R. T. Sanderson
Arizona
Stote University Tempe
I
I
A Rational Periodic Table
The general feeling about periodic tables, after a century of development and hundreds of snggested modifications, seems to be that the conventional "long form" commonly found on classroom walls and in nearly ail textbooks of chemistry is reasonably satisfactory, so why bother to try to improve it when perfection is admittedly unattainable? After fifteen years of teaching general chemistry, I must admit I am not satisfied. I am tired of apologizing for worn-out notions that persist only historically. I am impatient with the fundamentally meaningless wide physical separation in the chart between beryllium and boron, and between magnesium and aluminum. I t is embarrassing to try to rationalize the assignment of but a single group number to the three distinct transition groups that begin with iron, cobalt, and nickel. The inclusion of zinc, cadmium, and mercury among the transition elements has always been disturbing. The position of hydrogen deserves better thought. The widely prevalent inconsistencies in designating groups as A or B are little short of ridiculous. And finally, I dislike the inhibitions placed upon my own writings and teaching by the outmoded conventions. Let us suppose that we could ignore convention and tradition and extract all the good from past experience and present knowledge without contaminating it with the bad. What kind of a periodic table would result? This paper is written expressly to exhibit my conception of such a table, to explain its features, and to put on record an honest attempt a t a thoroughly rational table. The purposes of a periodic table may vary, and its design will vary appropriately. But let us consider the most common practical purpose: to organize the chemical elements in the most usefully systematic manner, so that both similarities and trends in structure and properties can best be taught to chemistry students. First and foremost, we want similar elements grouped together, and groups placed to represent continuous trends. We know now that similarity in properties results from similarity in atomic structure, and therefore electronically similar elements are logically to be grouped together. Since chemical bonding is the most important fundamental property of atoms, pfobably the most useful major division of the elements 1s baaed on their differences in bonding orbitals. Elements of one class use in bonding only the outermost shell. These we call the "major group" elements. They include all elements having inviolable penultimate shells of 2,8, or 18electrons, together with those having outermost shells of 2 or 8. All other elements can use both outermost orbitals and underlying shell d orbitals in bonding. These we call the "transition" elements, or
"subgroup" elements. They include all elements having partly filled d orbitals in the penultimate shell, and a few more which, although in the ground state their d orbitals are either completely empty or completelv filled, nevertheless can also use these orbitals in bonding. A rationai periodic table should separate these two kinds of elements physically for the simple reason that such a separation is pedagogically useful. The major group elements can very logically be discussed and compared together, apart from the transition element,^, and the latter also make a convenient, selfconsistent unit. As seen in the figure, the table makes this separation, showing clearly where the transition block would fit into the major group block. The troublesome matter of A or B designation is completely eliminated by designating the major groups by the usual numbers precedcd by M, and the transition groups similarly by T. Major Groups
Group number. The group number gives the number of outer shell electrons per atom, preceded by the letter n4. Thus the rare gas elements, whose recently discovered chemistry in no way weakens their rightful position as the corner post of the periodic table, end each period of the table as in the past but have the more logical group number M8. Hydrogen. I n recognition of its fundamental electronic dissimilarity to both the alkali metals and the halogens, and of its similarity in electronegativity to the elements of Group M4, especially carbon, hydrogen is placed over that group, but in a separate independent position, Thus hydrogen, whose outer shell is half filled, is close to carbon whose outer shell is half filled. These two alone of all the elements have neither outer vacancy nor outer electrons left over when all possible covalent bonds have been formed. Zinc, cadmium, and mercury. These elements are placed between calcium-strontium-barium and galliumindium-thallium because they use only the outermost shell~orbitalsin bonding. Unquestionably they have no reason for inclusion with the transition elements, but every bit as much right to major group membership as any of the elements that follow them in each period, to which they are electronically similar in having the closed penultimate shell of 18. Because of their placement in the table beside calcium, strontium, and barium, it is necessary to differentiate the two groups by calling zinc, cadmium, and mercury Group M2'. IS-Shell elements. Although there is probably no important advantage to designating all the major group elements beyond zinc, cadmium, and mercury as M number prime elements, it is important to make a Volume 41, Number 4, April 1964
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PERIODIC TABLE OF THE CHEMICAL ELEMENTS MAJOR
GROUPS
TRANSITION
COMPLETE
clear distinction between the two basic electronic types to he found in the major groups starting with M2. This is accomplished by leaving a small gap not only between Groups 312 and M Z ' , hut also between the 8shell and 18-shell elements within groups. Students need to recognize that they should not expect altogether consistent trends in properties down a group when the electronic type changes so drastically as from a penultimate shell of 8 to one of 18. A comparison of some properties of calcium and zinc, and of calcium chloride and zinc chloride, as given in Table 1, will illustrate very well the effect of filling in the underlying d orbitals 188
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Journal of Chemicol Education
LONG
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without extending the outermost shell. Notice that this change leads in the free elements to more compact atoms (lower covalent and ionic radii, higher density), more tightly held electrons (higher ionization energy and electronegativity, lower electrical conductance), and weaker metallic bonding (lower melting point, heat of fusion, boiling point, heat of vaporization, and heat of atomization). Zinc chloride is correspondingly less polar, less stable, and lower melting than calcium chloride. The justification for including l h h e l l elements with rare-gas-shell elements in the major groups is of course
that both types use the same kind of outer shell orbitals, principally s and p, in bonding. Consequently both tend to form the same kinds of compounds and comparison can usefully be made. Table 1.
Comoarison of Calcium and Zinc
Electronic structure Radius (A): metallic poy~lent lonlc Ionization energy (kcal/mole): first average of two Eleetranegativity Melting point ( T ) Heat of fusion (keel/male) Boiling point ( T ) Heat of vaporization (keal/mole) Heat of atomization (kcal/rnole) Electrical conductance
Ca
Zn
2-8-8-2
2-8-18-2
Transition Groups
Group number. The group number represents the number of electrons in each atom that are beyond the nearest lower rare gas structure. Since this is the number that determines the bonding characteristics of the element, its use is closely analogous to that of the major groups in which t,he number is not only the number of outermost shell electrons, but a t the same time also the number of electrons beyond the nearest lower rare-gas structure or rlosed 1Bshell structure. The T designates the transition series. The iron, cobalt, and nickel groups are appropriately numbered T8, TQ, and T10. The copper, silver, and gold group now becomes Group T11, since no matter how the electrons are distributed in the atoms, copper and silver atoms each have eleven electrons beyond the nearest lower rare gas structure. (Gold has 25 because of the interposition of the lanthanides.) There are no transition groups numbered T1 or T 2 because no transition elements have only one or two electrons beyond the nearest lower rare gas shell structure. Inner transition elements. The lanthanide and actinide elements, which present a transition within a transition, electronically, by the interruption of the d orbital filling in order to fill the underlying f orbitals, are most conveniently studied as two separate series and are therefore so segregated in the periodic chart. This vertical separation is in recognition of the well known chemical differences between the lanthanides and actinides as shown by the prevalence of higher oxidation states in compounds of the latter, especially of the first few members. Such differences are not necessarily a reflection of differences in ground state electronic configurations of the two series but probably result from the small energy differences between 5f and 6d orbitals in contrast to the larger energies required to shift electrons from 4f to 5d orbitals. It seems reasonable to define both series of these elements as those containing incompletely fiUed f orbitals in the shell underlying the penultimate. By this definition there are only 13 instead of 14, the 14th, lutetium, having filled f orbitals.
Lutetium and lawrencium. Although the change from inner transition element to transition element, defining the former as above, is not nearly as great as that from transition to 18-shell element, the situation here electronically is somewhat similar. Lutetium has no more reawn to be classed with the rare earth elements electronically than does lanthanum. For this reason, lutetium is placed in the same transition Group T3 with lanthanum rather than with the inner transition elements below. Presumably lawrencium can be treated in the same manner. Table 2 lists some properties of lanthanum and lutetium and their compounds that suggest the nature of the changes brought about by increasing the nuclear charge while filling in the f orbitals. It can be seen that lutetium, although higher melting than lanthanum, is somewhat more volatile and forms weaker bonds to chlorine and iodine. Table 2.
Comparison of Lanthanum and Lutetium
Electronic structure Radius (A): rnetaliii ionic (Ma+) Density (g/ml) Melting paint ('C) Boiling point (OC) TrieNoride, heat of formation (kcd/l/equiv) triiodide, heat of formation (keal/equiv)
La.
Lu
2-&18-18-9-2
2-8-18-32-9-2
The Complete Long Form
At the bottom of the figure is placed an assembled periodic table, the complete "long form" in its true meaning. Students can learn from this the order of filling in of electrons as the elements are built up in order of increasing atomic number. They can also see how some of the awkwardness of the conventional long form of the periodic table has been eliminated by separating the chemical elements into three blocks.
Prof. A. W. Cordes sends this photograph of an illuminated wall chart in use at the University of Arkansas, Fayetteville. Cost is moderate, ($270.00 excluding labor); materials used are readily available. Symbols can be lit individually. Construction dehils are available for interested readers who request them from Prof. Cordes.
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