Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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.


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 328° C (662° F), boils at 1740° C (3164° F), and has a specific gravity of 11.34; the atomic weight of lead is 207.20.

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.

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).


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.