THE ELECTROMOTIVE SERIES AND THE PERIODIC TABLE

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JOURNAL os CHEMICAL EDUCATION

228

F E E R W ~1927 Y,

THE ELECTROMOTIVE SERIES AND THE PERIODIC TABLE JOHN

R.SAMPEY, HOWARD COLLEGE, BIRMINGHAM, ALABAMA

Modern text-books of first-year chemistry do not emphasize an interesting relationship that exists between the electromotive series and the periodic table. In fact one of the most widely used texts in first-year college chemistry lists among the defects of the periodic system the absence of any relationship between these two important generalizations of the science. It is true the relation that may be derived from the order in which the elements stand in the two tables is limited, but it is sufficient to furnish a striking example of how our modern conceptions of the structure of atoms enable us to obtain a clearer insight into the mechanism of chemical reactions. In following our present-day practice of placing emphasis upon interpretations based on atomic structure the anthor has found this illustration very enlightening to students of first-year chemistry. Reference to a periodic table and to a table of the order of activity of the metals will show that in the alkali family the heaviest metal is the most active, the next heaviest is second in order of activity, and that this relation holds throughout this family. Immediately below lithium, the lightest and least active metal of the alkalies, we find in the electromotive series the following for the order of decreasing activity of the alkaline earth family: barium, strontium, calcium, and magnesium; turning to the periodic table we find this to he the same as the order of decreasing atomic weights. There is no such well-defined relation between the order of activity and the atomic weights of the other metals in the electromotive series, but turning to the most characteristic family of non-metals we find that the order in which the halogens stand in the family is the same order in which they stand in the electromotive series, the lightest being the most active. A rational explanation for this relationship may be found in recent developments of Bohr's theory of atomic constitution.' If metals are defined as elements whose atoms react by loss of valence electrons, then the most active element is the one that loses its valence electrons most readily. According to the electromotive series cesium should lose its one valence electron more easily than any of the other alkali metals; according to the postulates of Bohr's theory i t does so because the highly elliptical (61) orbit of its valence electron is pushed so far from the nucleus by the fifty-four other planetary electrons of the atom that the attractive forces between the nucleus and the valence electron are more weakened than is the case with any other members of 2 Niels Bohr, "The Theory of Spectra and Atomic Constitution," Cambridge University Press, London, 1922, p. 61.

the family. The (2J orbit of the valence electron of lithium is nearer to the nucleus than the orbits of the valence electrons of the other members of the alkali metals, and as a result the attractive force is greatest and the electron is most firmly held. A similar line of reasoning is employed to interpret the order of activity of the alkaline earth metals. In the halogen family the most active member is that one which most readily gains electrons, since non-metals react by gaining electrons; from Bohr's theory we should expect fluorine to be the most active, because the circular (24 orbits of its valence electrons are nearest the nucleus where the attractive forces are strongest. From this simple observation of the relation of atomic weights and the order of activity in the most characteristic families of the metals and non-metals, we are able to present to the student strong arguments for the necessity of obtaining a knowledge of atomic structure if he would search deeper into the mechanism of chemical reactions.