Journal of the American Chemical Society
Vol. 41, No, 6, pg. 868 (June 1919)
The problem of the structure of atoms has been attcked mainly by physicists who have given little consideration to the chemical properties which must ultimately be explained by a theory of atomic structure. The vast story of knowledge of chemical properties and relationships, such as is summarized by the Periodic Table, should serve as a better foundation for a theory of atomic structure than the relatively meager experimental data along purely physical lines.
Kossel1 and Lewis2 have had marked success in attacking the problem in this way. The present paper aims to develop and somewhat modify these theories. Lewis, rejecting the physical data as being insufficient or inconclusive, reasons from chemical facts that the electrons in atoms are normally stationary in position. These electrons arrange themselves in a series of concentric shells, the first shell containing two electrons, while all the other shells tend to hold eight. The outermost shell however may hold 2, 4, or 6, instead of 8. The 8 electrons in a shell are supposed to be placed symmetrically at the corners of a cube or in pairs at the corners of a regular tetrahedron. When atoms combine they usually hold some of their outer electrons in common, two electrons being thus held for each chemical bond. These electrons may form parts of both atomic shells of 8 electrons. By means of these postulates Lewis is able to give an extraordinarily satisfactory explanation of the periodic arrangement of the elements and to explain in detail most of their inert gases, the alkali and the alkaline earth metals, the halogens, boron, aluminium, scandium, carbon, silicon, nitrogen, phosphorus, arsenic, antimony, bismuth, oxygen, sulfur, selenium, and tellurium, a total of 35 out of the 88 known elements. The theory in its present form does not apply at all satisfactorily to any of the other elements.
The theory of atomic structure advanced in the present paper not only explains in a satisfactory manner the general properties and relationships of all the elements, but also gives a theory of the formation and structure of compounds which agrees excellently with the facts. It leads directly to a valence theory for organic compounds which is the exact equivalent of the ordinary theory. When applied to the structure of complex inorganic compounds it leads leads to a theory practically identical with that of Werner. In cases like those of the oxides of nitrogen, etc., which have not previously been explained by any theory of valence the results are thoroughly satisfactory. The structure of the nitrogen, carbon monoxide and hydrocyanic acid molecules are accounted for and new relationships are obtained.
Under these conditions the postulates underlying the theory receive strong support. In fact, the results seem to establish the fundatmental correctness of most of the postulates. The recent advances in the physics of the electron have been largely along the lines of Bohr's theory. It is generally assumed that the electrons are revolving all in one plane, in orbits about the nucleus. Such a view is wholly inconsistent with that of the present paper. Bohr's theory has had marked success in explaining and even in predicting new facts connected with the spectra of hydrogen, helium, and lithium, and must therefore contain important elements of truth.
It will probably be possible to reconcile the two theories. As has already been pointed out, Bohr's stationary states have a close resemblance to the cells postulated in the present theory. The series of numbers 1, 1/2, 1/9, 1/25 occur in much the same way in both theories.
The cellular structure postulated here also seems to be closely related to J.J. Thomson's1 theory of atomic structure in which he postulates tubes of force. It seems as though each cell in the present theory is analogous to the inner end of one of Thomson's cylindrical tubes of force. This view suggests that in an atom the electrons are acted on by a repulsive force inversely proportional to 1/t2 where t is the index number of the shell in which the electron is located. Thus instead of the force varying continuously, as in Coulomb's law, it varies discontinuously in proportion to 1, 1/4, 1/9, 1/25, etc. and only at large distances where t is very large does the force vary approximately continuously. In some such way we may hope to be led to a modification of Bohr's theory in which the electrons do not rotate about the nucleus.
The theory presented in this paper is essentially an extension of Lewis'1 theory of the "cubic atom". It may be most concisely stated in terms of the following postulates:
1. The electrons in atoms are either stationary or rotate, revolve or oscillate about the definite positions in the atom. In the most stable atoms, namely those of the inert gases, the electrons have positions symmetrical with respect to a plane, called the equatorial plane, passing through the nucleus at the center of the atom. No electrons lie in the equatorial plane. There is an axis of symmetry (polar axis) perpendicular to this plane through which 4 secondary planes of symmetry pass forming angles of 45 degrees with each other. These atoms thus have the symmetry of a tetragonal crystal.
2. The electrons in any given atom are distributed through a series of concentric (nearly) spherical shells, all of equal thickness. Thus the mean radii of the shells form an arithmetric series 1, 2 ,3, 4, and the effective areas are in the ratios 1 : 22 ; 32 ; 42.
3. Each shell is divded into cellular spaces or cells occupying equal areas in their respective shells and distributed over the surface of the shells according to the symmetry Postulate 1. The first shell thus contains 2 cells, the second 8, and third 18, and the fourth 32.
4. Each of the cells in the first shell can contain only one electron, but each other cells can contain either one or two. All the inner shells must have their full quotas of electrons before the outside shell can contain any. No cell in the outside layer can contain two electrons until all the other cells in this layer contain at least one.
5. Two electrons in the same cell do not repel nor attract one another with strong forces. This probably means that there is a magnetic attraction (Parson's magnetic theory) which nearly counteracts the electrostatic repulsion.
6. When the number of electrons in the outside layer is small the arrangement of the electrons is determined by the (magnetic?) attraction of the underlying electrons. But when the number of electrons increases, especially when the layer is nearly complete, the electrostatic, repulsion of the underlying electrons and of those in the outside shell becomes predominant.
7. The properties of the atoms are determined primarily by the number and arrangement of electron in the outside shell and by the ease with which the atom is able to revert to more stable forms by giving up or taking up electrons.
8. The stable and symmetrical arrangements of electrons corresponding to the intert gases are characterized by strong internal and weak external fields of force. The smaller the atomic number, the weaker the external field.
9. The most stable arrangement of electrons is that of the pair in the helium atom. A stable pair may also be held by: (a) a single hydrogen nucleus; (b) two hydrogen nuclei; (c) a hydrogen nucleus and the kernel of another atom; (d) two atomic kernels (very rare)
10. The next most stable arrangement of electrons is the octet, that is, a group of 8 electrons like that in the second shell of the neon atom. Any atom with atomic number less than 20, and which has more than 3 electrons in its outside layer tends to take up enough electrons to complete its octet.
11. Two octets may hold one, two, or sometimes 3 pairs of electrons in the common. One octet may share one, two, 3, or 4 pairs of its electrons with one, two, 3, or 4 other octets. One or more pairs of electrons in an octet may be shared by the corresponding number of hydrogen nuclei. No electron can be shared by more than two octets.
This theory explains the periodic properties of all the elements including those of the eighth group and the rare earths. It meets with success in explaining the magnetic properties of the elements, and applies as well to the so-called physical properties, such as boiling points, freezing points, electric conductivity, etc., as it does to the "chemical properties". It leads to a simple theory of chemical valence for both polar and non-polar substances. In the case of organic compounds the results are identical with those of the ordinary valence theory, which with oxygen, nitrogen, chlorine, sulfur, and phosphorous compounds, the new theory applies as well as to organic compounds, although the ordinary valence theory fails nearly completely.
This theory explains also the structure of compounds which, according to Werner's theory, are second order compounds with a coordination number equal to 5. According to the present theory, such compounds are to be regarded rather as typical primary valence compounds.
This valence theory is based on the following simple equation:
e = 8n-2p
Where e is the total number of available electrons in the shells of all the atoms in a molecule; n is the number of octets forming in the outside shells, and p is the number of pairs of electrons held in commmon by the octets. This equation is a complete mathematical statement of the primary valence requrements, not only in organic, but in inorganic chemistry.
The theory leads to very definite conceptions as to the positions of the electrons in the molecules or space lattices of compouds. The structures of molecules of nitrogen, carbon monoxide, hydrogen cyanide, and NO prove to be exceptional in that the kernels of both atoms in the molecule are contained within a single octet. This accounts for the practically identical "physical" properties of nitrogen and carbon monoxide, and for the abnormal inertness of molecular nitrogen.
The results obtained by the use of the postulates are so striking that one may safely reason that the results establish the fundamental correctness of the postulates.
These conclusions, however, are not easily reconciled with Bohr's theory of the atom, Bohr's stationary states have a rather close resemblance to the cellular structure postulated in the present theory. There are also striking points of similarity with J.J. Thomson's theory of the structre of atoms, in which he assumes that the attractive forces are limited to certain tubes of force.