Periodic table, table of the elements listed in order of increasing atomic number, arranged in rows and columns to illustrate periodic similarities and trends in physical and chemical properties. In 1869 Dmitri Mendeleev published the first fairly complete periodic table, which was later revised by Henry Moseley. The numbers and arrangement of the electrons in the atom are responsible for the periodicity of properties; hence the atomic number is the basis of ordering. Each row, or period, of the table contains elements that have the same number of electron shells. The number of electrons in these shells equals the element's atomic number; these numbers increase from left to right within each period. The elements are arranged in vertical columns, or groups, containing elements of similar atomic structure and properties, with regular gradation of properties down each group. The longer groups, with members in the first three (short) periods, are known as the main group; they are usually numbered IA to VIIA, and 0 for the noble or inert gases. The remaining groups, the transition elements, are numbered IIIB to VIII, IB, and IIb. The elements in group IA are called The alkali metals; in group IIA, the alkaline-earth metals; and in group VIIA, the halogens.
See also: Mendeleev, Dmitri Ivanovich.