Lead | Research & Encyclopedia Articles

Lead

Lead is the heaviest element in Group 14 of the periodic table, a group often known as the carbon family. Lead has an atomic number of 82, and atomic mass of 207.2, and a chemical symbol of Pb.

Properties

Lead is a heavy, soft, gray solid that is both ductile and malleable. The metal has a shiny surface when it is first cut, but it slowly tarnishes and becomes dull as it reacts with oxygen in the air. Lead is easily bent, cut, pulled, and otherwise worked to produce specific shapes. The melting point of lead is 621.3°F (327.4°C), its boiling point is about 3,180°F (about 1,750°C), and its density is 11.34 grams per cubic centimeter. Lead is not a good conductor of electricity, heat, sound, or vibrations. It is a moderately active metal that dissolves very slowly in water and in most cold acids.

Occurrence and Extraction

Lead is relatively common in the Earth's crust with an abundance of about 13-20 parts per million. It rarely occurs as a free element, and is found most commonly as a compound in the form of galena (lead sulfide; PbS), anglesite (lead sulfate; PbSO₄), cerussite (lead carbonate; PbCO₃), and mimetite (a mixed lead chloride arsenate; PbCl₂Pb₃(AsO₄)₂).

The largest producers of lead in the world are Australia, China, the United States, Peru, Canada, Mexico, and Sweden. In the United States, more than 90% of all the lead produced comes from a single state, Missouri. Lead is extracted from its ores by first converting the ore to lead oxide and then heating the oxide with charcoal (pure carbon): PbO₂ + C --heat CO₂ + Pb. The lead produced by this process is usually not very pure and can be further refined electrolytically. A significant amount of lead is now being recovered through recycling programs. For example, old car batteries were once just thrown away. Now they are sent to recycling plants where lead can be extracted and used over and over again.

Discovery and Naming

Lead is one of the oldest elements known to humans. One of the oldest lead objects known is a small statue found in Egypt from the First Dynasty, about 3400 B.C.. The metal is mentioned in many ancient manuscripts, including the Bible. No one is quite sure how lead got its modern name. At one time, it was known by the Latin term plumbum, from which it gets its chemical symbol.

Uses

Throughout history, lead has been used for many important applications, including water and sewer pipes, cable coverings, type metal, paints, food and tobacco wrappings, and gasoline additives. Over the past few decades, however, evidence of the toxic effects of lead on humans and other animals has begun to accumulate. Today, the element is recognized as a serious environmental hazard and, as a result, the lead industry has begun to undergo a dramatic change. Many of the products for which lead was used in the past are now being made of other elements.

On the other hand, some traditional uses of lead have not declined at all and, in many cases, have actually increased. The best example is the lead storage battery present in nearly every car and truck. Researchers have been looking for a substitute for the lead storage battery that would provide electrical current efficiently, but that would not pose an environmental hazard. So far, they have been largely unsuccessful.

A small percentage of lead is used to make compounds with many different uses, including insecticides, waterproofing materials, varnishes, stains for glass and ceramics, high explosives, specialized industrial paints, photography, cloud seeding, and semiconductors and photovoltaic devices.

Health Issues

Low levels of exposure to lead can produce symptoms such as nausea, vomiting, extreme tiredness, high blood pressure, and convulsions. Higher levels of exposure can cause brain damage that can result in mental retardation. Lead poisoning is usually not a serious problem for the general public, although certain groups are at special risk. For example, people who work in factories where lead is used may inhale lead fumes and accumulate increasingly large amounts of the metal in their bodies. Young children are also at risk for lead poisoning because they tend to eat materials such as dirt, paper, chalk, and paint chips that may contain lead compounds.