VOL. 2. No. 5
Wsssm STATUS OF ma IONIZATION THEORY
381
A COMPACT ARRANGEMENT OF THE PERIODIC TABLE WORTHH. ROOEBUSH, UNIVERSITY OP ILLINOIS, URBANA,ILL. While the numerous and complex physical and chemical relationships of the elements can be shown more completely by a space model, the most useful form of the periodic table is a two dimensional chart. Such a chart can be visualized and if the elements are arranged in a way that is compact and according to a system that is not too complex an enormous amount of information is conveyed to the eye of the advanced student at a glance.
JOURNAL OR CABMICAL EDUCATION
a82
MAY, 1925
The old Mendeleeff table had these virtues andwhile its defects are obvious in the light of recent knowledge, it is still in general use because no new airanzement has a ~ ~ e a r that e d is both simole and comnad. Some recently published tables have the disadvantage of attempting to show too much detail. Since the number of relationships that exist among the elements is so large we may feel sure that no arrangement will be devised that will show everything. It is necessary to sacrifice some relations in any arrangement that we may make and it seems to me that some of the farfetched chemical relations are among the least im~ortant. 1 A saksfactorv arrancement of the periodic table ought to fulfil a t least the following conditions: (1) be arranged according to atomic number; (2) preserve the Rydberg periods; (3) show the arou~inccof the electrons around the W. H. R o ~ ~ s u s ~ r nucleus; (4) show the number of valence electrons; (5) show the real chemical resemblances; (6) indicate the degree to which an element possesses the property which is commonly designated as electronegativity. Any other relations appear subordinate to those enumerated above. The accompanvincc . -table' is a tentative arranccement. I shall arobablv modify it a good deal but I &-, Y.rr r s 4 r 6 7 ( 8 , r a 3 r r r 6 r""""" believe that its generalform y. will prove permanently satL..,., isfactory. It is based upon LI a e c q 9 E 2.5' ,& b L B, ,",,* m Bohr's . conception of the mqo, n $ 2 F : 4 y grouping of the electron orv , a* ,: I:. 82, ;a". .,a*r*."r",. bits without necessarily ac5 :;,,g &E&+fd & ,kg,& ., -y cepting Bohr's views as to ,, , . , c . , - ~ , p ~ ! : " S . % ' p ..,. . " the motion of the electrons. ,,"."-."* ,,-..'9 &;*g ,2 ,, Such a group, be it made up of stationary electrons or electrons moving in orbits. we shall call a shell. following the terminology of G. N. Lewis. The arrangement consists of 16 columns and seven rows or periods of 'See Science, N . S., 59, 430 (1924).
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VOL.2, No. 5
THE'VALUE
OP
TEACHING VALENCE
383
elements. The atomic number and atomic weight are printed below the symbol for each element. Each row represents an additional shell of electrons and the maximum number of valence electrons is characteristic for each column starting with 1 for the alkali metals a t the left and running up to the maximum of eight possible to the platinum group, starting over again with 1 for silver and running up to 8 for the inert gases. The distinction between the valence and the number of valence electrons must be emphasized of course for the student. Hydrogen is placed with the balogens where it belongs and the number of valence electrons for hydrogen and helium is printed in parentheses above the symbols for the elements. The complete shell structure for the inert gases is shown a t the right. Some would prefer to designate the shells alphabetically as K, L, etc. Those elements whose atoms are capable of taking on the same valence and external (i. e., chemical) properties are assigned to a single place in the periodic table, e. g., the rare earths. It could be argued that cobalt is no more like iron than is manganese, but iron and cobalt have a potential maximum of eight valence electrons while manganese has not. The actual maximum numbers of valence electrons for the two middle columns is a variable quantity and this is indicated by the parentheses. Cerium is an element which might appear twice in the table. The only important chemical resemblances not shown are relations such as that of magnesium to zinc and this relation can probably best be shown by dotted lines dividing the table into zones of approximately equal electronegativity. If i t might be permissible to introduce a qualitative formula into a science which is rapidly becoming exact, we might represent the electro-
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negativity as a function of - where V is the number of valence electrons
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and S the number of shells. The basis of this formula is Coulomb's law and I believe that in a few years we shall calculate the energy changes in chemical reactions by means of it. No claim is made for originality in this arrangement and many alterations and additions will suggest themselves to the critic.
