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Basic Chemistry Concepts

Periodic Law

periodic law, astatine, valence shell, basicity, halogens

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Periodic Law, in chemistry, law stating that many of the physical and chemical properties of the elements tend to recur in a systematic manner with increasing atomic number. Progressing from the lightest to the heaviest atoms, certain properties of the elements approximate those of precursors at regular intervals of 2, 8, 18, and 32. For example, the 2nd element (helium) is similar in its chemical behavior to the 10th (neon), as well as to the 18th (argon), the 36th (krypton), the 54th (xenon), and the 86th (radon). The chemical family called the halogens, composed of elements 9 (fluorine), 17 (chlorine), 35 (bromine), 53 (iodine), and 85 (astatine), is an extremely reactive family.

Long-Form Table

The application of the quantum theory of atomic structure to the periodic law has led to the redesign of the periodic table in the so-called long form, which emphasizes this electronic interpretation. In the long-form table, each period corresponds to the building up of a new electronic shell. Elements that are directly in line with each other have strictly analogous electronic structures. The beginning and end of a long period represent the addition of electrons in a valence shell; in the central portion the number of electrons in an underlying shell increases.

The periodic law has been found to correlate a great many different properties of the elements, including such physical properties as melting and boiling points, densities, crystal structures, hardness, electrical conductivity, heat capacity, and thermal conductivity, and such chemical properties as reactivity, acidity or basicity, valence, polarity, and solubility.


Lewin, Seymour Z., M.S., Ph.D.

Professor of Chemistry, New York University.

Article key phrases:

periodic law, astatine, valence shell, basicity, halogens, crystal structures, chemical family, boiling points, fluorine, periodic table, valence, krypton, bromine, electrical conductivity, radon, argon, number of electrons, iodine, thermal conductivity, chemical properties, helium, precursors, xenon, chlorine, physical properties, acidity, neon, Progressing, hardness, redesign, long period, chemistry, regular intervals, elements, beginning, example, line, end, application

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