edited by JOHN H. WOTlZ Southern Illinois University Carbondale,Illinois 82901
Atomic Number before Moseley Jan W. van Spronsen University of Utrecht The Netherlands The year 1913 could he called the year in which the periodic system of the chemical elements received its theoretical f(,undation. In that y n r , Rohr developnl his atomic theory, Sod(ly and others put inrw;~rdthe "displacemrnt lnu," and the concept uf ianopy, nnd Moaeley arritngtd the eletncnts on the l~niisuftheir X-ray ;pet tm. '\I1 this led t o n Imir re~isionof ideas about the periodic system. Instead of atomic weight, nuclear charge (that is, atomic number) could he used as the classifying factor. Many problems with the periodic system, such as the incorporation of the radioactive disintegration wroducts. were solved in one blow. It wasthe Dutchman Antonius Johannes van den Broek (1870-1926) who introduced the idea of the atomic number of an element in the years before 1913 ( 1 4 ) .Van den Broek was interested in finding a system which could incorporate all the existing elements. In 1907,he published aperiodic system based on the alpha particle (5). Rutherford had concluded Irtm his rrir;irch on ;adioactiw radiation thst alpha parrides werc h~tilding-blocksof matter. L:. that rime, however, thwr was no certainty about the nature of an alpha particle. Van den Broek assumed it to be one half of a helium atom and named it an alphon. He then arranged the elements by ascending atomic weight but supposed that atomic weights should increase by a constant two mass units (i.e., by one alphon). Thus, he obtained a periodic system, the alphade system, with space for 120 elements-8 groups and 15 periods (Fir - .=. 1- ,) .. \
As far back as the vear of van den Broek's birth (1870). Mendeleev had suggested that a three-dimtmsional tnhlr niicht be the best way togiveall i f t h e chemical elemmt. their proper places. Van den Broek was the first to remember this and in 1911 he constructed a three-dimensional system in order to express better the periodicity of the properties of the elements (6). His earlier 15 periods were reduced to 5 and each of three elements in a group are to he thought of as standing perpendicular to the plane of the diagram (Fig. 2). He called this a "cubic" system. On the basis of Rutherford and Barkla'sl conclusion that the nuclear charee of an atom must be half of its atomic weight, van den ~ r o e k hypothesized a t the end of 1912 that the order number of an element of the periodic system is equal to the number of positive charges in the nucleus of the atom (4). This was hefore Moseley's work. True, he did not prove this hypothesis experimentally but i t was immediateiy accepted hy the world of physics. The relation between atomic number and atomic weight did not prove to he as simple as van den Broek first thought. Research later showed that the relationship is not linear. As can 1 Charles Glover Barkla (1877-1944), English physicist. In 1906 he showed that when X-raw were scattered hv. narticular elements they . a heam uitha p ~ n c t m r i n gpower chararreristiu uf the el.men, I n 191: he was awarded the h b r l I'rirr in phyriri. ~~
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106 / Journal of Chemical Education
Figure 1. Periodic system based on a-particles (van den Broek-1907)
Figure 2. Cubical (alphades)system, in which ihe 120 elements, in groups of perpandicukr ar the plane of the drawing (Van den Broek1911).
three, are
he seen from the cases of argon and potassium, tellurium and iodine, and cobalt and nickel, i t is not even monotonic. His periodic system also turned out to be incorrect (note for example, the multiple rare earths such as Gd*,Gdz, Gds and Dyl, Dyz, Dya), hut his principle is right and in contemporary periodic systems the elements are arranged according to ascending nuclear charge.
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Acknowledament The author wishes to express his gratitude to Russell F. Trimhle of the Southern Illinois University a t Carbondale for his assistance in the preparation of this manuscript. Literature Cited
