RADIOACTIVITY and fhe PERIODIC TABLE' A n Introduction to the Study of Atomic Structure and Isotopes i n Elementary Chemistry
J. F. KING AND P. H. FALL2 Williams College, Williamstown, Massachusetts
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N AN elementary course in chemistry, radioactivity and the periodic table serve as an excellent introduction to a study of the modern theory of atomic structure. The historical development of radioactivity, including an understanding of the Soddy-Fajans Disintegration Hypothesis, "catalyzes" the student's grasp or understanding of the concepts of atomic structure, atomic numbers, mass numbers, and isotopes. The contributions which Rutherford and his school of workers made to the rapid evolution of the modern theories of atomic structure are delightfully revealed in a most illuminating way in the excellent biography of R ~ t h e r f o r d . ~Any instructor will find it both stimulating and helpful t o review in his class these contributions and developments when taking up the subject of atomic structure, isotopes and the periodic table. Once the student has grasped the significance of the SoddyFajans Disintegration Hypothesis, his "feeling" for atomic weights, atomic numbers, mass numbers, and isotopes becomes sufficiently definite to go into some of the details of the periodic table. In order to show the students in our department how the forty naturally occurring radioactive isotopes, as well as the 263 known stable isotopes, fit into the periodic table, we have worked out an adsaptation of a commonly used periodic chart of the elements. A great many charts have been proposed to illustrate how the loss of an alpha particle lowers the atomic number two units, whereas the loss of a beta particle raises the atomic number one unit. In the first case above, the disintegration product falls into the periodic group two units to the left of its immediate ancestor, whereas the disintegration product resulting from the loss of a beta particle falls into a periodic group one unit to the right of its immediate ancestor. This is really the SoddyFajans Rule. Some charts have Group zero elements in the middle but it has been our experience that such a chart confuses the student because i t is so different from the periodic chart with which he is usually somewhat familiar. In order to utilize the usual group arrangement, a
Hubbard-designed Periodic Chart4was glued to a rotatable galvanized iron (20-gage) drum, 16" in diameter and 43" high. As the accompanying two photographs show, the chart was mounted on the drum so that there
is a regular change in atomic number as the drum is rotated. The spiral-like arrangement of the originally rectangular chart was easily realized by joining the lower left edge of row (period) 1 to the upper right edge of row (period) 2, and so on down the entire horizontal 'Presented before the Division of Chemical Education at the rows, when the left and right edges of the rectangular A. C. S. in Cincinnati. Ohio. A ~ r i l ninetv-ninth meetine of the . . Hz,-1940. chart were joined together on the cylindrical drum. Present address: Hiram College, Hiram. Ohio. "Rutherford," The Macmillan Company, New York Citv. 1939. "EYE,
Published by W. M. Welch Msnufacturing Company. Chicaao. Illinois.
The chart was glued to thedrum by means of a thin film of regular furniture glue. At the upper edge of the space allotted to each ele-
ment which has any known isotopes, was fastened, by means of a small brass hing,e, a rectangular brass rod, I/,' wide and approximately '/16" thick. To each separate rod were attached small .cup hooks-the number of hooks corresponding to the number of puhlished5 isotopes of the particular element. In each case the length of the rod attached depended upon the number of hooks to he fastened to it. The hooks were spaced an inch apart on each rod. From these hooks can be suspended, by means of small holes, lucite rectangles (21/n1' X 4%" X l / B " )on whichareprinted thesymbols, atomic numbers, and mass numhers of all the isotopic species of the elements. The atomic numbers are printed in red (using Duco lacquer, Mansion Maroon, 246-52926) and the mass numbers, as well as the symbols, are in black (using Duco lacquer, 246-2048-R). On each of the lucite plates of the 263 known stable isotopes there is also given the per cent abundance of the particular isotope. This is printed with India ink on a small strip of scotch cellulose tape placed under the ' L~VINCOODAND SEABORG, Rw. Mod. P h y f . , 12, 30 (1940).
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mass number. Similarly, on each of the lucite plates of the forty naturally occurring radioactive isotopes, the "half-life" of the particular radioactive isotope is printed on a small strip of scotch tape. Furthermore, on each of these plates is indicated the immediate ancestor of the particular radioactive isotope and also the immediate descendant of this isotope, as well as the rays given off in the formation and in the disintegration of the particular radioactive isotope. An example of a lucite plate for a radioactive isotope is given in the accompanying diagram. As a further aid in observing or "spotting" the radioactive isotopes, when all the stable and naturally occurring radioactive isotopes are hung on their appropriate hooks (making a total of 303 plates on the chart), the upper and the left-hand borders of the plates designating the naturally occurring radioactive isotopes are painted red. This border is visible in the accompanying photographs. It is a simple matter to follow through the disintegration of an entire "family" of any of the three radioactive series. This is done by placing on their proper hooks the lucite plates, one by one, as we follow through the disintegration series. By such a procedure the student can readily observe thes hifting of the elements, in the periodic group, resulting from a loss of an alpha particle or the loss of a beta particle. Furthermore
with this type of chart i t is easy for the student to see that any one of the respective disintegration products of a radioactive element is an isotope of an element normally listed in the periodic table. This we find highly advantageous. We have found this demonstration model to be of great value, not only in clarifying the points already indicated, but also in making evident, even to the casual observer of the assembled chart, the relative numbers of isotopes of the different elements, as well as the abundance of isotopes in the even-numbered periodic groups and the scarcity of isotopes in the oddnumbered periodic groups. This is very apparent from the illustrations.
