Let Us Make the Table Periodic J. Arthur Campbell Harvey Mudd College, Claremont, CA 9171 1 Most chemistry classrooms display a wall chart lahelled "Periodic Table of the Elements". What does a student see? Well, there is a set of rectangular boxes arranged in rows and columns, each box containing some alphabetical symbols and numerals that are difficult (even impossible) to read. Nor is any of the symbols periodic. Not even the format is truly periodic, no matter how one views it. Even more serious in the long run is the lack of great usefulness of the svmbols and numbers. Elements are difficult to locate, and the atomic numhers and weights say little about chemistry. Yet a major emphasis in chemistry, and perhaps the major difficulty in learning chemistry, is the interpretation of observations in terms of atomic behavior. But neither the atomic weight nor number, as such, has much to do with chemical properties and reactivity. Even stoichiometry requires much mire than atomic weights and numbers for real comprehension. Some wish to chanee the form of the table. I would sueeest that changing the content is equally important. ~ o r e & a n 100 forms of the table have been suggested since Newlands, Mendeleev, and Meyer got the idea started (1-3). It is likely the suggestions will continue to come (4,5). In the 1930's there was a surge, accompanied by much pounding on the academic table as to whose form was right. Recently there has heen a large attempt to table any further change as shown bv numerous letters to Chemical and Engineering News. I have never seen all the suggested forms, but each of the ones I have seen shows no obvious periodic function. The form varies, but the emphasized content of each hox is usuallv still a nonoeriodic chemical svmbol for the element, an atomic numbk, and an atomic weight. None of the symbols is directlv. or ohviouslv. correlated to the form of the tahle. The for&& itself is seidom periodic more than twice, sometimes onlv once. perhaps this interprets why such an aperiodic table is not readilv used and understood by students. Recall also that a further problem with wall ch& is that even the aperiodic symbols are hard to read from much of the classroom. So at least two problems exist: legibility and rationality. Here is a suggested approach to increasing both legibility and rationality. Use a format and set of symbols that are visible from a large classroom distance and that are obviously periodic. Wall charts are difficult to make, so for some yearn I have photncopied a single sheet, and given one to each student, marked "DO NO'I' WRITE ON THIS SHEET". One side bears a truly ~eriodictable, shown here as the figure. (A wall chart, of course, would be more professionally designed, with less symbol overlap, some color [for example, to indicate relative electronegativity], and much more visibility. But I am stuck with the 8.5 X 11limitation in black and white.) The other side bears the standard alphabetical list of the elements with their symbols, atomic numbers, and atomic weights. This alphabetical list serves as an excellent index to the periodic table chart. The numerical side of the photocopy also contains a set of standard values for the most used fundamental physical constants. Thus, I do not expect the student to memorize these numbers, symbols, and format of the table. What use would such memory be in 20 years? Even in 1 year? Murh moredesirable is the student's learning, for -A
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the long term, to understand and use intelligently these materials for correlating knowledge at the chemical, that is, the atomic level. For most people concentration on the atomic level is a main characteristic and Dower of what we call chemistry. But not all seem to agree 16), though I do. I will first discuss the symbols, then the format.
Symbols
Notice that most of the symbols used in the figure are periodic from row to row and are nonperiodic (that is, are familial) in the columns. The most visible symbols are the circles (should he spheres) showing relative atomic sizes and ionic sizes. It is true that "best" values of such radii vary from researcher to researcher, hut seldom more than a few per cent (7). The values used here are half the smallest internuclear distances in crystals. For most of the radii the elementarv. or monatomic. m t a l is used. For some. such as oxygen andnitrogen, crys&l&e compounds are used. Atomic and ionic sizes are clearlv periodic in the rows. and familial (increasing) in the colum&. The rows show rather smooth trends, with large values at each end and a small maximum in atomic sizes near the d-to-p transitions of columns 12 and 13. The next most visible symbols are the valence electron symbols (s'-~, pl", dl-'O, fl-14), each clearly allotted to a family value for each column. In the rows, each is also readily related to the column and is periodic from row to row. For the s column the valence electron numbers are 1or 2. For the p columns they are the last digit of the column minus 2 (for the selectronsJ. For thed columns the number of d electrons is usually the column number minus 2. And for the f families the number off electrons is usuallv iust the number of the element in that f row. Valence eieitron numbers are, of course, one of the most difficult items to correlate closely with the format of any periodic table. Ionic charges are also clearly familial (constant) in the columns, and periodic (vary in a repeating fashion) from one row to the next. The onlv aperiodic svmhols are the elementarv svmhols " and atomic nLmbers (ihich are correlated with the normal index table also handed out on the rear of this sheet).and the underlining of the elements that have only radioactive isotopes. The barred rectangle at atomic number 114 sueeests thk next element most licely to be synthesized accordiig to current theory. So the symbols are selected to be those most useful to a chemistry student both in rationality and legibility. The lack of perfection in rationality is inherent not only in the numbers of d and f electrons. The student sees that hydrogen (H) and helium (He) have some ambiguity in position as shown by their dotted and solid-line boxes. Other periodic symbols could be added (but not on a wall chart) (8.9).Incidentally, a few of my symbols have small errors, which I encourage students to find. ~~~
Format
There has been, and is now, a great set of arguments on whose periodic table is best. Most of the arguments have little logic. (A unique hest is hard to define also.) I agree with scientists such as Fernelius in his excellent article (lo),and do not areue for anv articular format. but do uree usine sizes as a k a i n presented set of data. knd, having chose; sizes for use, the format in thefigure works well. Volume 66 Number 9
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The overall sequence, of course, is based on atomic numbers. The four blocks (sl-=, ~ l - dl-lo, ~ , P I 4 ) are based on valence electrons, and the breaks in the table correspond to those hlocks. They also correspond to appreciable changes in the rate of size variation. The f hlock has a very slow and small variation in atomic sizes. The d block has a bit larger and faster variation. The n block has a still lareer and faster one, and the s block (smail though it i d has tKe fastest and lareest variation in size in the rows. Variations in each block, both familial and row, are readily correlated with valence shells and the extent to which the outer electrons are shielded from the nucleus. The increases in size that occur within each lone row near columns 12 and 13 are readily correlated to the d : t ~ transition, -~ as is the large jump fr& p6 (noble gases) to s' (alkali metals). Of cour8e. it must be emphasized that the 05 to o"umo in size is nrimarilv due to a shift from covalen&adii'in>5 to van der w a d s radii in p6. Properties
The valence electron block format has the considerable virtue, especially when atomichonic sizes are used, of correlating very well with many, actually most, chemical properties and reactivities: metallic, covalent, ionic bonding; polarities; color of elements and compounds; oxidation states; redox potentials; acidhase properties; solubilities; melting and boiling points; entropies, free energies, enthalpies; positive ions &dsmaller than their neutral atoms, negative ions are larger, isoelectronic ionslatoms decrease in size with increasine atomic number. ~1mos;'every treatment in a chemical text is enhanced if tied to atomic sizes. After all, chemical forces are coulombic
A mOdlfled perlodlc table wim symbols and data showing perlodlc properties
740
Journal of Chemical Education
and spin related at the most fundamental levels. As a result there are very few, if any, chemical properties that are not closely related to size and valence shell occupancy. (Atomic weights and numbers have much more remote, if any, correlation.) Summary
There is no overwhelmingeffort here (after all, how many would respond positively?) to argue for the format of the figure. What I do urge is that we stop basing the visible periodic table on data that led to its discovery about 100 years ago. Instead, let us concentrate on the current data, sizes and valence electrons, which are periodic and provide simple and understandable bases for beginning, and advanced, students to understand ohsenratious of the properties and behavior of the world they see. For this, they really need to know something about the components of that world. Atomic/ionic size works. Let us make the periodic table of the elements live up to its name. Feel free to photocopy the figure as a first step. Literature Clted
Puddepbatt, R J.; Mowhan, P. K. Tho Periodic Toble of the Elements, 2nd ed.: Clmndon: 0Xfo.d. 1986. 2. Mazurs, E. G. Gmphic Rrpreaonfofian of the Period System during One Hvndmd Years. 2nd 4. Uni ;v. of Alabama 1974. 3. Spronaen,J. W. Van TheP~riodirSrat~maf thDChDmieolElemenfr,AH~toryofthe 1.
Finf Hundred Yeom;Elsevier: A m a t d a m , 1969. 4. ~d hoccommitfoe,ACS, H. Bent, Cbair (appom+d 1988).aoeChem.Eng.Neus1987, (April W.31; 1986. (Jan.27).21. 5. Guonther. W. B. J. Chem. Edue. 1987.M.9-10. 6. Bent,H. J Chem.Edu.1986,63,878-379. 7. Mason.J.J. Chem.Edue. 1'388.65.17-20. 8. Stmng, J. A. J. Chsm.Educ. 1386.63.834-836. 9. Goth, G. W . J. Chem. Edur. 1986,63,836837. 10. Femelius, W. C. J. Ckm. Educ, 1986.63.263-266.
