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Lead, city, United States

Lead (lēd), city (1990 pop. 3,632), Lawrence co., W S.Dak., in the Black Hills; laid out 1876 after the discovery of gold there, inc. 1890. It is the site of the famous Homestake Mine, which was in operation from 1877 to 2001.

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lead, chemical element

lead, metallic chemical element; symbol Pb [Lat. plumbum]; at. no. 82; at. wt. 207.2; m.p. 327.502°C;; b.p. about 1,740°C;; sp. gr. 11.35 at 20°C;; valence +2 or +4. One of the oldest metals used by humanity, lead was known to the ancient Egyptians and Babylonians. The Romans used it for pipes and in solder. It was one of the first metals mined in North America, where it was sought after especially for making shot.

Properties and Isotopes

Lead is a dense, relatively soft, malleable metal with low tensile strength. It is a poor conductor of electricity and heat. Lead has a face-centered cubic crystalline structure. It is below tin in Group 14 of the periodic table. Although lead has a lustrous silver-blue appearance when freshly cut, it darkens upon exposure to moist air because of the rapid formation of an oxide film; the film protects the metal from further oxidation or corrosion. All lead compounds are poisonous (see lead poisoning). Lead resists reaction with cold concentrated sulfuric acid but reacts slowly with hydrochloric acid and readily with nitric acid.

The element has four naturally occurring stable isotopes, three of which result from the decay of naturally occurring radioactive elements (thorium and uranium). Since this decay takes place at a constant rate, it is possible to predict either the maximum age of a lead-containing rock or its composition at some earlier date, as long as the rock has not been chemically altered. There are 25 known radioactive isotopes of lead, some of which occur naturally in small amounts.

Natural Occurrence and Processing

Although lead is seldom found uncombined in nature, its compounds are widely distributed throughout the world, principally in the ores galena, cerussite, and anglesite. Australia, the United States, Canada, and Russia are among the chief producers of lead. In the United States galena (a lead sulfide ore) is mined in southern Missouri, with some ore coming from the western states. The ore is concentrated by the flotation process and is then refined by electrolysis or by smelting. About one third of the lead used in the United States is so-called secondary lead, i.e., lead and lead alloys reclaimed chiefly from automobile batteries.

Uses

The single most important commercial use of lead is in the manufacture of lead-acid storage batteries (see battery, electric). It is also used in alloys such as fusible metals, antifriction metals, solder, and type metal. Shot lead is an alloy of lead, antimony, and arsenic. Lead foil is made with lead alloys. Lead is used for covering cables and as a lining for laboratory sinks, tanks, and the "chambers" in the lead-chamber process for the manufacture of sulfuric acid. It is used extensively in plumbing. Because it has excellent vibration-dampening characteristics, lead is often used to support heavy machinery and was used in the foundations of the Pan Am Building built over Grand Central Station in New York City. Lead is also employed as protective shielding against X rays and radiation from nuclear reactors.

Lead has many commonly used compounds. Commercially important are the lead oxides, which have many uses. Litharge is lead monoxide, PbO; red lead is lead tetroxide, Pb₃O₄; lead peroxide or dioxide, PbO₂, is used in matches, as a mordant in dyeing, and as an oxidizing agent. White lead, 2PbCO₃·Pb(OH)₂ (basic lead carbonate), is an important pigment used in paints, putty, and ceramics. Chrome yellow, PbCrO₄, is a bright yellow pigment. "Sublimed white lead," PbSO₄·Pb(OH)₂ (basic lead sulfate), is also used as a pigment. Lead acetate (sugar of lead) is used as a mordant, and lead azide, Pb(N₃)₂, is employed as a detonator for explosives. Lead arsenate is used as an insecticide. Tetraethyl lead, used as a antiknock compound in gasoline, is now banned for environmental reasons in the United States and other countries.

Although lead and most of its compounds are only slightly soluble in water, the use of lead pipe to carry drinking water is dangerous, since lead is a cumulative poison that is not excreted from the body (see lead poisoning). The "lead" of lead pencils does not contain lead; it is a mixture of graphite and clay.

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lead

Metallic chemical element, chemical symbol Pb, atomic number 82. Lead is a soft, silvery white or grayish, malleable, ductile, dense metal that conducts electricity poorly. Its stable isotopes are all end products of radioactive decay of uranium and other heavy elements. Known since ancient times, lead is so durable and resistant to corrosion that Roman lead pipes are still usable. Lead is used in roofing, as cable coverings, and in pipes, conduits, and structures. Other uses are in storage batteries, ammunition, and low-melting-point alloys (e.g., solder, pewter) and as shielding against sound, vibrations, and radiation. Lead is rarely found free in nature; its major ore is the sulfide galena (PbS). Because it and its compounds are poisons (see lead poisoning), lead-based paints and gasoline additives have been phased out in many countries. Lead in compounds has valence 2 and 4; an oxide (litharge, PbO) is the most widely used. Lead compounds are added to lead crystal (see glass), glazes, and ceramics and are used as pigments, drying agents for paints and varnishes, insecticides and herbicides, and fireproofing agents and in matches, explosives, and pyrotechnics. Almost half of all lead is recovered from recycled scrap. The “lead” in pencils is graphite.

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lead

A metal pin that extends out from a chip which plugs into a socket or is soldered onto a circuit board. See pin, socket mount, surface mount and lead frame.

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lead¹

1. the act or prerogative of playing the first card in a round of cards or the card so played

2. the principal role in a play, film, etc., or the person playing such a role

3. Music an important entry assigned to one part usually at the beginning of a movement or section

4. a wire, cable, or other conductor for making an electrical connection

5. Boxing

a. one's habitual attacking punch

b. a blow made with this

6. Nautical the direction in which a rope runs

7. a deposit of metal or ore; lode

8. the firing of a gun, missile, etc., ahead of a moving target to correct for the time of flight of the projectile

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lead²

1. a heavy toxic bluish-white metallic element that is highly malleable: occurs principally as galena and used in alloys, accumulators, cable sheaths, paints, and as a radiation shield. Symbol: Pb; atomic no.: 82; atomic wt.: 207.2; valency: 2 or 4; relative density: 11.35; melting pt.: 327.502?C; boiling pt.: 1750?C

2. a lead weight suspended on a line used to take soundings of the depth of water

3. 

a. graphite or a mixture containing graphite, clay, etc., used for drawing

b. a thin stick of this material, esp the core of a pencil