Lead

subject = Chemistry
title = Lead

Lead is a lustrous, silvery
metal that tarnishes in the presence of air and becomes a dull bluish
gray. Soft and flexible, it has a low melting point (327 °C). Its chemical
symbol, Pb, is from plumbum, the Latin word for waterworks, because of
lead\'s extensive use in ancient water pipes. Itsatomic number is 82; its
atomic weight is 207.19.

Lead 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 damage
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.

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. 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.

Lead 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, 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.


Tetraethyl lead or tetramethyl
lead (PbEt4) 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. 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.


More
Uses

· the metal and the dioxide are used in storage batteries, cable
covering, plumbing, ammunition
· manufacture of PbEt4 - an antiknock
compound in petrol.
· environmental concern with lead poisoning, (and
cheaper unleaded petrol prices) is slowly resulting in less use of lead
in petrol
· the metal is very effective as a sound absorber, a radiation
shield around X-ray equipment and nuclear reactors
· used extensively
in paints, although recently the use of lead in paints has been drastically
curtailed to eliminate or reduce health hazards
· the oxide is used
in producing fine "crystal glass" and "flint glass" with a high