CIATION ACHERS
THE PERIODIC TABLE AND ARRANGEMENT OF THE EXTRANUCLEAR ELECTRONS' LAURENCE S. FOSTER Belmont, Massachusetts
IN TEE teaching of science it is important to emphasize the sn'entific method and to use it as convincingly as possible. I t is especially important to avoid teaching theories as though they were facts. Our ideas of atomic structure are still in the classification of theory; no one has ever seen an atom and our picture of the arrangement of the extranuclear electrons is still being evolved. I t does not seem to me to be good science teaching to give the beginning student, very early in his first science course, the very inadequate picture of an atom of neon.as a dot surrounded by a couple of circles with more dots on them and then to proceed to compare this structure with the structure of an atom of sodium which has another circle, another dot, and as a "consequence" a valence of one. The correct approach is to present the student mith enough information to afford h i the opportunity of asking, "Why does that happen?" He can learn the answer as a theory that is a possible and a plausihle explanation of the chemical phenomena observed. In chemistry, it is possible to cover a lot of ground using purely descriptive material and eventually reaching a point when it becomes profitable to summarize the information into a law-the Periodic Law-before asking the question, "Why do elements behave like that?" I t should become apparent to the student that the Periodic Law is one of the really great generalizations in chemistry and, also, that it is one of the triumphs of the modern theory of atomic structure that it gives a reasonable explanation of the observed periodicity. It is recognized, of course, that the theory of the arrangement of the electrons permits the explanation of numerous other phenomena, but this merely makes the theory all the more convincing. In chemistry, the most important facts to be explained are the variations in the physical and chemical properties of all the
elements and their compounds as well as their similarities. The important place occupied by the periodic table in the evolution of the theory of atomic structure cannot be overemphasized. The periodic table is worthy of a much larger place in the chemistry course than it has been receiving lately and it should certainly he illustrated very carefully before its explanation in terms of a theory of atomic structure is attempted.= In his book "Explaining the A t ~ m , "the ~ late Selig Hecht gave a very useful type of periodic table for class demonstration purposes. It is illustrated in Figure 1. The 96 elements are recorded on a tape in order of their serial or atomic number. By coiling the t,ape to bring like elements under one another, a helical arrangement of the elements is achieved. It is a lot easier to do this now than it was in Mendeleev's time, since we can take the inert gas family as a guide. Mendeleev had no such guide and made some mistakes. He put the elements Cu, Ag, and Hg in one group and classified Au along with B, Al, and U. Mendeleev also put Pb in the alkaline earth group along with Ca, Sr, and Ba, a natural mistake based upon the fact that Pb commonly has a valence of two. If we choose the inert gases, which are convincingly members of one family, as the central column, the rest of the elements fall into the places currently assigned to them. It is remarkable that Mendeleev did as well as he did, and it is especially noteworthy that he placed Te and I, as well as Co and Ni, in their proper order, in spite of the reversal of the order of their atomic weights, in seeming violence of his Periodic Law. He sensed that chemical behavior is more fundamental than atomic weight and he actually used the correct order even though the importance of the serial or atomic number was not recognized for nearly a half century. What do we mean by periodicity? On Selig Hecht's tape, now spiralling around the inert gas family, we
'Based on a. paper presented at the 9th Annual Summer Conference of the New England Association of Chemistry POSTER,L. S., THISJOURNAL, 16. 409 (1939). Teachers, Wellesley College, August, 1947, as part of a symWECHT, SELIO,"Explrtining the Atom," Viking Press, New posium on atomic stmeturc. York, 1947. 28B
JOURNAL OF CHEMICAL EDUCATION
have the halogens on one side, the inert gases in the middle and the alkali metals on the other side. These groups of elements are readily recognized as families by anyone who 'has received a brief introduction to chemical properties. The nitrogen, oxygen, and alkaline earth families are equally easy to identify and fall into family groups with obvious periodicity on the coiled tape. As we go farther from the inert gases on either side, the chemistry becomes less familiar and, in addition, the family similarities are not so marked. For introductory courses it is sufficient to focus attention on the elements in the region of t,he inert gases. Let us count the elements:
changed from time to time in tune with the advance of knowledge? Is it doing the student a service to give him only a very rudimentary picture of atomic structure, to give it to him as immutable fact, and to give it to him so early that he cannot readily see its pertinence? I think that the answers to t,hese questions are obvious. What shall we teach? After laying an adequate factual background of chemistry, we can then give a very simple story of atomic structure, showing what it does and what i t does not explain, and teach that the picture is incomplete. Since the inert gases are inert, we can assume with the Bohr theory that they possess completed shells, that the alkali metals have an addiFrom H to He there are 2 el~msot3 tional shell containing one electron that is readily reLi t o Ne 8 moved, and that the halogens have an outer shell that Na to A 8 lacks one electron from being complete. We can K to Kr 18 Rb to Xe 18 bring in evidence other than chemical to add weight Cs to Rn 32 to the theory. But, if the student asks how many Fr to element 118 32 electrons there are in the inner shells, what is the The numbers 2 , 8 , 18, and 32 are numerically identical answer and what proof can be presented for it? I t with numbers that appear in the interesting mathe- took a Nils Bohr, a G. N. Lewis, and an Irving Langmatical series based on the values of 2 n2,where n = 1, muir to elaborate the theory so far, but they did not at fimt give the correct answer to this apparently simple 2, 3, 4, etc.; namely, 2, 8, 18, 32, 50, 72, etc. When the beginning student has reached this point, question. It remained for a Welsh chemist, not he now has something that needs explaining. There sufficiently well known in this country, named Charles is the extraordinary order of the periodic table with R. Bury, to complete the p i ~ t u r e . ~Langmuir had the remarkably simple relationship in the number of made the simple assumption that the inert gases would elements that occur between inert gases. What theory contain outer shells with 2, 8, 18, 32, etc., electrons, can be advanced to explain these and myriad other corresponding to the number of elements between observations about chemical elements? Teachers of members of the inert gas family. I t was known, howchemistry readily give an answer, but how valid is the ever, that the maximum state of oxidation of elements answer they present? Is the theory of the arrangement is eight and that certain elements actually form comof the electrons about the nucleus of an atom to be pounds with this valence number; e. g., osmium in classified as a fact, or is it a hypothesis that is being ' B ~ R YC., R., J . Am. Chum. Sac., 43, 1062 (10213.
Reproduced from " E r ~ l a i n i n g the Atom." by Selig Hecht, the Viking Preea. Ino., New York
Fig-
1
MAY. 1949
OsOl has a valence of eight. Furthermore, Abegg and Badlander in 1899 had advanced a rule stating that the sum of the hydrogen valeucy of an element and its maximum oxygen valency is often equal t,o eight and never exceeds eight. The following quotation from "Introduction to Theoretical Chemistry," by W. B. Meldrum and F. T. Gucker,s gives proper recognition to Bury's contribution: "In 1921, Bury at the University College of Wales, elaborated the Lewis-Langmuir static atom model and concluded from chemicul evidence that the outermost layer of an atom contained at most eight electrons and not, as Langmuir has supposed, the full number to which it could finally be built up. He was thus led to conclude that inner building occurred in the transitional elements, the number of electrons in the outer shell remaining the same, while successive electrons filled up a lower shell. He also postulated that a deeper inner building occurs in the rare earths, almost like that which me accept today. He assigned to lanthanum an electron configuration in successive shells of 2, 8, 18, 8, 3, . . . and concluded that the last rare earth element must have a configuration of 2, 8, 18, 32, 8, 3 . . .. In his own words, 'between lutecium (71) and tantalum (73) an element of atomic number 72 is to be expected. This would have a structure 2, 8, 18, 32, 8, 4 and would resemble zirconium.' In the same year, Bohr independently published almost exactly the same structure.. .. Coster and von Hevesy looked for this element in zirconium minerals and using X-ray diffraction techniques, discovered hafnium (72). . . . Its discovery represents a brilliant application of the theory of the electronic arrangement in atoms. It was not until it was realized that element 72 was not a rare earth that a successful search for it was instituted." Arguments have been presented for the use of the long form of the periodic table because of the clearer relationship it has to the fundamental structure of the elements,=and recently the long form has been brought un t,o date.0 -
American Book Company. New York, 1936, p. 56G. e F o s ~ ~L. n , S., THISJ O T T R N23, A ~603 (1946). 6
Simmons' has recently presented a novel arrangement of the periodic table that has much merit from a pedagogic standpoint, since one needs to refer in elementary courses to the right portion only. The table offered to teachers by Merck and Company, Rahway, New Jersey, also presents Deming's table of valences, that includes both the common and the less familiar states of oxidation. Teachers should reflect on how seldom they have pointed out to their students the existence of any compounds that show an element in an unfamiliar valence state. Is it not more important to teach that silver can have a valence of two and even three, and that iron forms compounds in which its state of oxidation is 4+ (ferrites) and 6+ (ferrates) than to pay homage to an over-simplified theory of atomic structure? It will pay teachers to study the table of valences and to tack copies a t various places in their laboratories to remind them that a tremendous lot of chemistry has gone into setting it up. Honr little of this chemistry do we impart in one short year! The close relationship between the periodic table and the theory of the arrangement of the extranuclear electrons presents, thus, one of the best opportunities for teaching an appreciation of the scientific method, and it is hoped that this brief presentation will stimulate teachers to become more familiar with the properties of all the 96 elements, the structures they are believed to possess, and the correlation between these structures and the chemical behavior each exhibits. When discrepancies of more than a minor nature arise, more work needs to be done to correct the theory. Students should he made to realize that there are still problems to be solved. It is more important to call unexplained phenomena to their attention than to present a glittering picture of past triumphs. When properly introduced, the study of the periodic table, its history and its role in the development of a theory of atomic structure, can he t.he most stimulating part of the course.
