PERIODIC LAW

Figure 1: Modern version of the periodic table of the elements. To see more information about an 1/4 in chemistry, the generalization that there is a recurring pattern in the properties of the elements when they are arranged in order of increasing atomic number-i.e., the total number of protons in the atomic nucleus. The periods (horizontal rows) of the periodic table illustrate these relationships. (See Figure 1.) The initial discovery, which was made by Dmitry I. Mendeleyev in the mid-19th century, has been of inestimable value in the development of chemistry. It was not actually recognized until the second decade of the 20th century that the order of elements in the periodic system is that of their atomic numbers, the integers which are equal to the positive electrical charges of the atomic nuclei expressed in electronic units; and in subsequent years great progress was made in explaining the periodic law in terms of the electronic structure of atoms and molecules. This clarification has increased the value of the law, which is used as much today as it was at the beginning of the 20th century, when it expressed the only known relationship among the elements. Figure 1: Modern version of the periodic table of the elements. To see more information about an 1/4 in chemistry, the generalization that there is a recurring pattern in the properties of the elements when they are arranged in order of increasing atomic number-i.e., the total number of protons in the atomic nucleus. The periods (horizontal rows) of the periodic table illustrate these relationships. (See figure.) The elements, arranged in order of their increasing atomic numbers, are divided into seven periods reflecting periodicities in electronic structure and in chemical and physical properties. There are three short periods of 2, 8, and 8 elements; three long periods of 18, 18, and 32 elements, all of which are known; and a further long period predicted to contain 32 elements with atomic numbers from 87 to 118 (successful efforts to prepare elements with atomic numbers as high as 110 had been claimed by the late 1980s). The progression of elements along a period represents a systematic occupation by electrons of orbitals, or electron shells. A period is completed when all the electron orbitals within a given energy level are filled. The formulation of the periodic table began during the 1860s, when the Russian chemist Dmitry I. Mendeleyev undertook a detailed investigation of the relationship between the properties of the elements. In 1869, on the basis of an extensive correlation of the properties and the atomic weights of the elements, he proposed the periodic law and devised a tabular arrangement of the elements to show the observed relationships. Lothar Meyer, a German chemist, developed a similar classification of the elements independently of Mendeleyev at about the same time. Mendeleyev's periodic table made it possible to observe many types of chemical relations hitherto studied only in isolation. It was not, however, widely accepted at first. The new classification system had gaps, but Mendeleyev predicted that they would be filled by elements not yet discovered. The validity of his predictions became apparent with the discovery of three new elements within the next 20 years. During the 1920s it was recognized that the order of elements in the periodic table is that of their atomic numbers. In subsequent years considerable progress was made in explaining the periodic law in terms of the electronic structure of atoms. This clarification greatly increased the value of the law. Additional reading J.W. van Spronsen, The Periodic System of Chemical Elements (1969), is a thorough treatment of the history of the periodic system and the periodic table. Linus Pauling, The Nature of the Chemical Bond and the Structure of Molecules and Crystals, 3rd ed. (1960, reissued 1989), provides a general discussion of atomic and molecular structure and of valence in relation to the periodic system and the elements. Other works include Edward G. Mazurs, Graphic Representations of the Periodic System During One Hundred Years, rev. 2nd ed. (1974); D.J. Clouthier et al., Relationships and Mechanisms in the Periodic Table (1989); and Richard J. Puddephatt and P.K. Monaghan, The Periodic Table of the Elements, 2nd ed. (1986). Linus C. Pauling The Editors of the Encyclopdia Britannica