Periodic Law

V

Electron Shell Theory

In the periodic classification, noble gases, which in most cases are unreactive (valence = 0), are interposed between highly reactive metals that form compounds in which their valence is +1 on one side and highly reactive nonmetals forming compounds in which their valence is -1 on the other side. This phenomenon led to the theory that the periodicity of properties results from the arrangement of electrons in shells about the atomic nucleus. According to the same theory, the noble gases are normally inert because their electron shells are completely filled; other elements, therefore, may have some shells that are only partly filled, and their chemical reactivities involve the electrons in these incomplete shells. Thus, all the elements that occupy a position in the table preceding that of an inert gas have one electron less than the number necessary for completed shells and show a valence of -1, corresponding to the gain of one electron in reactions. Elements in the group following the inert gases in the table have one electron in excess of the completed shell structure and in reactions can lose that electron, thereby showing a valence of + 1.

An analysis of the periodic table, based on this theory, indicates that the first electron shell may contain a maximum of 2 electrons, the second builds up to a maximum of 8, the third to 18, and so on. The total number of elements in any one period corresponds to the number of electrons required to achieve a stable configuration. The distinction between the a and b subgroups of a given group also may be explained on the basis of the electron shell theory. Both subgroups have the same degree of incompleteness in the outermost shell but differ from each other with respect to the structures of the underlying shells. This model of the atom still provides a good explanation of chemical bonding.

VI

Quantum Theory

With the development of the quantum theory and its application to atomic structure by the Danish physicist Niels Bohr and other scientists, most of the detailed features of the periodic table have found a ready explanation. Every electron is characterized by four quantum numbers that designate its orbital motion in space. By means of the selection rules governing these quantum numbers and the exclusion principle of Wolfgang Pauli, which states that two electrons in the same atom cannot have all four quantum numbers the same, physicists can determine theoretically the maximum number of electrons required to complete each shell, confirming the conclusions inferred from the periodic table.

Further development of the quantum theory revealed why some elements have only one incomplete shell (namely, the outermost, or valence, shell), whereas others may have incomplete underlying shells as well. In the latter category is the group of elements known as the rare earth elements, which are so similar in properties that Mendeleyev had to assign all 14 to a single place in his table. The rare earth group includes the elements in the lanthanide series.

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VII

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.

See also Elements, Chemical; Isotope.

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