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The Chemistry Student
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THE STRUCTURE OF MATTER. VI. THE PERIODIC CLASSIFICATION OF THE ELEMENTS COMPLETED
In the October number1 we began a discussion of the periodic properties of the elements in the light of our present theories of atomic structure. It is recommended that the reader refiesh his memory of that article before proceeding with the present one. It will he recalled that we began the construction of a periodic table and that the chart facing page 1320 depicted one period of two elements, two periods of eight elements each, and part of a fourth period. It was in the fourth period that we began t o encounter difficulties, culminating in .. the impossibility of fitting iron (atomic number 26) into the eighth column of our table.
The Fourth Period It is evident that we are entering upon a period longer than eight. If we begin with potassium @ud count the elements in the order of their atomic numbers until we.. arrive a t krypton, the next noble gas after argon, we find that we have run through a series of eighteen. (See Table I.) How shall we distribute eighteen elements across eight columns and come out even a t the end? It is true, of course, that many periodic arrangements of the elements contain more than eight columns. At the outset, however, we have said that the maximum number of electrons which can occupy the valence or outermost shell of any atom is eight. The inclusion of additional columns, therefore, would not get us out of our immediate difficulty. The ideas which led to a satisfactory solution of the problem arose over a period of time and should be credited to Rydberg, Langmuir, Lewis, and others. Bury2 finally assembled them, made an important addition or two of his own and offered the simple explanation which we shall now examine. The necessary postulates may be summed up briefly as follows: 1. The maximum and the stable number of electrons for the first "shell" or "layer" outside the nucleus is two. 11. The Periodic Classification of the Elements," 1 "The Structure of Matter. Tnrs JOU~AI., 5, 1312-20 (Oct.. 1928). 2
3. Am. Chen. Soc., 43, 1602 (1921).
TABLEI ATOMIC NUMBERS 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
i4 25 26 27 28 29 30 31
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminum Silicon Phosphorus Sulfur Chlorine Areon Potassium Calcium Scandium Titanium Vanadium Chromium Mgnganese Iron Cobalt Nickel Copper Zinc Gallium
32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62
Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Columbium Molybdenum
... Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Cesium Barium Lanthanum Cerium Praseodymium Neodymium Illinium Samarium
63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92
Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutecium Hafnium Tantalum Tungsten
... Osmium Iridium Platinum ., Gold Mercury Thallium Lead Bismuth Polonium
...
Radon
-
. . Radium Actinium Thorium Protoactinium Uranium
2. The maximum and the stable number for the second layer is eight. 3. The third layer can be occupied by a stable group of eight electrons. Except when the third layer is also the valence layer, however, the maximum number of electrons for this shell is eighteem3 4. A fourth layer of electrons may begin to form before the third layer has completed its maximum group of eighteen. 5 . Certain elements are able to exist in more than one form. In other words, they are able to transfer electrons from the third to the fourth layer and vice versa. The last two postulates are very important for they show us how we can a Some chemists prefer t o divide the group of eighteen into two or more layers. However, since little is definitely known about the actual spatial arrangement of these electrons, we shall, for the sake of simplicity, adhere t o Bury's original scheme.
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distribute eighteen elements through eight columns and a t the same time they offer an explanation for the behavior of certain elements of variable valence. Let us begin our fourth period anew. The structures previously presented for potassium, calcium, and scandium need not be altered. For these elements the electron layer arrangements which we may designate as (2, 8, 8,1), (2, 8, 8, 2), and (2, 8, 8 , 3 ) , respectively, seem to be quite stable and to account satisfactorily for their chemical behavior. When we come to titanium, however, we find that a transition from the eight-electron to the eighteen-electron third layer is beginning. Titanium exhibits not
only the valence of four which we previously attributed to it, but also valences of two and three. The succeeding seven elements also display transition properties, and then, beginning with zinc, we proceed in the normal manner to fill out the period. Table I1 sets forth structures which Bury proposed to account for the characteristics of the elements of the first "long period."
The Fifth Period An examination of succeeding elements, preliminary to beginning the fifth period, reveals the fact that this group should he somewhat like the fourth. Beginning at rubidium and counting until we arrive at a noble gas (xenon), we again g m through a series of eighteea4 A consideration of the chemical properties of these elements, however, shows that we need postulate only three transition members in the series rather than eight as in the previous period. Table I11 indicates the structures assigned
by Bury to the elements of the fifth group. In order to conserve space we have omitted the first three electron shells of these elements, which contain in every case two, eight, and eighteenelectrons, respectively, and have noted only the two outermost layers. The electron layers of atoms are frequently designated by letters, beginning with K for the innermost and proceeding outward alphabetically. Thus, for the fifth group we may write: "hat is, if we include in our count the blank space left far the supposedly missing element number 43.
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K-2 L-8 M-I8 -as indicated in Table I11
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The fact that ruthenium extends into the eighth column should not be taken to indicate that it ever acts like a noble gas. This structure is included to account for the tetroxide which may be represented electronically as follows:
.. o : .. .. : .. :O:Ru:O: .. .. .. :o:
The Sixth Period The sixth period again calls for an extension in length. Thirty-two elements are included in the series from cesium to radon, the last of the noble gases. The additional postulates necessary to account for this fact are : (i. The fourth or N electronic layer may be occupied by stable groups of eight, eighteen, or thirty-two electrons. Thirty-two electrons constitute the maximum possible. As before, it is assumed that electrons may be added to outside shells before the stable inner groupings are completed. 7. The fifth or 0 electronic layer may contain stable groups of eight or eighteen electrons. These groupings also may remain incomplete. while electrons are added to outer shells. Atoms of variable structure are e possible. We can therefore look for two transition groups in the sixth period. The first will correspond with the change from an N layer of eighteen electrons to an N layer of thirty-two electrons. This change takes place through fourteen of the rare earths. The second transition is from an 0 layer of eight electrons to an 0 layer of eighteen electrons. It is in most respects similar to the N layer transition from eight to eighteen electrons which we noted in the fifth period. Here four elements are involvedosmium, iridium, platinum, and gold. For several transition members of the rare earth group, Bury postulated variable structures. The remaining members of the series are supposed to vary from each other only by successive additions of one electron to the N layer as one proceeds down the list.5 Bury's structures for these elements may be indicated as follows: 5 It is understood, of course, that corresponding additions of protons are made to the nucleus. In the present article we have avoided unnecessary complications by ignoring isotopes, although many of the elements discussed are isotopic. If the student so desires, he can refer to the first volume of the "International Critical Tables" and construct for himself isotopic diagrams similar to those shown in the second article of 5,1312-20 (Oct., 1928). this series. THls JOURNAL,
JOURNAL oos CHEMICAL EDUCATION
346
FEBRUARY. 1929
The complete plan for the sixth period (but with the rare earths condensed) is shown in Table IV. Osmium is introduced into the eighth column, not to show any relationship with the noble gases but to account for its tetroxide, as was the case with ruthenium in the fifth period. TABLEIV THESIXTHPERIOD K-2, L-8, M-18 2
3
56 Ba N-I8 0-8 P-2 62, 63 Rare Earths
57 La N-18 0-8 P-2 %71
Rare Earths
* 76 0 s N-32 0-14 P-2 77 In N-32 0-15 P-2 78 PT N-32 0-16 P-2 79 Au N-32 0-17 P-2 80 H g N-32 0-18 P-2
76 0 s N-32 0-13 P-3 77 Ir N-32 0-14 P-3 78 PT N-32 0-15 P-3 79 Au N-32 0.16 P-3
4
58, 59 Rare Earths 72 Hf N-32 0-8 P-4 76 0 s N-32 0-12 P-4 77 IR N-32 0-13 P-4 78 Pt N-32 0-14 P-4
5
6
73 Ta N-32 0-8 P-5
74 W N-32 0-8 P-6 76 0 s N-32 0-10 P-G 77 IR N-32 0-1I P-6 78 PT N-32 0-12 P-6
1 A
1
H 1 .m77 K-I 3 Li 6.939 11-2
L 1
=--
I1
Na 22.997 K-2 L-8 M-1 19
K 39.09t K-2 L-8 M-8 N-l
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-29 Cu 63.57 K-2 L-8 M-18 N-1
-
37
Rb 85.44
K-2 L-8 M-18 N-8 0-1 -
PD K-2 L-8 M-I8 N-15
47
0-3
4g
In
lo?.@
K-2 L-8 M-18 N-18 0-1 55 Cs 132.81
K-2 L-8 M-18 N-18 04 P-1 -
40
114 8 K-2 L-8 M-18 iV-18 0-3
67 La 138 91 K-2 L-8 M-18 N-18 0-8 P-3 68-71
Rare Earths K-2 L-8 M-18 N-18 to 32 0-8 P-3
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79 Au 197.2 K-2
58 M-18 N-32 0-18
P-I -
87
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The Seventh Period The seventh period is but a fragment and may be easily disposed of in the first six columns of our table. There has been some speculation as to whether this fragment represents the beginning of a new period of eighteen or thirty-two theoretical elements. On the whole, its members seem to resemble most closely the elements occurring in the preceding periods of eighteen. Such speculations need not concern us greatly, however, for it is quite evident that these elements approach, and probably reach in uranium, the ultimate limit of size and complexity compatible with a degree of stability necessary for existence. Even these elements break down spontaneously, giving rise to the phenomenon known as radioactivity. Periodic Tables, Charts, and Models Our completed table may be assembled as shown on the folder herewith. Attention to the symbols printed in bold-faced type will reveal that if some means can be found for disposing of the transition members of the rare earth group and the nine other transition element& &bn, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, and platinum, the remaining elements can be arbitrarily distributed in regular series across an eight-column table. In the modern form of MendelCeff's table the rare earths are condensed (much as we have condensed them) ?nd the o t h e r ~ i n eelements listed above are placed in a ninth column O n the right. Some people prefer an eighteen-column table. The possibli forms of tabulatioril' are almost innumerable and nearly every form has ??me merit which particularly endears i t to its maker. Systems which require space models are almost as numerous; most of them are - riations of the helix. The table which we have employed here is not recommended for general use; it is not nearly compact enough, We have chosen this form not because of its merits as a periodic chart but because it illustrates clearly the structures'of the elements and the reason for %e periodicity of their properties. The student will not find it time wasted to arrange a table or model of his own if he is interested in such poject. References to charts and models which have been described in THISJOURNAL are listed below.
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References 1. Geo. W. Sears, "A New Form of Periodic Table as a Practical Means of Correlating the Facts of Chemistry," THISJOUFXAL, 1, 173-7 (Oct., 1924). 2. M. Courtines, "A Model of the Periodic Table," Ibid., 2, 107-9 ((Feb., 1925). 3. W. H. Rodebush, "A Compact Arrangement of the Periodic Table," Ibid., 2, 381-3 (May, 1925). 4 . H. A. Geaque, "A Classification of the Elements with Respect to Their Properties," Ibid., 2, 4 6 H (June. 1925).
5. Ira D. Garard. "A Simole Rule far the Classification of the Elements," THIS JOURNAL. 3, 542-6 (M&, 1926): 6. C. J. Monroe and W. D. Turner, "A New Periodic Table of the Elements," Ibid., 3,1058-65 (Sept., 1926). 7. 0. J. Stewart, "Another Attempt to Base a Classification of the Elements on Atomic ~ t i a u r e , "Ibid., 5. 57-63 (Jan., 1928)
Errata Twp errors in previous articles of this series have been called to our attention.
On page 1316 of the October number, line eleven, Prout is incorrectly written "Praust." On page 1479 of the November number, line two, tetravalent is incorrectly written "tervalent."
