Lead (original) (raw)

Lead is a chemical element in the periodic table that has the symbol Pb and atomic number 82. A soft, heavy, toxic and malleable true metal, lead has a dull gray appearance and is bluish white when freshly cut but tarnishes to dull gray when exposed to air. Lead is used in building construction, lead-acid accumulators, bullets and shot, and is part of solder, pewter, and fusible alloys. Lead is the heaviest stable element.

Notable Characteristics

Lead has a bright luster and is a ductile, very soft, highly malleable, bluish-white metal that has poor electrical conductivity. This true metal is highly resistant to corrosion. Becuse of this property, it is used to contain corrosive liquids (e.g. sulfuric acid). Lead can be toughened by adding a small amount of antimony or other metals to it.

Applications

Early uses of lead included building materials, pigments for glazing

ceramics, and pipes for transporting water. The castles and cathedrals of Europe contain considerable quantities of lead in decorative fixtures, roofs, pipes, and windows. Lead is the fifth most widely used metal (in its elemental state) after iron, aluminium, copper and zinc. Common uses include:

• in lead-acid batteries, in electronic components, cable sheathing, ammunition, in the glass of CRTs, ceramics, leaded glass (see glass making), lead piping (not much used these days, although leaded solder was legal for use on drinking water pipes into the 1980s (US)), in paint (banned since 1978 in the US, and gradually in the UK from the 1950s to the 1970s, although older painted surfaces could be up to 50% lead by weight), casting alloys, pewter, solder, and dentistry filling. Also used as flashing on roofing to protect joins from rain. In gasoline (petrol) as tetra-ethyl lead and tetra-methyl lead to reduce knocking (also called pre-detonation, pre-ignition or pinking) from the 1920s, in 1986 leaded gasoline was banned from sale in the United States, in the EU in 1999.

Lead is a superconductor with critical temperature _T_c=7.20 K (-265.95 °C).

History

Lead has been used by humans for at least 7000 years, because it was (and continues to be) widespread and easy to extract, as well as easy to work with, being both highly malleable and ductile as well as easy to smelt. Lead was mentioned in the

Book of Exodus. Alchemists thought that lead was the oldest metal and associated it with the planet Saturn. Lead pipes that bear the insignia of Roman emperors are still in service. Lead's symbol Pb is an abbreviation of its Latin name plumbum.

By the mid-1980s, a significant shift in lead end-use patterns had taken place. Much of this shift was a result of the U.S. lead consumers' compliance with environmental regulations that significantly reduced or eliminated the use of lead in nonbattery products, including gasoline, paints, solders, and water systems.

Extraction

Native lead does occur in nature, but it is rare. Currently lead is usually found in ore with zinc, silver and (most abundantly) copper, and is extracted together with these metals. The main lead mineral is galena (PbS), which contains 86.6% lead. Other common varieties are cerussite (PbCO3) and anglesite (PbSO4). But more than half of the lead used currently comes from recycling.

In mining, the ore is extracted by drilling or blasting and then crushed and ground. The ore is then taken through a process developed in Australia in the 19th century at Broken Hill. A flotation process separates the lead and other minerals from the waste rock (tailings) to form a concentrate by passing the ore, water and certain chemicals through a series of tanks in which the slurry is constantly mixed. Air is blown through the tanks and lead sulphides attach to the bubbles and rise to form a foam which can be removed. The foam (which is around 50% lead) is dried and then sintered before being smelted to produce a 97% lead concentrate. The lead is then cooled in stages which causes the lighter impurites (dross) to rise to the surface where they can be removed. The molten lead bullion is then refined by additional smelting with air being passed over the lead to form a slag layer containing any remaining impurities and producing 99.9% pure lead.

Isotopes

Lead has four stable, naturally occurring isotopes: Pb-204 (1.4%), Pb-206 (24.1%), Pb-207 (22.1%) and Pb-208 (52.4%). Pb-206, Pb-207 and Pb-208 are all radiogenic, and are the end products of complex decay chains that begin at U-238, U-235 and Th-232 respectively. The corresponding half-lives of these decay schemes vary markedly: 4.47 × 109, 7.04 × 108 and 1.4 × 1010 years, respectively. Each is reported relative to 204Pb, the only non-radiogenic stable isotope. The ranges of isotopic ratios for most natural materials are 14.0-30.0 for Pb-206/Pb-204, 15.0-17.0 for Pb-207/Pb-204 and 35.0-50.0 for Pb-208/Pb-204, although numerous examples outside these ranges are reported in the literature.

Precautions

Lead is a

poisonous metal that can damage nervous connections (especially in young children) and cause blood and brain disorders. Long term exposure to lead or its salts (especially soluble salts or the strong oxidant PbO2) can cause nephropathy, and colic-like abdominal pains. Its historical use by the Roman Empire for water piping (and its salt, lead acetate, also known as sugar of lead, as a sweetener for wine) is considered by some to be the cause for the dementia that affected many of the emperors. The concern about lead's role in mental retardation in children has brought about widespread reduction in its use. Paint containing lead has been withdrawn from sale in industralised countries, though many older houses may still contain substantial lead in their old paint: it is generally recommended that old paint should not be stripped by sanding, as this generates inhalable dust. Lead salts used in pottery glazes have on occasion caused poisoning, when acid drinks, such as fruit juices, have leached lead ions out of the glaze. It has been suggested that what was known as "Devon colic" arose from the use of lead-lined presses to extract apple juice in the manufacture of cider. Lead is considered to have particularly nasty consequences for mothers in spe, i.e. girls and young women. For that reason many universities do not hand out lead in girls' analysises.

The earliest pencils actually used lead, though pencil `leads' have been made for the last couple of centuries from graphite, a naturally occurring form (allotrope) of carbon.

Language Derivations

The Latin plumbum has given birth to a number of terms in the English language:

• Plumbing, or system of piping, derives from the fact that pipes were once made of lead.
• Plumb bob or plummet, a small, pointed body of metal the weight of which is used to draw a string vertical under tension, refers to the fact that they were originally made from lead.
• Plumb wall is so-said because a plumb bob is used to find the vertical.
• Plumbing the depths derives from the use of the lead weight to draw the sounding line down to the bottom of the water body (or to the end of the line if the water's really deep!).
• Plumb crazy may derive from the fact that lead poisoning can cause insanity; or, according to the OED, from a U.S. sense of plum (derived from plumb) meaning 'completely'.
• Plumbism is the medical term for lead poisoning.
• Aplomb comes from the French � plomb, meaning plumb vertical, and therefore confident and cool.

The origin of the name of the fruit called a plum is not related.

see also: Lead poisoning

External Links

• WebElements.com - Lead
• EnvironmentalChemistry.com - Lead
• Global Phaseout of Leaded Gasoline