Lead
From Encyclopædia
Lead is a lustrous, silvery
metal that tarnishes in the presence of air and becomes a dull bluish gray. Soft and malleable, it has a low melting point (327 deg C). Its chemical symbol, Pb, is derived from plumbum, the Latin word for waterworks, because of lead's extensive use in ancient water pipes. Its
atomic number is 82; its
atomic weight is 207.19.OccurrenceThe Earth's crust is about 15 ppm lead. Four stable lead isotopes exist in nature; three of them are end products of the radioactive decay of
uranium and
thorium. It is assumed that all of the fourth isotope, that exists in rocks has been there since the rocks were formed. The most valuable lead ore is galena, which is almost always mixed with other valuable ores. The production of pure lead from ores is a complex process in which the raw material undergoes several purifications. Much of the
world production of bismuth, arsenic, antimony, and silver comes from the production of lead. Lead is also recycled from scrap
metal.ToxicityLead and lead compounds can be highly toxic when eaten or inhaled. Although lead is absorbed very slowly into the body, its rate of excretion is even slower. Thus, with
constant exposure, lead accumulates gradually in the body. It is absorbed by the red blood
cells and circulated through the body where it becomes concentrated in the soft tissues, especially the
liver and
kidneys. Lead can cause lesions in the
Central nervous system and apparently can damage the
cells making up the blood-brain barrier that protects the brain from many harmful chemicals (see BRAIN).Symptoms of lead poisoning include loss of appetite, weakness, anemia, vomiting, and convulsions, sometimes leading to permanent brain damage or death. Children who ingest chips of old, lead-containing paint or are exposed to dust from the deterioration of such paint may exhibit symptoms. The newborn of mothers who had been exposed to lead levels even considered
safe for children appear to show signs of slowed mental
growth. Levels of environmental lead considered nontoxic may also be involved in increased
hypertension in a significant
number of persons, according to studies released in the mid-1980s. As a result, the U.S. Centers for Disease
Control in recent years have been revising downward the levels of environmental lead that it would consider
safe. At one time, lead poisoning was common among those who worked with lead, but such workplace hazards have been largely curtailed (see POLLUTANTS, CHEMICAL).UsesLead has been used by humans since ancient times. It was used in ancient
Egypt in coins, weights, ornaments, utensils, ceramic
glazes, and solder. Lead is mentioned in the Old Testament. The Romans conveyed drinking water in lead pipes, some of which are still in operation. Roman slaves extracted and prepared the lead, and Pliny describes a disease among the slaves that was clearly lead poisoning. Because of their potential toxicity, lead water pipes are no longer being installed. The greatest single use of lead
metal today is in the plates of storage batteries for automobiles.The protective oxidation layer formed by lead in contact with such substances as air, sulfuric acid, and
fluorine makes it highly resistant to corrosion. For this reason, lead has been used to make drainage pipes and lead chambers in sulfuric acid factories. It is also used as a roofing material. The softness and malleability of lead make it useful for sheathing telephone and
television cables. Lead is used in solder because of its low melting point. When combined with tin, lead forms solder alloys that are stronger than lead alone, with melting points lower than those of either original
metal.Lead has the highest
density of all
metals in common use, which, for example, makes it useful as a counterweight in the keels of
ships. Because of their high
density, lead bullets and shot encounter little air resistance and thus achieve excellent striking power. Shot is produced by allowing molten lead to drip
Down from heights up to 38.10 m (125 ft). The drops become spherical and are condensed by the cooling action of the air before being collected in a tank filled with water or oil. Lead's
density and softness also make it highly suitable for damping sound and vibrations. To isolate them from vibration, heavy machinery and even whole buildings are placed on lead blocks. Because the effectiveness of shielding against gamma and
X rays depends largely on the
density of the shield, lead is used in the protective shielding of X-ray machines and
nuclear reactors.Lead can be alloyed with many
metals. Lead alloys are important because they resist attack by sulfuric acid and are harder than pure lead.Lead CompoundsLead exists in the divalent or tetravalent oxidation state in compounds. A
number of lead compounds are important in the paint industry. At one time, lead pigments were often used in ceramic
glazes and interior paints; but because lead pigments are toxic, their use is now restricted. White lead is the most important lead pigment. Lead dioxide is a vigorous oxidizing
agent that is sometimes used in match heads.Tetraethyl lead or tetramethyl lead has often been added to gasoline to improve
engine efficiency and reduce gasoline consumption in automobiles. Because of the toxic effect of lead on the environment, however, plans call for phasing out this use (see
GASOLINE). Lead azide is sensitive to striking and is highly explosive; it is frequently used as a detonator of
explosives. Lead iodide is a
light yellow substance that is used as a dye in such processes as coloring bronze. It has
light-sensitive properties comparable to those of silver salts.Stephen FleishmanBibliography: Chisholm, J. J., and O'Hara, D. M., Lead Absorption in Children (1982); Harrison, R. M., and Laxen, D. P., Lead Pollution (1981); Hehner, N. E., and Ritchie, E. J., Lead Oxides (1974); Nriagu, J. O., Lead and Lead Poisoning in Antiquity (1983); Rochow, E. G., and Abel, E. W., The Chemistry of Germanium, Tin and Lead (1975); Smith, M., et al., eds., Lead Exposure and
child development (1989).
