Lead

I

Introduction

Lead, symbol Pb (Latin plumbum, a lead weight), dense, bluish-gray metallic element that was one of the first known metals. The atomic number of lead is 82; the element is in group 14 (or IVa) of the periodic table (see Periodic Law).

Lead was mentioned in the Old Testament (see Bible). It was used by the Romans for making water pipes, soldered with an alloy of lead and tin.

II

Properties

Metallic lead is a soft, malleable, ductile metal. When gently heated it can be forced through annular holes or dies. It has low tensile strength and is a poor conductor of electricity. A freshly cut surface has a bright silvery luster, which quickly turns to the dull, bluish-gray color characteristic of the metal. Lead melts at 327.46°C (621.43°F), boils at 1749°C (3180°F), and has a specific gravity of 11.35; the atomic weight of lead is 207.2.

Lead is soluble in nitric acid but is little affected by sulfuric or hydrochloric acids at room temperature. In the presence of air, it slowly reacts with water to form lead hydroxide, which is slightly soluble. Lead is toxic when taken internally; although ordinary water usually contains salts that form a coating on pipes, inhibiting the formation of soluble lead hydroxide, pipes used for carrying drinking water should not contain lead.

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Lead occurs naturally in eight isotopic forms, of which four are stable and four radioactive. The stable isotopes, lead-206, lead-207, and lead-208, are, respectively, the end products of the uranium, actinium, and thorium series of radioactive decay; lead-204, also stable, has no natural radioactive precursors (see Radioactivity).

III

Occurrence

Lead is widely distributed all over the world in the form of its sulfide, the ore galena. Lead ranks about 36th in natural abundance among elements in Earth's crust. Ores of secondary importance are cerussite and anglesite. The principal method of extracting lead from galena is to roast the ore—that is, convert it to the oxide, and reduce the oxide with coke in a blast furnace (see Metallurgy). Another method is to roast the ore in a reverberatory furnace until part of the lead sulfide is converted to lead oxide and lead sulfate. The air supply to the furnace is then cut off and the temperature raised; then the original lead sulfide combines with the lead sulfate and lead oxide to form metallic lead and sulfur dioxide.

Waste material, such as battery scrap, recovered from various industrial processes, is also smelted and constitutes an important source of lead. Because galena often has other minerals associated with it, the crude lead, or pig lead, that is obtained from the smelting processes contains metals such as copper, zinc, silver, and gold as impurities. The recovery of precious metals from lead ores is often as important economically as the production of lead itself. Silver and gold are recovered by the Parkes process, whereby a small amount of zinc stirred into molten lead dissolves the precious metals. This molten alloy then rises to the surface of the lead as an easily removed scum, and the zinc is removed from the silver or gold by distillation. Pig lead is often purified by stirring molten lead in the presence of air. The oxides of the metallic impurities rise to the top and are skimmed off. The purest grades of lead are refined electrolytically.

IV

Uses

Lead is used in enormous quantities in storage batteries and in sheathing electric cables. Large quantities are used in industry for lining pipes, tanks, and X-ray apparatus. Because of its high density and nuclear properties, lead is used extensively as protective shielding for radioactive material. Among numerous alloys containing a high percentage of lead are solder, type metal, and various bearing metals. A considerable amount of lead is consumed in the form of its compounds, particularly in paints and pigments.

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