemoglobin, leading to a build-up of the precursor chemical — this can paralyze the gut, resulting in constipation and abdominal pain, and cause a build-up of fluid in the brain, causing headaches. Over a longer period, it causes anemia and neurological problems.
Chronic lead poisoning has been a significant problem due to the widespread use of lead in applications that have allowed it to enter the environment. For example, metallic lead was formerly used in water pipes and lead compounds have been used in paints. These uses have been discontinued in most countries, and lead piping replaced by non-toxic alternatives. The biggest source of lead in the environment has been the compound tetraethyl lead, which was added to gasoline to achieve smoother combustion. Due to concerns about the health effects of lead in the environment, particularly on children in urban areas, leaded gasoline has also been phased out in many countries.
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