The two preceding chapters have shown that in most cases the collateral relationships which are now interpreted in the light of the nuclear theory will not be altered, except as to the nature of the language in which they are described, by the collapse of that theory and the consequent necessity of substituting something else, and that in the most notable instance where one of the subsidiary structures does fall along with the nuclear concept itself, the structure that is destroyed is one whose loss need not be mourned. There is, however, one other casualty that warrants some special comment.
The objective of the presentation in this volume is to show that the nuclear theory of the atom, the concept of the atom as a minute positively charged nucleus surrounded by electrons bearing oppositely directed (negative) charges equal in magnitude, is not valid in the Iight of existing factual knowledge. No attempt has been made in the preceding pages to inquire into the validity of the electrical theory of matter in general, since this is, to some degree at least, a separate question, inasmuch as a demonstration that the concept of an atom constructed of certain specific kinds of charged particles arranged in a specific manner is no longer tenable does not necessarily exclude the possibility that electric charges may still be important factors in the atomic structure, nor does it necessarily invalidate the assumption that the inter-atomic forces have an electrical origin. It is probably apparent to most readers, however, that the powerful arguments now available against the nuclear hypothesis, particularly those which stem from the experimental discovery that the electron is not a permanent “building block” type of entity, but a particle that can be created or destroyed with relative ease, will also deliver the coup de grace to the electrical theory of matter as a whole, when these arguments are appropriately developed. In order to avoid leaving the impression that this conflict with a popular and generally accepted theory in some way weakens the force of those arguments, it will be desirable to take a brief look at the inter-atomic aspects of the electrical theory.
To account for the existence of solid substances, at least two forces of an inter-atomic character are required, irrespective of the theoretical viewpoint from which the subject is approached. As indicated in Chapter 2, the weight of evidence now indicates that the atoms are widely separated and that the inter-atomic distance simply represents the point of equilibrium between the attractive and repulsive inter-atomic forces. The currently popular electrical theory postulates that the atoms are held in contact by the electrical forces of attraction, but this does not eliminate the need for a repulsive force; it merely puts this force inside the atom. There still has to be a force resisting deformation. This requirement of two opposing forces imposes a very severe restriction on the formation of theories of the constitution of matter, since the number of known types of force is extremely limited. Aside from the forces known to result directly from motion, such as centrifugal force, there are only three kinds of force of which we have any definite knowledge: gravitational, electric and magnetic. Of these three, the only one that appears to be strong enough to account for the cohesion of solids is the electric force. This, of course, creates a strong predisposition on the part of the scientific community to look favorably on any theory which attributes the observed phenomena to electrical causes, and to be very tolerant of the shortcomings of such a theory.
In view of this background, the discovery early in the nineteenth century that certain substances, on being dissolved, separate into positively and negatively charged ions, was naturally accepted as definite proof that matter is composed of charged particles and that the force of attraction between unlike charges furnishes the explanation of the cohesion of solids. The more recent finding that electric charges of all kinds are easily created and easily destroyed has, however, cut the ground out from under these conclusions. It is now generally admitted that at least some of the ionic charges must be created in the solution process, and since there is no good reason to believe that two different mechanisms are operative, this gives rise to a rather strong presumption that allionic charges are created in the solution process and that no charges exist prior to solution.
The new information throws an altogether different light on the situation within the solid. As long as it seemed certain that the positively charged sodium ions and negatively charged chlorine ions which we find in salt solutions were obtained from the solid sodium chloride, then since we know that there is a force of attraction between oppositely directed charges, it was only natural to conclude, without going into the matter very deeply, that this force of attraction accounts for the cohesion of the solid sodium chloride structure. But if we approach the same question with the benefit of present-day knowledge, which tells us that the ions which we find in solutions are probably created in the solution process, so that we now have to face the basic issue as to whether or not any such ions are actually present in the NaCl crystal, the picture looks very much different. We are now confronted with the awkward fact that the existence of positive and negative charges in contact or in very close proximity is totally foreign to the known behavior of electric charges. Everything that we actually know about such charges indicates that they destroy each other on contact, and that mixtures of oppositely charged particles can exist only under conditions such as those in solution, where the charges are created as fast as they are destroyed.
Furthermore, there is no other evidence of the existence of solid ions that can stand up under critical examination. The complete lack of merit of one of the arguments commonly advanced for this purpose was pointed out in the introductory chapter. The same text from which this argument was taken goes on to characterize the high melting points of compounds of the NaCl type as evidence that they are composed of ions. But when this new argument is fully developed in the text we find that all it amounts to is that the higher melting points indicate that the force of cohesion is greater in these substances than in compounds of the organic type, and hence it is deduced that the nature of the cohesive force must be different. The conclusion is then drawn that since it is different, it must be ionic: an excellent example of the kind of reasoning that has to be used to bolster this electrical theory. The truth is that the solid state contributes no evidence of the existence of ions, and without the benefit of the positive knowledge that the occurrence of ions in solution was formerly supposed to provide, but no longer does, a consideration of all of the available facts must lead us to the conclusion that the atoms are not charged in the solid state, and that their cohesion is attributable to something other than electrical forces.
When we thus take off the rose-colored spectacles before looking at the ionic theory, it is also evident that the superficially attractive features of this theory apply only to one class of compounds, and that outside of this one class it is practically lost. The theory explains the cohesion of the so-called ionic” compounds as the result of the attractive forces between ions of opposite charge, but there are “non-ionic” compounds, even more numerous than the ionic variety, which possess cohesive forces that, so far as we can determine, are qualitatively identical with those of the ionic compounds. It thus becomes necessary to advance two totally different explanations for what is, to all appearances, the same problem. This is typical of what the electrical theory encounters on every hand. If we attempt to get down to details, the situation becomes even worse. Here the theorists cannot even agree as to the manner in which the theory should be applied. Physicists give us one answer, says V. F. Weisskopf, chemists another, but “neither of these answers is adequate to explain what a chemical bond is.65
The physicists who are attempting to apply the latest quantum concepts to the problem have been singularly unsuccessful, if we appraise their results by any realistic standards. Some very broad claims on their behalf are often made by overenthusiastic supporters, the following from G. G. Hall being a typical example: “Quantum mechanics... gives the solution, in principle, to almost every chemical problem,” but the true significance of such statements becomes apparent when Hall goes on to say, “Very unfortunately, however, there is an enormous gap between this solution in principle and the practical calculation of the properties of any specific molecule.66
The chemical theory is more completely developed but, as Weisskopf says, it does not answer the essential question either. This theory is based on the hypothesis that ions are created by the transfer of electrons from one component of the solid structure to the other. Since the inert gases have no tendency toward chemical combination, the originators of this theory deduced that the numbers of electrons presumably contained by these elements, 2, 10, 18, etc., must be unusually stable, and as the elements immediately adjoining the inert gases are very reactive and have unit positive and negative valences, it was further deduced that these elements have the ability to lose or gain electrons, thereby attaining the stable electron content of the inert gas and at the same time acquiring positive and negative charges respectively. Thus potassium, with 19 electrons, is assumed to lose one, reducing it to the same 18 status as argon and producing a singly-charged positive ion. Similarly chlorine, with 17 electrons, is assumed to accept the electron lost by potassium, producing a singly-charged negative ion, and increasing the chlorine electrons to the stable value 18. The cohesion of the compound KCl is then explained by the attraction between the positive and negative charges. So far as simple compounds of the KCl type are concerned, this theory is quite plausible, or perhaps it would be more accurate to say that the theory would be plausible if someone could come up with a reasonable explanation as to how the ions are originally produced. The test of such a theory, however, is not how well it agrees with the particular set of facts that it was specifically designed to fit, but how well it agrees with the other facts in the area it purports to cover, and this one runs into serious difficulties as soon as it gets beyond the KCI class of compounds. For instance, vanadium forms three binary compounds VO, VN, and VC, which crystallize in the same simple cubic structure as KCI, and so far as we can tell, are held together by the same kind of forces. But the theory that seemed so plausible in the case of KCI has a hard time explaining the composition of any one of these compounds, to say nothing of producing any explanation for the fact that this element has a different valence in each. In none of these cases does the number of electrons presumably transferred to the electronegative component leave vanadium with the 18 residue which is supposed to be so significant in the theory of the KCl structure.
By the time we reach the “non-ionic” compounds practically all resemblance to the original theory has been lost. Here the concept of achieving stability by transfer of electrons from one component to the other is clearly inapplicable. In the compound ClF, or the chlorine molecule Cl2, neither component is in a position to give up an electron, since each is one electron short of the inert gas figure to begin with. A new idea is therefore introduced into the theory: that of “shared electrons.” It is postulated that each chlorine atom in Cl2 contributes one electron to a sort of common pool, under joint management, so to speak. Each atom then has 16 electrons of its own and is entitled to claim both of those in the joint account, making a total of 18 each, thus complying with the postulated requirements for stability.
For present purposes, an extended analysis of this “shared electron” concept is not necessary. It should be sufficient to point out first, that this is purely an invention designed to fit the existing situation, and not something derived from the underlying atomic theory; second, that it has all of the earmarks of a crude contrivance, indeed it is about as weird an idea as was ever advanced in the name of science; and third, that it makes no attempt to accomplish the primary objective of a theory of chemical combination, an explanation of the nature of the cohesion, and devotes itself entirely to considerations of a secondary nature.
The inability of the electrical theory to furnish a consistent and comprehensive explanation of the cohesion of solids, together with the absence of any valid evidence of the existence of ions in the solid state, indicates that it will be necessary to abandon this theory as a whole, along with the concept of the nuclear atom. As the situation now stands, there is nothing tangible to support any aspect of the electrical theory of matter. The theory maintains its standing only by reason of the inertia of familiar habits of thought and the general reluctance to abandon an explanation, however unsatisfactory it may be, as long as there is nothing available to take its place. This is one of those instances in which the scientist allows himself to be governed by his emotional reactions as a human being rather than by the objective principles of science. A strong reluctance to admit ignorance is a pronounced human characteristic, and in everyday life one of the most devastating retorts that can be made to a critic is to ask, “Have you anything better to offer?” From a strictly scientific standpoint, on the other hand, such a question is completely irrelevant; if the accepted explanation proves to be wrong and we have no acceptable substitute for it, good scientific practice requires us to admit that we simply do not know the answer.
Furthermore, even though this means that the scientific profession is faced with the difficult task of finding some other theory to replace the electrical theory of cohesion, a task which scientists can hardly be expected to welcome, it should be remembered that this necessity of finding some other explanation of the cohesion of solids has been present all the time; it has merely been pushed to one side and ignored. Even the most enthusiastic supporter of the electrical theory must admit that the theory does not pretend to explain the cohesion of all solids, and as long as the coverage is incomplete the search is not ended. None of the interpretations of the electrical theory thus far devised gives us any reasonable explanation of the cohesion of metals, for instance. As Weisskopf puts it, in an admission which follows the statement previously quoted, “I must warn you I do not understand why metals hold together.” This situation cannot be ignored indefinitely. Sooner or later the issue must be faced, and since it will then be necessary to look for a completely new explanation in any event, the demise of the electrical theory does not change the over-all problem materially. In all probability, when we finally do discover the correct explanation of the cohesion of metals, we will have the correct explanation of solid cohesion in general.
Although the electronic theory of chemical combination is a distinct failure in the primary task of such a theory, explaining why the combinations hold together, it does a better job on some of the other aspects of the subject, particularly in connection with the properties portrayed by the periodic table. It should therefore be noted that whatever value these accomplishments of the theory may possess will not be lost when the electrical theory of matter has to be discarded, as these items can be transferred bodily to any new theory that is set up. The reason is that the structure of the electronic theory is almost entirely independent of the terminology employed, and replacement of the electron concept by something else is a matter of changing labels rather than of rebuilding the structure of the theory.
Most of the real knowledge in this area is numerical. As one chemistry textbook says, “The most important test which the theory of electronic configurations must meet, in order to satisfy the chemist, is that of providing an explanation for the periodic law,67 and the periodic table which expresses this law is a representation of a purely numerical relationship. Here we find that if we arrange the elements in the order of atomic number so that they fall into successive periods and into eight periodic groups, the chemical properties of these elements are then related to their positions in this table. Thus the elements in group one of the short periods and of the first half of the long periods constitute the well-known series of alkali metals: lithium, sodium, potassium, etc. The chemical properties of any one of these elements are very similar to those of the other members of this series, but quite different in many respects from those of other elements. Each of the alkali elements, for instance, has a positive valence of one, which is a condensed way of saying that an atom of one of these elements will form a stable compound of the NaCl class, or some equivalent, with an element of negative valence one, that two such atoms will form a stable compound similar to Na2O with an atom of negative valence two, that such an atom will acquire a one-unit positive charge if the compound containing it dissociates on being dissolved, and so on.
The important fact that should be realized, so far as the point now at issue is concerned, is that all of these relationships are purely numerical, and that the numbers express the full extent of our actual knowledge in this field. The name “electron” is a purely arbitrary label derived from theory and attached by the theorists to the numbers that represent the actual meaning. This label does not enter into the relations in any way-all of these relations are based on dimensionless numbers-and replacement of the label electron” by some other label will not alter these relations in the least.
It should also be noted that the numerical relations were not derived from the atomic theory. These relations are primarily a result of the periodicity; that is, of the existence of distinct groups of elements. But there is nothing in the nuclear theory of the atom to indicate the existence of such groups. The nuclear hypothesis does not require, or even infer, such an arrangement. The only numerical property of the electrons that is inherent in this hypothesis is the total number of electrons in the atom. The further conclusion that the electrons are arranged in groups or shells is simply an ad hoc assumption formulated to fit the observed facts. “It became clear at once,” says Slater, that several different assumptions were required to give a consistent statement of the principles underlying the structure of the periodic system.68 In other words, the theory does not explain the facts; the facts explain the theory. It is the existence of the experimental facts with respect to the periodic properties, chemical and spectroscopic, that has dictated the form of the assumptions and has thus shaped the electronic theory. The important feature of that theory is not what came from the atomic theory-the label “electron” that does not enter into the mathematical expressions at all-but what was put in by means of the assumptionsthe specific numbers of elements in the successive groups.
Present-day textbooks tend to give the impression that the knowledge in this field is a product of the electron theory, but this is not true; the knowledge came first and the electron theory was hooked on afterward. The periodic table was devised by Mendeleeff in 1869, a full quarter of a century before the electron was discovered. The actual procedure which was followed was not to arrive at an answer by developing the consequences of the basic theory, as it is now commonly portrayed, but to devise a scheme for filling in the gap between the atomic theory and what necessarily has to be the end result: the existing chemical knowledge. It is quite obvious that any other theory of atomic structure that might be proposed can be connected with the periodic table and other chemical properties by exactly the same means; the only difference, aside from minor details, will be that the numbers expressing such factors as the positions of the elements within the periods will no longer be identified by the name “electron” but by some other designation. Whatever degree of validity the electronic thory may possess will then be equally applicable to this new theory.
When the correct theory finally appears, the situation will, of course, be quite different. The correct theory of atomic structure must necessarily be of such a character that it not only carries with it a “built-in” explanation of the inter-atomic forces, but also an explanation of the periodic groupings, which will permit the theory of the periodic system to be derived directly from the atomic theory without the necessity of any ad hoc assumptions. In the meantime, since the chemists have to content themselves with something less than this correct theory, there is no reason to believe that there will be any particular difficulty in fitting the chemical picture to whatever new theory of the atom the physicists may devise. It should actually be a relief to be rid of the necessity of dealing with such patently forced concepts as “shared electrons” and “resonance.
All the way through these three chapters which have been devoted to a discussion of the various collateral subjects that are tied in to the atomic theory in current scientific thought, essentially the same situation has been encountered. Instead of being products of the atomic theory, as is now generally contended, the developments in these collateral fields-atomic energy levels, periodic properties of the elements, mathematical correlation of x-ray spectra, etc.-have taken place independently, and the connection with atomic theory exists only through the agency of the language that is used to describe the observed facts and the conclusions derived therefrom.
If we appraise the general situation in these fields critically, it is apparent that most of the knowledge that does exist is purely mathematical. The matter of interpretation of the mathematical relationships thus becomes extremely important and, as H. Margenau points out in a recent work, since the non-mathematical information of a factual nature is so meager, it is seldom possible to subject an interpretive hypothesis in these areas to any conclusive tests. The physicist thus “has an embarrassing amount of freedom in making his interpretations,69 as Margenau expresses it. Under these circumstances common prudence certainly calls for the exercise of particular care in distinguishing these untested interpretations from established facts. Had such a policy been followed in the past, this present discussion would have been wholly unnecessary, as it is quite obvious that the identification of the units that enter into the theory of spectra, the theory of chemical combinations, etc., as electrons” is simply one of these unverified interpretations.
It has been apparent ever since Moseley published his findings in 1913 that the atomic number is an important basic feature of the atom; that we have, as Moseley said, “a fundamental quantity, which increases by regular steps as we pass from one element to the next.” But Moseley and his contemporaries were under the impression that they had “positive knowledge” of the existence of an atomic nucleus, and they therefore identified these Moseley units, as we may call them, with the positive charge on the hypothetical atomic nucleus, and by extension, with the number of electrons necessary to neutralize this positive charge. From then on, the genuine significance of the Moseley units, which has become more and more apparent as the scope of physical knowledge has widened, has been mistakenly attributed to protons and electrons, and has constituted the bulk of the “evidence” offered in support of the present-day atomic theories.
If we analyze this situation rather than just taking it for granted that current thinking is correct, we find that the significant statements that are now made about atomic electrons are actually statements about the Moseley units, and they are valid only to the extent that the word “electron” can be used as a synonym for “Moseley unit.” Whenever an attempt is made to go beyond this point and introduce some meaning appropriate to the experimentally observed electron but not to the Moseley unit, this leads to trouble. The endless difficulties that have been experienced in the attempt to treat the atomic electron as a particle, which have culminated in the complete surrender of the front line theorists, who now say that it is not a particle, but only a “symbol,” have already been discussed. The explanation is that the real electron, where we actually know it, is a particle, but the Moseley unit is not. Similarly, the impasse that is faced whenever any attempt is made to apply electronic theory to the metals is a result of trying to force one of the characteristic properties of electrons, their negative charge, into a situation where it does not belong. The units of the so-called “electronic theory,” the units which arrange themselves into periods and groups, are actually Moseley units, not electrons, and there is no reason to believe that the Moseley units are charged; on the contrary, the fact that current theory finds it necessary in some cases to identiry them with the negatively charged electron, and in other cases to identify them with the positively charged proton, strongly suggests that they are not charged. The complete indifference of the cohesive forces in the solid state to the direction of the hypothetical ionic charge (an atom of an electropositive element will join another atom of the same element, or an atom of another electropositive element, just as readily as it will join an atom of an electronegative element) points to the same conclusion. Use of the term electron” instead of “Moseley unit” therefore introduces from the language that is employed, a foreign element, the electric charge, that is not present in the phenomenon itself. This, of course, has the effect of confusing the whole situation.
The collapse of the nuclear theory of the atom does not destroy the electronic theory of chemical combination or other similar theories; it merely necessitates abandoning the use of the terms “electron” and “nuclear charge” in connection with these theories, and recognizing that the units to which these terms are currently applied are in reality Moseley units: entities about which little is known with certainty, other than that they are units of atomic number, but are not individual particles and are not associated with electric charges. One of the requirements of a fully satisfactory theory to replace the nuclear theory of the atom is that it should provide an adequate explanation of the origin and nature of these Moseley units. It is quite possible that this development may result in some rather drastic revisions of existing thought, but in the meantime, until such a theory appears, these various side branches of the atomic theory can continue along substantially the same lines as heretofore simply by substituting “Moseley unit,” or some equivalent term, in those place where the expressions “electron” or “nuclear charge” are now employed.