An arrangement of the chemical elements by symbol in a geometric pattern designed to represent the periodic law by aligning the elements in periods so that the corresponding parts of the several periods are adjacent. When the elements are aligned in order of increasing atomic number, they constitute a succession of period, each beginning with an alkali metal (one electron in the outermost principal quantum level) and ending with an element of the helium family, helium, neon, argon, krypton, xenon, and radon (each having eight electrons in the outermost principal quantum level, except for helium, which is limited to two). Each helium family element is followed directly in atomic number by an alkali metal, which begins a new period.
The advantage of placing each successive period so that it is adjacent to the preceding period in its corresponding parts is that similar elements are thus brought together in groups. For example, the alkali metals that begin each period have similar properties that correspond to their atomic structure, each having one outermost shell electron. In a periodic table, the elements exhibit a steady trend in properties from the beginning of each period, where they are metals, to the end, where they are nonmetals. Within each group the elements are quite similar. A given element can be predicted, on the basis of its position in a periodic table, to resemble the other elements of its group and be intermediate in properties between its adjacent neighbors within its period. The chief function of a periodic table is to serve as a fundamental framework for the systematic organization of chemistry (R. T. Sanderson).See Mendeleyev, D. I.Note:Dr. Glenn T. Seaborg predicted the existence of synthetic superheavy elements 107–168 on the basis of theoretical calculations. Numbers 122–153 he called “superactinides.”
