An IMPROVED PERIODIC TABLE W. F. LUDER Northeastern University, Boston, Ma~sachusetts
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OR half a century instructors and students have struggled with the inconsistencies of the periodic table. These inconsistencies arise from the attempt to confine it to eight columns, one of which con-
tains fifteen rare earths crammed into one elementspace. Many monstrosities have been proposed in the effort to overcome these difficulties, but none has succeeded. Now a t last a new periodic arrangement of the
elements has appeared which eliminates the disadvantages of the old chart without introducing any new difficulties of interpretation. The new arrangement suggested by R. L. Ebel' not only fits the elements into their proper places in a wholly consistent manner, but the basis on which it is done is more fundamental than the old one. The accompanying periodic table is an elaboration of Ebel's arrangement involving a change from vertical to horizontal periods and the addition of the electron configurations for the last three shells of each atom.
is given by taking as the ordinate on the chart the shell in which the differentiating electron lies and by taking as the abscissa its number in that shell. For example, Ga comes next to Ca because the differentiating electron for Ga is the third in the N-shell. The differentiating electron for Ca is the second in the N-shell, yet the atomic numbers of the two are 31 and 20. The reason for the gap is that there are eight electrons in the M-shell of the calcium atom and eighteen in the Mshell of the gallium atom. (The word shell is used because it is non-committal about the nature of energy
Ebel's proposal is to use our knowledge of the structure of the atoms to determine their position in the table--rather than their atomic numbers. An examination of the accompanying chart reveals a t once that there are several breaks in the order of increasing atomic numbers. These correspond to breaks in the electron configuration of the atoms. The electron configuration of the atom determines its position in the table. More specifically, the location of the "differentiating electron" fixes the position of the element. That position
levels.) When all the atoms are thus arranged according to the position of their differentiating electrons, they fall into three separate groups. The division depends upon whether the differentiating electrons occur in the outermost shell, the second from the outermost, or the third from the outermost. These three groups Ebel calls the "representative elements," "the related metals," and the rare earths. It is this grouping which makes his chart so clear and simple and emphasizes distinctly the correlation of the periodic table with atomic structure which he has so ingeniously achieved. A few minutes spent in studying the chart thoroughly
EBBL, R. L., "Atomic structure and the periodic table," J. CHEM.EDUC., 15, 575 (1938).
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will suggest all sorts of interesting possibilities in teach- about students asking why Mn is not a non-metal. ing the relationship of the properties of the elements to At first glance one might wonder how the valences of the "related metals" are to be explained, hut second their atomic structure. The only inconsistency present in the new arrange- thought will remind one that the old table does not ment is a minor one, which will cause neither the student explain them a t all. The new tahle reveals the fact nor the instructor any difficulty if the instructor is care- that they require explanation. Furthermore, it proful to point it out properly. In fact, it may he used as vides the necessary clue for the explanation. One has an illustration of principles already learned. In a few only to refer hack to the preceding inert gases, keeping instances the positions of the elements on the tahle are in mind that there are (generally speaking) two elecnot uniquely fixed by the locations of the differentiating trons in the o u t m o s t shell of each of the related metelectrons. For example, it appears that for Cu there als. The significance of the numhers 2;8, 18, 32 as are two differentiating electrons. One is in the M- critical points in electron configuration stability is also shell and the other is in the N-shell. Asimilarsituation given its proper emphasis by the very shape of the new exists for Ag and Au. The characteristic number of periodic table. After the usual introduction, discussion of the electrons in the outermost shell for the "related metals" is two, but Cu, Ag, and Au have only one. By referring achievements, and pointing out the inconsistencies of to the preceding elements this behavior seems entirely the old periodic tahle based on atomic numbers, the new logical from the standpoint of knowledge already ac- one based on electron configurations can now he introquired hy the student. Cu has 2.8.18.1 electrons. duced to college freshmen or to physical chemistry stuNi has 2.8.16.2. The student may imagine Cu with dents with a feeling of great satisfaction to both in2.8.17.2 electrons, keeping in mind its valence of two. structor and students. Newlands' Law of Octaves, He will he reminded of the tendency, exhibited by the Mendel&eff's Periodic Table and his remarkable prehalogens, of nearly completed shells to 6ll up with elec- dictions, the fitting in of the inert gases, the X-ray extrons from other atoms. He can then see that he might periments of Moseley, usually arouse the student's have expected that one of the two electrons in the out- keen interest in the correlation of the properties of the ermost shell of his imaginary 2.8.17.2 copper atom elements with their atomic structure. His interest in what promised to he one of the grandest laws of nature would drop down into the next shell. All the other much more serious difficulties have been is only to he disappointed when he looks beyond the eliminated in the new chart. The "related metals" twentieth element. It is obvious to him that using have been cleared out of the representative elements, eighteen columns instead of eight eliminates one diffiyet the arrangement of the representative elements has culty only by introducing another which is perhaps been left intact. The position of the rare earths not even greater. Moreover, the eighteen-column tahle only fits consistently into the whole scheme, but helps still ignores the problem of the rare earths. But when to complete the explanation of the other electron con- the new chart is presented to the student, after such an figurations. The so-called transition elements which .introduction, he can see that, when the periodic table is are usually lumped together in "Group VIII" have built up on the fundamental chemical basis of electron their proper place. The location of the non-metals configuration, i t really is one of the greatest triumphs may he pointed out, and the instructor need not worry of all science.
