chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled Periodic Table). The vertical columns, or groups, are numbered from I to VIII, with a final column numbered 0. Each group is divided into two categories, or families: one called the a series (the representative, or main group, elements); the other the b series (the transition, or subgroup, elements). All the elements in a group have the same number of valence electrons and hence similar chemical properties. The horizontal rows of the table are called periods. The elements of a period are characterized by the fact that they have the same number of electron shells; the number of electrons in these shells, which equals the element's atomic number, increases from left to right within each period. In each period the lighter metals appear on the left, the heavier metals in the center, and the nonmetals on the right. Elements on the borderline between metals and nonmetals are called metalloids.

Group Ia (with one valence electron) and group IIa (with two valence electrons) are called the alkali metals and the alkaline-earth metals, respectively. Two series of elements branch off from group IIIb, which contains the transition elements, or transition metals; elements 57 to 71 are called the lanthanide series, or rare earths, and elements 89 to 103 are called the actinide series, or radioactive rare earths; a third group, the superactinide group (elements 122–153), is predicted to fall outside the main body of the table, but none of these has yet been synthesized or isolated. The nonmetals in group VIIa (with seven valence electrons) are called the halogens. The elements grouped in the final column have no valence electrons and are called the inert gases, or noble gases, because they react chemically only with extreme difficulty.

In a relatively simple type of periodic table, each position gives the name and chemical symbol for the element assigned to that position; its atomic number; its atomic weight (the weighted average of the masses of its stable isotopes, based on a scale in which carbon-12 has a mass of 12); and its electron configuration, i.e., the distribution of its electrons by shells. The only exceptions are the positions of elements 103 through 116; complete information on these elements has not been compiled. Larger and more complicated periodic tables may also include the following information for each element: atomic diameter or radius; common valence numbers or oxidation states; melting point; boiling point; density; specific heat; Young's modulus; the quantum states of its valence electrons; type of crystal form; stable and radioactive isotopes; and type of magnetism exhibited by the element (paramagnetism or diamagnetism).

See P. W. Atkins, The Periodic Kingdom: A Journey into the Land of Chemical Elements (1997).

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