JOURNAL oa CHEMICAL ZDUCATION
542
MAY,1926
A SIMPLE RULE FOR THE CLASSIFICATION OF THE ELEMENTS IRA D. GARARD,NEW JERSEYCOLLEGEPOR WOMEN, RUTGERS UNIVERSITY, NEWBRUNSWICK, N. J.
Most of the schemes for classifying the elements are based on Mendelejeff's Periodic Law or some modification of it. The advantages of these systems are discussed in text-books and are well-known. I n order to benefit by these advantages, however, the average chemist must have his table ready-made because of the difficultiesin making one, which are as follows: (a) The periods are of different lengths and so the properties are not periodic functions of the atomic weights. (b) The weights reverse the positions of argon and potassium, cobalt and nickel, and tellurium and iodine. ( c ) Each group contains two families into which the elements can he placed only by a knowledge of their properties. The second defect is removed by the use of atomic numbers instead of the weights, but the first and thud persist. For these reasons the present paper proposes a scheme which may be used to place the elements in the usual classes without recourse to any properties except the atomic numbers. It is, therefore, of maximum service in learning or using the chemistry of the less known members of the families.
The Basis of Classification The present classification is made from the atomic numbers, that is, the number of electrons outside the nucleus of the various atoms. Furthermore, the arrangement of these electrons is taken t o be in layers which require, passing from the nucleus outward, 2,8, 8, 18, 18,32, and 32 electrons, respectively, for saturation.' The assumption is now made that the chemical nature of an element is determined by the number of electrons in the outer layer or rather by the state of saturation of that layer. The first case is that of complete saturation, represented in the first layer by the element with 2 orbitular electrons (He) ; in the second layer by the element with 2 8 (Ne) ; in the third by that with 2 8 8 (A) ; and so on. These, having the outer layer saturated, show no tendency t o lose electrons, and likewise, none to gain them in a layer which contains no electrons in the neutral atom. These elements, therefore, are inactive chemically. The second case is that of those elements which have one electron in I), Na (2 8 1) and so on. I n the outer layer, that is, H(1), Li (2 this case the tendency of the element is to lose its one outside electron and become positive, with a valence of one in combination. In the cases which follow there will be, for example, in every layer except the &st, the elements with two, three, four, five, six, and seven electrons, and so we might expect a positive valence of two, three, four, five, six, ' Langmuir, J. Am. Ckem. Sac., 41, 868 (1919).
+
+ +
+
+ +
VOL.3. No. 5 A SIMPLE RULEPORTRE CLASSIPICATION OF THE EL&MSNTS
643
and seven, respectively. This in most cases does, of course, exist. However, as the layer approaches saturation there is an increasing tendency to gain enough electrons to saturate it completely. For example, the element having seven electrons in the second layer (F) is characterized by the strong negative valence of one. Between those elements whose outer layers contain one electron and those which lack one for complete saturation, there are several elements whose properties are in some instances determined by the number of electrons present in the outer layer and in other instances are determined by the number of electrons lacking for complete saturation of that layer. These principles are made the basis of the present classification. In the case in which the second layer is the outer one, there are the following elements: Symbol
L i B e B
At. No.
3 1
Electrons in outer layer
4
2
5
3
N
C 6
7 4
5
F
0 8
9 6
1
7
Ne 0
n
These elements may be placed in families either because of the outer electrons present which are 1 , 2 , 3 , 4 , 5 , 6 , 7 , or n; or because of those which are lacking which are the same in the reverse order and so are represented by n-7, n-6, 72-5, n-4, n-3, 7 - 2 4 , and n-1, where n is the number of electrons required to saturate the layer, in this case, eight. This makes a decision necessary since any element may fall into either of two families (except the inert gases which all fall in the n family). That is, carbon might be in the 4 group or in the n-4 group, nitrogen in the 5 or the n-3 group and so on. For the purpose of this decision the following arbitrary rule is the most useful: If the number of electrons id the outer layer is less than forty per cent of the number required for satnration, the element belongs in the direct family (1, 2, 3, etc.), $more than forty per cent, in the indirect family (n-7, n-6, n-5, etc.). This rule covers fifteen families and includes all of the elements which involve the first three and the seventh as outer layers and most of those whose valence electrons are in the fourth, fifth, and sixth layers. The rule is very simply applied. Take, for example, aluminum which has three electrons in the third or outer layer (Table I). Since this layer requires eight electrons for saturation and three is less than 40 per cent of eight, aluminum belongs in the 3 family, while silicon which contains more than 40 per cent of eight electrons in the outer layer belongs in the n-4 family. I n Table I, those elements which cannot be placed in the above families are inclosed in braces. The fifteen families so far mentioned do not include all of the elements whose outer electrons are beyond the third layer. I n the fourth layer, the 7 group takes manganese and the n-7, copper, which leaves nickel, cobalt, and iron that do not fall into the above families. Since the number of
TABLEI
THEATOMICNUMBERSOB
H He
1 1 2 n
second
layer
Li Be
3 1 4 2
a :: N 0
F Ne
7 8 9 10
5 6 7 n
THE
ELEMENTS AND THE NUMBER OP ELECTRONS IN THE OUTERLAYERS
19 20 Sc 21 Ti 22 V 23 Cr 24 M n 25 Fe 26 Co 27 Ni 28 Cu 29 Zn 30 Ga 31 Ge 32 K Ca
1
1 2 3 4 5 6 7 8 9 10 11 12 13 14
37
Rb Sr Yt Zr Cb Mo
!
Ru Rh Pd Ag Cd
In Sn Sb
Te I Xe
'
38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 n
Seventh layer
87 88 89 Th 90 U-Xe 91 U 92
Ra Ac
1 2 3 4 5 6
electrons in the outer layer is greater than 40 per cent of 18, these elements must fall into "indirect" families and so the list is extended to n-8,n-9, and n-10. These families have no members involving only the first three layers because here these values would be zero or negative. This extension provides for all the elements except those whose atomic numbers lie between 61 and 76. I n other words all of the ninety-two elements except fourteen are classified in families as shown in Table 11.
VOL.3, NO. 5 A SIMPLE RULE
POR ME CLASSIPICATION OF THE ELEMENTS
545
TABLE If THEFAMILIES OF CHEMICAL ELEMENTS
"-10
n~~
n~8
1
2
3
Li
Be
B
Na K Rb Cs
Mg Ca
..
Ka
Al Sc Yt La Ac
n-7
n-6
n-5
Sr Ba
4
Ti Zr
Ce Th
5
V Cb Pr U-Xz
n ~ 4 n-3
6
7
Cr Mo Nd
Mn
..
..
U n-2
n-1
H C
Fe Ru 0s
Co Rh Ir
Ni Pd Pt
Cu Ag Au
Zn Cd Hg
Ga In
TI
Si Ge Sn Pb
N P As Sb Bi
0
F
S Se
Cl Br I
Te Po
..
n
He Ne A
Rr Xe Rn
Discussion In this classification there are some of the usual difficulties. There are, however, no more than are common to all such tables. The disposition of all the elements common to both tables corresponds to that of Lewis2 with the exception of carbon and silicon which he places with titanium and zirconium. The advantage of the table lies in the fact that any of these seventyeight elements may be placed in its proper family from its atomic number. For example, molybdenum bas the atomic number 42. Of these 42 electrons, 2 are in the first layer, 8 in the second, 8 in the third, and 18 in the fourth which leaves 6 in the fifth. Since G is less than 40 per cent of the 18 required to saturate this layer, the element belongs in the "direct" families and, therefore, in the 6 family. Likewise the atomic number of indium is 49 and by the same calculation we find 13 electrons in the fifth or outer layer. This is more than 40 per cent of 18 and so the element belongs in the "indirect" n-5 family. The fourteen elements not included in Table I1 are Sa, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, and Ma. These are mainly the rare earths whose disposition in tables of this kind is only tentative. Since the first of these has eight electrons in a layer requiring thirty-two for saturation and thelast lacks twelve of completing that number, i t is scarcely to be expected that the tendency to gain or lose a definite number of electrons can be predicted. However, some of these can be pretty well placed by extending the "indirect" families. Thus n-11 is masurium, n-12 is tungsten, n-13 tantalum, n-14 hafnium, n-15 lutetium, n-16 ytterbium, and n-17 thulium. If these elements be placed in Table I1 where Lewis, "Valence and the Structure of Atoms and Molecules," Chemical Catalog Co., New York, 1923.
.
these values would fall in the fourth and fifth layers, they fall into the 7, 6, 5, 4, 3, 2, and 1families, respectively. This violates the 40 per cent rule in the fourth and fifth layers but not in the sixth where these elements lie. They are fairly well fitted to those families by their properties, although ytterbium becomes an alkaline earth and thulium an alkali metal which is unlikely although they have been so classified.3 Samarium, the first of these unassigned elements contains eight electrons in the sixth layer. If this were placed with those in the fourth and fifth layers having the same number, it would come in the n-10 or iron family, europium with cobalt, gadolinium with nickel, terbium with copper, dysprosium with zinc, holmium with gallium, and erbium with tin. While this extension of the rule finds little theoretical justification, there is some similarity of properties, especially in the earlier members. This arrangement of the rare earths is a t best merely a suggestion since their properties show them to be more nearly related to each other than to the other families. The state of saturation of the outer layers in these elements is such that they have no exact counterparts among the elements of lower atomic number. Summary A rule is given whereby all of the elements except the rare earths can be placed in the proper chemical families by means of the atomic number and the number of electrons required to saturate each layer as given by Langmuir. Mellor, "A Comprehensive Treatise of Inorganic and Theoretical Chemistry," Volume V, p. 617 (1924).
