National Geographic : 1989 Dec
Chemists' keyboard: the periodic table Building blocks of all matter, 103 elements provide the raw compo nents for forging tomorrow's materials. (There are at least five additional man-made elements, produced in minute quantities and existing then only briefly.) These basic substances differ as widely as buoyant hydrogen and heavy iridium-twice as dense as lead. But when arranged in ranks in the periodic table, here simpli fied, atoms of these disparate elements reveal family relation ships that bring order to the physical world and provide an invaluable guide to chemists in their labs. Each key bears an element's symbol; the full name appears on the key's right side. The front side lists the element's atomic number-the number of protons in its nucleus. Beginning with hydrogen, at upper left, these numbers increase sequentially along the horizontal rows, known as periods: helium, 2, upper right; lithium, 3, beneath hydrogen, and so forth. Family similarities unite ele ments aligned in vertical col umns. For instance, lithium and the five other metals beneath it share traits of softness, lightness, low melting point, high heat con ductivity, and similar reactions in forming chemical compounds. Exceptions are the elements known as lanthanides and acti nides, smaller dual keyboard, whose family ties run horizontally (see note at right). Colored blocks on the keys, coded at right, identify some of the elements that play important roles in materials, both traditional and exotic. Hydrogen compounds similar to those found in petroleum and liv ing organisms are the foundation materials of many plastics. Lithium, the lightest metal, finds growing use alloyed with alumi num for aircraft. Molten lithium reacts explosively with water, making processing dangerous.