subject = Chemistry
title = Lead
papers = Lead is a lustrous, silvery
gray. Soft and flexible, it has a low melting point (327 ?C). Its chemical
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
up the blood-brain barrier that protects the brain from many harmful chemicals.
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
Lead has been used by humans since ancient times.
It was used in ancient Egypt in coins, weights,
ceramic glazes, and solder. Lead is mentioned in the Old Testament. The
conveyed drinking water in lead pipes, some of which are still in operation.
extracted and prepared the lead, describes a disease among
toxicity, lead water pipes are no longer being installed. The greatest
The protective oxidation layer formed by lead in contact
and fluorine makes it highly
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
used in solder
because of its low melting point. When combined with tin, lead forms solder
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
in the keels of ships. Because of their high density, lead bullets and
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
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
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
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
yellow substance that is used as a dye in such processes as coloring bronze.
light-sensitive properties comparable to those of silver salts.
? 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
? 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 refractive
index for achromatic lenses
? used by the Romans for plumbing
(the decline of the Roman empire is attributed to lead in the water supply!)
? used to contain corrosive liquids
? cable covering
? shield against X-rays
? oxide used to produce crystal