Electron Configuration as the Basis of the Periodic Table


production of a large number of modifications of the. Mendeleeff chart (7). Yet in practically none of these variations has any departure from the ori...
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Electron Configuration as the Basis of the Periodic Table V

W . F . LUDER

Northeastern University, Boston, Massachusetts

D

ISSATISFACTION with the original form of the penod~c . . table of the elements has resulted in the production of a large number of modifications of the Mendeleeff chart (7). Yet in practically none of these variations has any departure from the original proposal of Mendeleeff been made. With but one or two exceptions all of the charts are based primarily upon the order of increasing atomic numbers. In view of the many difficulties and inconsistencies which still remain in all periodic tables so arranged, i t would seem that the time has come for a radical modification of the fundamental proposition. The basis of a more satisfactory periodic chart is to be found in the arrangement of the electrons in the various levels and sublevels according to the quantum theory. Several attempts have been made to do this and a t the same time retain the order of increasing atomic weights. The resulting charts have eliminated most of the old difficulties and have introduced few new ones, but they have an unnecessarily complicated and cumbersome appearance. To secure compact simplicity, it seems necessary to abandon the restriction that the elements must he arranged in order of atomic numbers throughout the whole tahle. The sole basis for the chart herein advocated is the electronic configuration of the atoms, which is, after all, the only logical basis for a chemical chart of the elements. DEFECTS OF THE MENDEL~EFFCHARTS

The various modifications of the Mendeleeff chart have been classified (7) as follows: (1) Short chart, (2) Long chart (Werner type), (3) Long chart (Bayley type), (4) Spiral (Baumhauer type), (5) Helical (Harkins type), (6) Miscellaneous. Most of those included in the spiral, helical, and miscellaneous types are too cumbersome for convenient use. Practically all the tables now in use are modifications of the long or short charts. Zmaczynski (9) lists as defects of the short chart:

1. Unrelated elements such as chlorine and manganese are placed in the same group. 2. No clear separation of metals from non-metals is possible. 3. Elements which give colorless and diamagnetic ions are not distinguished from those which give colored and paramagnetic ions. 4. The table has no place for the rare earth metals. 5. The table does not reflect the electron configuration of the atoms.

At least two more should be added to this list: (6). the doubtful position of hydrogen (since hydrogen resembles the halogens in forming negative ions it has. no definite place in the tahle); (7) the failure of the tahle to retain beyond the third period the continuous variation of properties from active metal to active nonmetal as electrons are added to the outermost shell of the atoms. When the addition of electrons to the outermost shell is broken off a t the two 4s electrons of calcium so that the third shell may be completed with 3d electrons beginning with scandium, no chart base$ solely on an order of increasing atomic numbers can solve the difficulty. The long chart most often proposed as an alternate form of the periodic tahle (2) (Figure 1) overcomes the first three of the defects, but still suffers from the last four. Moreover, the first three defects are eliminated only by introducing another, which, in the opinion of some, is even more objectionable. When the table is merely stretched out to eighteen columns, the short second and third periods must he split to make room for the so-called "transitional" elements. The continuous gradation of properties: from active metal to active non-metal, which is characteristic of the successive addition of electrons to the outermost shell of the atoms and which has always been considered one of the most important features of the periodic table, is completely overlooked in the simple eighteen-column chart. Whereas the short chart is defective in this respect beyond the thud period, the long chart ignores it entirely. Some advocates of the long chart have claimed it t o be based on atomic structure and that it is the modern form of the periodic table. The long chart is hardly modem, since MendelM himself first proposed it in 1872 (7). The fact that it ignores the hreaks in electron configuration at calcium, strontium, barium, lanthanum, and radium, and, even more important, that it introduces non-existent hreaks between beryllium and boron, and between magnesium and aluminum effectively disposes of the claim that the long chart is based on atomic structure. In short, the long chart eliminates three of the seven major defects of the short tahle, leaves three more unchanged, and greatly exaggerates the seventh. It would seem, therefore, that there is little net gain in the use of the eighteen-column chart. The spiral and helical forms of the Mendele& chart in many cases do eliminate some of the seven defects, but practically all of them violate one of the most

!1

CmrP Numbers Pctiod

--I

1 2 Elements

/

8

Ib

I

Inert

2b

Gaae3

--

--

---------

--

--------

.-

----

He 2

H

1 -Li

11

8 Elcments

Be

3 4 --

IU 8 Elemcnts

Na

K

Transitional Elements

Ca 20

21

Rb

Sr

Y 39

37 38 Cs 55

VI 32 Elements

Ba

VII 6 Elements

87

Cr

-Fe

Mn

Co 27

NL

~h 45

~d

22 23 24 25 26 28 291 --

18 Kr

C u t Zn

36

30

1-

Zr 40

Cb 41

MO

~a

42

43

RU 44

A ~ ;c d

46

47, - - ..HI Ta W Re 0s IT Pt AY 72 73 74 75 76 79; 77 78 - - --, ------I-

r.

Ra 88

Th 90

Ac

89

48

1-

La;5*7O *:LU 57 I,& -. :71

56 --

-

V

TI

Sc

19 --

V 18 Elements

Nr 10 A

Mg

11 12 , -

IV 18 Elements

-

U

Pn 91

Hp

80

I

"I "I" ---

92

[Token from 1. C a a ~Eoac., . 16, 409 (193911

FIGURE 1.-LONGCAART

important requirements of a convenient periodic table, namelv: comoact sim~licitv. Most of the miscellaneous tipes lis&d by the ~ u i m (7) s suffer from the same fault. Only one of them, that of Mitra (5), will be noted here. I t is one of the early attempts to base the periodic table on electronic configuration. REQUIREMENTS OF A SATISFACTORY PERIODIC SYSTEM

A satisfactory periodic system of the elements should eliminate all seven defects of the atomic number chart without introducing any additional difficulties. Furthermore, it should have a compact simplicity in its arrangement. This seems to be impossible without abandoning the arbitrary requirement that all the elements throughout the whole table must succeed each other in the order of their atomic numbers, but

2 8 2 8 8 2 8 2 8 18 2 8 1 8

Na Mg Al

Ne A

K

18 18 18

1

Xe

S

CI

Ga G e

Se

Ti

V

Y

Zr

Cb* Mo* Ma* Ru* Rh* Pd* Ag*

Cr* Mn

Fe

Co

Ni

Cu*

As Se Br

Rb Sr

Kr

8

P

Ca

zn 8

Si

that requirement is merely a matter of habit and there appears to be no valid reason why chemists should continue to be handicapped by it. This is especially true now that tables of the arrangement of electrons by quantum number are so well known. A brief consideration of one of these tables is sufficient to make any discemiug chemist willing to discard the strict atomic number order for the periodic table. These requirements can be met by changing the basis of the periodic system f r m that of the order of atomic numbers to that of the electron configuration of the atoms. Furthermore, when this is done Mitra's (5) idea of combining the periodic table with the quantum number table is fully realized. The chart should not only overcome all existing defects without introducing new ones, but should "at the same time indicate without recourse

I

Cd In Cr Ba

Sn Sb T e I

32 H g

TI Pb Bi

Po

..

I

I

I Ce Pr

Lu

Hf

Ta

W

Re

OE

IT

Pt

Au*

..

..

I TmYb

I

to a second table, the nature of the electron configuration which is responsible for the periodic classification and the group division" (5). E-IER

ATTEMPTS To BASE THE PERIODIC SYSTEM ON ELECTRON CONFIGURATION

Since the tabulation of the auantum numbers for all the atoms, several writers haveproposed periodic charts based on electron configuration. One of the earliest of these was that of W t n (5). Mitraps table is an eight-column table laid on its side. The groups are horizontal, rather than vertical. The use of only eight columns makes the table complicated and prevents the elimination of several of the seven defects. One of the best of the earlier electron configuration tables is that of Gardner (3) (Figure 2). This chart overcomes the first five of the seven defects, but is prevented from being completely successful by rigid adherence to the order of increasing atomic numbers. In order to maintain this order the chart has to be stretched vertically and the last of the p, d , and f electrons fail to follow their natural sequence. However, it should be noted that this is the first chart which properly emphasizes the existence of four types of elements: (1) those with all electron groups complete (the inert gases); (2) those with one incomplete group (the representative elements); (3) those with two incomplete groups (the "transition" elements); (4) those with three incomplete groups (the rare earth elements). The asterisk "marks elements for which the 'normal' atom is thouzht to have onlv one electron in the outermost group, but as practically all these give divalent ions, the point is of minor interest chemically." So far as the chemist is concerned all the "transition" elements have two electrons in their outermost shells. "The upper limits of covalencies 8, 6, and 4 are marked by heavy horizontal lines." I t is evident that this

curvature and that when straightened out it bears considerable resemblance to Werner's long chart (7). He is able to retain the order of atomic numbers by replacing potassium, rnbidium, and cesium in the first group with copper, silver, and gold, and by placing o.,.tUm I

: 1 1 l ?TI l 1

.

.

-

* , -

l(?)l? 1 ' 1

-

chart comes close to fulfilling all the requirements of a satisfactory periodic system. Zmaczynski (9) uses a spiral imposed on a cone instead of a cylinder, then cuts the cone at the inert gases for plane representation (Figure 3). A glance at his table will show that it was unnecessary to retain the

zinc, cadmium, and mercury in the second group at the expense of calcium, strontium, and barium. The similarity between the two charts proposed independently at about the same time by Gardner in England and Zmaczynski in Russia1 would be remarkable were it not for the fact that both are thoroughgoing attempts to base the periodic system on electron configuration. Both use 32 columns. Both emphasize the existence of several types of elements. (Zmaczynski calls them "normal," "abnonnal," and "twiceabnormal" referring to the electronic structures of the representative elements, "transition" elements and rare earths.) Both overcome five of the seven defects. But both fail to provide a definite place for hydrogen or to arrange the representative elements consistently. The reason for this failure is the reluctance to abandon the order of atomic numbers. Apparently Van Rysselberghe (8) was the first to recognize the necessity of departing from the order of atomic numbers (Figure 4). However, as will be seen, it is unnecessary to deviate as widely as Van Rysselberghe did. The table does emphasize the fact that "chemical properties depend more upon the azimuthal . -

-

1 Although his chart was not published in this country until 1937, Zmaczynski states in a footnote (9) : "After I had used the table in my classes for four years it was printed in 1934 by the

White-Russian State Publishing House. Minsk."

quantum number I of the last electron added than on the principal quantum number n." But this can be shown equally as well without breaking up the table quite so much. Further examination of the chart will reveal that the table overcomes some of the defects of the atomic number charts, but introduces one or two of its own. It does not seem to be so good as Gardner's which was apparently unknown to Van Rysselberghe. The clue to a satisfactory arrangement of the periodic system according to electron configuration was given by R. L. Ebel (1) in 1938 (Figure 5). In Ebel's table the basis of the arrangement is the position of the "differentiating electron." Plotting the atoms according to the positions of their differentiating electrons makes it possible to build up a compact, selfcontained, completely satisfactory periodic chart. Ebel's three "levels" can be combined into one table (4). Experience in using the resulting table ib the classroom has confirmed its usefulness and has led to its combination with the quantum number table (Figure 7). THE ATOMIC STRUCTURE CHART OP THE ELEMENTS

The chart shown in Figure 7 overcomes all seven of the defects of the atomic number charts without introducing any new difficulties and, in addition, combines the quantum number table with the periodic system. Considering the atoms to be built up by the addition of one electron a t a time as the atomic number increases by one unit of positive charge, the differentiating electron (1)in a particular atom is the additional electron as the atomic number is increased one unit. In other words, the differentiating electron is the additional electron which makes an atom different from the atom of the preceding element in order of increasing atomic numbers. For example, the differentiating electron for Be is the second electron in the second shell. This THE PERIODIC TABLE Ths Rc9rcscnloliua Elrmrnls (Lcud I ) 2

3

4

5

6

7

The Reloled Mclnls (Lewd 2 )

Thc rmc rorlhs (Leuel 3 ) ElrLronsl9 20 Orbits N Ce Pr

21

22

Nd I1

23

24

25

26

27

28

29

30

31

32

Sm Eu Gd Tb Dy Ho Er Tm Y b Lu

ITakmfrarn 3. CHBI. FIGUR~ 5.-EBEL'S

EDVC.,15, 575 (1938)l

electron differentiates Be from Li which has only one electron in the second shell. The position of an atom in the chart is given by plotting the location of its differentiating electron. The shell (principal quantum number n) in which the differentiating electron lies is taken as the ordinate; its number in that shell is taken as the abscissa. For example, Al occupies its position, because its differentiating electron is the third in the thiud shell (principal quantum number n = 3). This electron is the first 39 electron. The differentiating electron for Si is the second 3p electron, or the fourth electron in the third shell. So on across the chart until the 39 sublevel is filled with the eighth electron in the third shell for argon. The differentiating electron for Sc is the ninth in the third shell, the first 3d electron, so Sc must be placed in the ninth column and thiud row. The differentiating electron for Ti is the tenth in the third shell and so on across to Zn where the 3d level is completed. From Sc to Zn electrons are being added to the second from the outermost shell. Most of these elements contain two electrons in the outermost shell. The exceptions, Cr and Cu, in which according to the quantum number table one of the outer electrons

has dropped back into the next shell below, will be considered subsequently. With Ga the fourth shell resumes filling up from two to eight for Kr. Ca and Ga are adjacent in the chart, hut the atomic numbers are 20 and 31. This is the first break in the order of atomic numbers necessitated by arranging the table according to electron configuration. At this point i t may be convenient when presenting the chart for the first time to students, to refer briefly to an energy level diagram such as that of Pauling (6) (Figure 6). This seems unwise for elementary students. A brief reference to the spectroscopic tabulation of quantum numbers is sufficient. With more advanced students the graphical representation of the energy sublwels in Figure 6 saves considerable verbal explanation. Visualization of the 3d sublevel lying between the 4s and 4p sublevels and of the 4d lying between the 5s and 5 p sublevels, and so on, is quite convincing. Similar interruptions in the order of atomic numbers occur between Sr and In, La and Hf, Ba and TI, and after Ra. The one between La and Hf occurs after the ninth electron (the first 5d electron) in the fifth shell is

added to La. The addition of electrons to the 5d subshell is halted here until the 4f orbitals are completed with Lu. Thus the rare earths have a logical position in the chart. They are so much alike because their differentiating electrons are added to the third from the outermost shell. Following through the chart in this manner makes evident the fact that Mitra's goal (5) of combining the quantum number table with the periodic system has been fully realized. The electron configuration of any element can he read off the chart aha glance. Even the shape of the chart indicates the manner in which electron shells are built up. The most remarkable feature of the arrangement according to the position of the differentiating electron is the way in which the elements fall logically into three separate groups. The representative elements in which the differentiating electrons are in the outermost shell form the "skeleton" of the periodic table. These elements exhibit the whole range of properties from active metal to active non-metal and beyond to inert gas. The valence electrons are in the outermost shell with a complete shell just below. The continuous

gradation of representative properties persists throughout, instead of ending with Ca as is the case in the atomic number charts. To be sure, some atoms have 18 instead of 8 in the preceding shell, but as pointed out by Van Rysselberghe (8) "chemical properties depend more upon the azimuthal quantum number, 1, of the last electron added than on the principal quantum number." In the related metals (a term proposed by Ebel (1) which seems more logical than "transitional elements") the differentiating electrons are in the second from the outermost shell. The d orbitals are being completed while the s orbital is complete in most cases. The exceptions such as Cu and Cr in which the s orbital contains one electron in the "normal" atom are, as Gardner pointed out, "of minor interest chemicauy" since practically aU of them do form divalent ions. In other words, for most purposes the related metals may be considered as all having two electrons in the outermost shell. As a result they are all much alike. Except for Zn, Cd, and Hg which have completed d orbitals, the related metals exhibit variable valence, paramagnetism, and colored solutions. This behavior is due to the small energy difference between the two electrons in the outermost sheU and the incomplete shell just below. In the rare earths, which now have a place in the chart, the differentiating electrons are in the third from the outermost shell. The 4f orbitals are being fiUed in while two 6s electrons remain in the outermost shell and one 5d electron remains in the second from the outermost shell. Therefore, aU the rare earths are very much alike chemically and exhibit their common valence of three. The positions of H and He give hydrogen a definite place in the periodic table. Two electrons in the first shell are very stable. Therefore, He is an inert gas. When it is placed next to hydrogen according to its electron configuration, there is no difficulty in pointing out why hydrogen behaves both like the alkali metals and like the halogens. The inclusion of He in the second column does not mean that it might be confused with the alkaline earth family; it merely emphasizes

the stability of two electrons in the fust shell, a condition which persists throughout all the other atoms. The non-metals are separated from the metals by the heavy line running from lower right to upper left across the representative elements. This line divides the representative elements almost equally between metals and non-metals. Non-metals do not occur elsewhere in the table. The periods remain the same in this chart as in the atomic number charts, running from an alkali metal to an inert gas. The sequence of electron'.addition in each period is indicated in the first column on the lefthand margin on the table. Other features of the chart will be evident upon further study and especially upon use in the classroom. CONCLUSION

Use of the atomic structure chart of the elements in his own classes during the last three years, fust with freshmen, then with physical chemistry students, has convinced the author that such a table is a great improvement over the Mendel6eiI type. Invariably the students are much better satisfied with it than with the older charts. Similar reports have come from a few high-school teachers who have tried it. I t may not be wise to go into excessive detail in elementary courses, but experience indicates that the use of the chart itself is advantageous at any level of instruction. This is to be expected. The atomic structure chart does much more than eliminate the obvious defects of the Mendele& charts because it is founded upon a more fundamental basis--electron configuration. LITERATURE CITED

EBEL. J. CHEM.EDUC.. 15.575 (1938). FOSTER, %bid.,16,409 (1939). GARDNER, Nature, 125, 146 (1930). LUDER, J. CHEM.EDUC., 16,393 (1939). Mrrrth, Phil. Mag., 11, 1201 (1931). P A ~ I N"Nature O, of the chemical bond," Cornell University Press. Ithaca. N. Y., 1939. QUAMAND QUAM, J. CREM. EDUC., 11,27,217,288 (1934).

VANRYSSELBERGHE, ihid., 12,474 (1935). ZMACZYNSK 14,232 I , ~ ~(1937). ~~.,