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Lead                                                                                                                                   

Standard Sate: solid at 298K 

Color: bluish white  

Lead is a bluish-white lustrous metal. It is very soft, highly malleable, ductile, and a relatively poor conductor of electricity. It is very resistant to corrosion but tarnishes upon exposure to air. 

(Pb), a soft, silvery-white or grayish metal in Group IVa of the
periodic table. Lead is very malleable, ductile, and dense and is a
poor conductor of electricity. Known in antiquity and believed by the
alchemists to be the oldest of metals, lead is highly durable and
resistant to corrosion, as is indicated by the continuing use of lead
water pipes installed by the ancient Romans.

A brief treatment of lead follows. For a discussion of the properties
and applications of lead, see Chemical Elements: Carbon group
elements. For full treatment of lead and lead mining and refining, see
Industries, Extraction and Processing: Low-melting metals.

Occurrence, properties, and uses.


Rarely found free in nature, lead is present in several minerals; but all
are of minor significance except the sulfide, PbS (galena, or lead
glance), which is the major source of lead production throughout the
world. Lead may be extracted by roasting the ore and then smelting it
in a blast furnace or by direct smelting without roasting. Additional
refining removes impurities present in the lead bullion produced by
either process. Almost half of all refined lead is recovered from
recycled scrap.

When freshly cut, lead oxidizes quickly, forming a dull gray coating,
formerly thought to be lead suboxide, Pb2O, but now recognized as a
mixture of lead and lead monoxide, PbO, which protects the metal
from further corrosion. Similarly, although lead is soluble in dilute
nitric acid, it is only superficially attacked by hydrochloric or sulfuric
acids because the insoluble chloride or sulfate coatings that are
formed prevent continued reaction. Because of this general chemical
resistance, considerable amounts of lead are used in roofing, as
coverings for electric cables placed in the ground or underwater, and
as linings for water pipes and conduits and structures for the
transportation and processing of corrosive substances.

Lead has many other applications; the largest is in the manufacture
of storage batteries. It is used in ammunition (shot and bullets) and as
a constituent of various low-melting alloys, such as solder, type metal,
and pewter. In the construction of large buildings, lead sheets are
used in the walls to block the transmission of sound; and pads of lead
and asbestos are used in the foundations to absorb the vibrations
caused by street traffic and other sources. Because lead effectively
absorbs electromagnetic radiation of short wavelengths, it is used as a
protective shielding around nuclear reactors, particle accelerators,
X-ray equipment, and containers used for transporting and storing
radioactive materials.

Lead and its compounds are toxic and are retained by the body,
accumulating over a long period of time--a phenomenon known as
cumulative poisoning--until a lethal quantity is reached. In children the
accumulation of lead may result in cognitive deficits; in adults it may
produce progressive renal disease. See also lead poisoning. (See
poison.)

Lead has four stable isotopes, all of which are the end products of
the radioactive decay of other elements; their relative abundances
are: lead-204, 1.48 percent; lead-206, 23.6 percent; lead-207, 22.6
percent; and lead-208, 52.3 percent. More than 20 radioactive
isotopes have been reported.


Compounds.


Lead shows valences of +2 and +4 in its compounds. Among the
many important lead compounds are the oxides: lead monoxide, or
lead(II) oxide, PbO; lead dioxide, or lead(IV) oxide, PbO2; and trilead
tetroxide, Pb3O4. Lead monoxide exists in two modifications, litharge
and massicot. Litharge, or alpha lead(II) oxide, is a red or reddish
yellow solid, has a tetragonal crystal structure, and is the stable form
at temperatures below 488 C (910 F). Massicot, or beta lead(II)
oxide, is a yellow solid and has an orthorhombic crystal structure; it is
the stable form above 488 C. Both forms are insoluble in water but
dissolve in acids to form salts containing the Pb2+ ion or in alkalies to
form plumbites, which have the PbO22- ion. Litharge is produced by
air oxidation of lead. Except for tetraethyllead [Pb(C2H5)4], an
organic compound that has been used as a gasoline antiknock
additive, litharge is the most important commercial compound of lead;
it is used in large amounts directly and as the starting material for the
preparation of other lead compounds. Considerable quantities of
lead(II) oxide are consumed in manufacturing the plates of lead-acid
storage batteries. High-quality glassware contains as much as 30
percent litharge, which increases the refractive index of the glass and
makes it brilliant, strong, and resonant. Litharge is also employed as a
drier in varnishes and in making sodium plumbite, which is used for
removing malodorous thiols (a family of organic compounds
containing sulfur) from gasoline.

Lead(IV) oxide, found in nature as the brown-to-black mineral
plattnerite, is commercially produced from trilead tetroxide by
oxidation with chlorine. It decomposes upon heating and yields oxygen
and lower oxides of lead. Lead(IV) oxide is used as an oxidizing
agent in the production of dyestuffs, chemicals, pyrotechnics, and
matches and as a curing agent for polysulfide rubbers. Trilead
tetroxide (known as red lead, or minium) is produced by further
oxidation of lead(II) oxide. It is the orange-red to brick-red pigment
commonly used in corrosion-resistant paints for exposed iron and
steel. It also reacts with iron(III) oxide to form a ferrite used in
making permanent magnets.

Another economically significant compound is lead(II) acetate,
Pb(C2H3O2)2, a water-soluble salt made by dissolving litharge in
strong acetic acid. The common form, the trihydrate
[Pb(C2H3O2)23H2O], called sugar of lead, is used as a mordant in
dyeing and as a drier in certain paints. In addition, it is utilized in the
production of other lead compounds and in gold cyanidation plants,
where it primarily serves to precipitate soluble sulfides from solution.

Various other salts, most notably basic lead carbonate, basic lead
sulfate, and basic lead silicate, were once widely employed as
pigments for white exterior paints. Since the mid-20th century,
however, the use of such so-called white lead pigments has
decreased substantially because of a concern over their toxicity and
attendant hazard to human health. The use of lead arsenate in
insecticides has virtually been eliminated for the same reason. atomic
number 82 atomic weight 207.19 melting point 327.5 C (621.5 F)
boiling point 1,744 C (3,171.2 F) density (20 C) 11.29 g/ml oxidation
states +2, +4 electron config. 2-8-18-32-18-4 or 6s26p2 or
1s22s22p63s2 3p63d104s24p64d104f 145s2 5p65d106s26p2

"lead" Encyclop�dia Britannica Online.

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