periodic law

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periodic law,

statement of a periodic recurrence of chemical and physical properties of the elementselement,
in chemistry, a substance that cannot be decomposed into simpler substances by chemical means. A substance such as a compound can be decomposed into its constituent elements by means of a chemical reaction, but no further simplification can be achieved.
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 when the elements are arranged in order of increasing atomic numberatomic number,
often represented by the symbol Z, the number of protons in the nucleus of an atom, as well as the number of electrons in the neutral atom. Atoms with the same atomic number make up a chemical element. Atomic numbers were first assigned to the elements c.
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. Such an arrangement in the form of a table in which the groupings of elements having similar properties are easily identified is called the periodic system or the periodic tableperiodic table,
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
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Pioneering Periodic Arrangements of the Elements

Laws of Triads and of Octaves

Early in the 19th cent., a number of chemists had noticed certain relationships between the properties of elements and their atomic weight. In 1829 J. W. Döbereiner stated that there existed some three-element groups, or triads, in which the atomic weight of the middle element was the average of the other two and the properties of this element lay between those of the other two. For example, calcium, strontium, and barium form a triad; lithium, sodium, and potassium, another. The English chemist J. A. Newlands found (1863–65) that if the elements are listed according to atomic weight starting with the second, the 8th element following any given element has similar chemical properties, and so does the 16th. This became known as the law of octaves. About the same time, A. E. de Chancourtois arranged the elements according to increasing atomic weight in the form of a vertical helix with eight elements in a turn, so that elements having similar properties fell along vertical lines.

The Periodic Table

D. I. Mendeleev was the first to state the periodic law close to its present form. He proposed in 1869 that the properties of elements are periodic functions of the atomic weightatomic weight,
mean (weighted average) of the masses of all the naturally occurring isotopes of a chemical element, as contrasted with atomic mass, which is the mass of any individual isotope. Although the first atomic weights were calculated at the beginning of the 19th cent.
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 and grouped the elements accordingly in a periodic system. Working independently and not aware of Mendeleev's work, Lothar Meyer arrived at a similar system, publishing his results about a year after Mendeleev's. When Mendeleev devised his periodic table a number of positions could not be fitted by any of the then known elements. Mendeleev suggested that these empty spaces represented undiscovered elements and by means of his system accurately predicted their general properties and atomic weights.

Introduction of Atomic Numbers

The work (1913–14) of H. G. Moseley on the X-ray spectra of elements (see X rayX ray,
invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light. The wavelength range for X rays is from about 10−8 m to about 10−11
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) led to the present form of the periodic law. He found that the wavelength of the X-radiation of elements decreased with increasing atomic weight. However, the relationship was not a strict one. He assigned a new set of numbers, called atomic numbers, to the elements he had studied, so that there was a relation between the wavelength and the atomic number. The atomic number is the number of positive charges, or protons, contained in the atomic nucleus (see atomatom
[Gr.,=uncuttable (indivisible)], basic unit of matter; more properly, the smallest unit of a chemical element having the properties of that element. Structure of the Atom
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) or, equivalently, the number of negative charges, or electrons, outside the nucleus in a neutral atom. The periodic law can be explained on the basis of the electronic structure of the atom, which is believed to be the main factor underlying the chemical properties and many of the physical properties of the elements. In turn, the electronic structures of atoms have been successfully accounted for by the quantum theoryquantum theory,
modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.
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In spite of its great success, the periodic system that had been introduced by Mendeleev had some discrepancies. Arranged strictly according to atomic weight, not all elements fell into their proper groups. Better arrangement could be made if the positions of certain neighboring couples were interchanged. For example, to suit the chemical order of the table, the inert gas argon (at. wt. 39.948) should come before the chemically active metal potassium (at. wt. 39.0983). Through Moseley's work, it was found that although the atomic number of an element is roughly half its atomic weight, the atomic weight does not always increase with increasing atomic number. The discrepancies occur just for those elements where Mendeleev's law failed. Based on atomic number, the periodic law now has no exceptions. Although all the missing elements in the periodic table have been found (with the aid of the periodic table itself), the table retains its usefulness to the chemist as a reliable check for disputed or uncertain data concerning some of the known elements.

Periodic Law


a fundamental law that explains the periodic change in the properties of chemical elements in terms of increasing nuclear charge within these elements’ atoms.

D. I. Mendeleev discovered the periodic law in 1869 by observing the relationship between the atomic weights and properties of those elements that were known at the time. Mendeleev first used the term “periodic law” in November 1870, and in October 1871 he proposed his final formulation of the law: “the properties of elements, and therefore, the properties of the simple and complex bodies that are formed by these elements, are in periodic dependence on the elements’ atomic weights” (Periodi-cheskii zakon: Osnovnyestat’i, 1958, p. 111). The periodic system of the elements that was developed by Mendeleev is a graphic or tabular representation of the periodic law.

The physical significance of the periodic law became clear only after it was established that the nuclear charge of an atom of an element increases by a single charge unit with respect to the nuclear charge of an atom of the neighboring element in the periodic system. The nuclear charge of an atom is numerically equal to the ordinal number, or atomic number Z, of the corresponding element in the periodic system, that is, the number of protons in the nucleus. In turn, the number of protons equals the number of electrons in the corresponding neutral atom.

The chemical properties of atoms are determined by the structure of the atoms’ outer electron shells, which change periodically with increasing nuclear charge; thus, the basis of the periodic law is the concept of change in the nuclear charge—not in the atomic mass—of the atoms of the elements. The graphs for the periodic change of several physical quantities that depend on Z, for example, ionization potential, atomic radius, and atomic volume, are a striking illustration of the periodic law. No general mathematical expression of the periodic law has been formulated.

The periodic law has enormous scientific and philosophical significance. It provides a framework for examining the interaction of all elements and for predicting the properties of elements that have not yet been discovered. It has helped to direct many useful scientific questions concerning the structure of matter in chemistry, physics, geochemistry, cosmochemistry, and astrophysics. The law is a vivid manifestation of the general laws of dialectics, specifically, the law of the transformation of quantitative change into qualitative change.

periodic law

[¦pir·ē¦äd·ik ¦lȯ]
The law that the properties of the chemical elements and their compounds are a periodic function of their atomic weights.

periodic law

the principle that the chemical properties of the elements are periodic functions of their atomic weights (also called: Mendeleev's law) or, more accurately, of their atomic numbers