e
RECENT HISTORY OF THE NOTION OF A CHEMICAL SPECIES' JACK J. BULLOFF Rensselaer Polytechnic Institute, Troy, New York
WE HAVE
grown to accept the terms "element," before long, cathode rays, radioactivity, and the Brow"compound," and "mixture" as simple terms, simply nian movement indisputably established the particudefined. Timmermans, in a workz that is a must for late nature of matter. Ostwald half-recanted and Wald every thinking chemist, shows how complex these retreated into silence. Out of this misadventure came an idea we would do terms really are. The definitions of a chemical individua!, and of well to think of more often. This is the concept of chemical purity, have undergone recent revision in "hylotropy." A material that cannot be fractionated two directions. On the one hand, quantum and nu- into components by a physical means is said to be clear chemistry have challenged the doctrine that chemi- "hylotropic" with respect to that means. A chemical cal elements are homogeneous entities. On the other individual exhibiting hylotropy to several physical hand, studies of the structure and stoichiometry of means of differentiation over an appreciable range of solids invite a change in our ideas of definite propor- thermodynamic coordinates (P, V , T, etc.) is defined as a "compound." To Ostwald and Wald an element tions in chemical combinations. was hylotropic for an infinite range of thermodynamic LAST ATTACK ON THE ATOMIC THEORY variables. These definitions had two advantages over those then Over a century ago Proust bested Berthollet in the mighty struggle that admitted Dalton's ideas into the current, and over some that still clutter the textbooks fabric of chemistry. Today, we take these concepts of our time. First, a clearly macroscopic criterion was used to define materials in which the usual chemical for granted, and view this struggle as an old bygone. I t was as this century opened that a powerful at- interest is macroscopic. It serves little to use nontack purposing to banish the atomic theory waslaunched operational definitions in science. Is it not more to the point to tell a student that the material in a certain by Wilhelm Ostwald and Frantisek Wald. Ostwald, the father of physico-chemical literature, beaker cannot be fractionated without decomposition one of the three musketeers of the ionic theory, and an than to inform him pontifically that only one kind of eloquent and incisive arguer, was a figure of massive molecule can be found in the liquid in the beaker? Second, these definitions are in a form based upon ' and honored prestige. Wald was a chemist of no mean attainments.= Ostwald opposed the atom on the prin- thermodynamics, and under the sway of an idea of ciple that its existence was undemonstrated, and on the quantitative verification. Hylotropy can be deterbasis that the long chain of reasoning from postulated mined by a Third Law entropy evaluation. We can atomism to the phenomena of the laboratory was not sum up by saying that at least Ostwald and Wald altogether convincing. Wald held that the phase had formulated a criterion of a chemical species. rule of J. Willard Gibbs could so define a phase of conCOMPONENTIAL DIVERSIFICATION stant composition that the law of definite proportions would follow without atomistic assumption^.^ Quantum and nuclear chemistry demonstrated that Ostwald, already a confirmed opponent of atomism, chemical elements were not hylotropic. Separation felt that Wald had made a constructive addition to of monatomic metal vapors into spin components, his cause, and immediately joined forces with him.5 diatomic gaseous elements into ortho-para states, Theirs was a noble enterprise; had it succeeded we elements into isotopic components, and the interconwould have had macroscopic systems defined in such a version of nuclides have removed the doctrine of the manner as to be invariant to future discoveries of the immutability of chemical elements from the concepfine stmcture of matter. It was too noble to last: tual frame work of theoretical chemistry. We retain the idea of hylotropy to a means rather than the idea 1 Presented at the 121st Meeting of the American Chemical of infinite hylotropy in ordinary chemical operations. Society, Buffalo, New York, March, 1952. 2 TIMMERMANS. "Chemical Species," Chemical Puhlishing Co., These new discoveries leave our ideas as to how comNew York, 1940. pounds are formed about the same as before. What SOhituary notices: C h m . Zlg., 54, 905 (1930); Collection Czechoslou. Chem. Cmnmuns., 3, 3 5, 9, 49 (review) (1930); they have added is a further choice of components. I n this new chemistry we can, by selecting from three C h a . Obzw, 5, 281, 283 (1930). ' Chern. Ztg., 30,463-7g (1897); 31, 756, 770 (1898); 32,299, hydrogen isotopes, three carbon isotopes, and three 767, 1249, 1276 (1899). Z. phyaik. Chem., 22, 253 (1897); 23, oxygen isotopes, build a variety of methanols or form78 (1897); 63, 307 (1908). J. C h a . Soc., 85, 506 (1904); Z . physik. Chem., 69, 506 aldehydes. But, essentially, these would be methan o l ~or formaldehydes respectively. (1909).
FEBRUARY, 1953
The operations of usual chemistry have given us the concept of purity on a mass basis. The operations of nuclear, radio-, and radiation chemistry lead to other concepts of purity. Since the Nuclear Age has reached a mass-production scale in fission, and promises t o reach it in fusion, these concepts will rapidly become increasingly familiar. Since there is no reason to conclude that the art of exploiting breeder reactors differs from the other industrial arts of our technologic civilization, the amount, variety, and use of nuclidic products is due for a logarithmic increase. In improving a hydrogen-bomb assembly line, we are concerned with the isotopic as well as the chemical purity of a sample of tritium. In the assay of a fission product from a breeder reactor run we are concerned with removing those elements whose radiation cannot he disentangled from that of the nuclide of interest by our assay devices; it is activity purity we are concerned with. If we want to determine an ultramicroscopic trace impurity by irradiation in an accelerator beam, we have to make certain that other similarly excitable material is absent or accounted for; we want activation purity. If we are projecting a new type of self-sustaining assembly with a severe space problem accompanying, we want nothing present that would interrrupt nuclear multiplication chains and are concerned with cross-section purity. In the more complex world of tagged molecules and traced ions other ideas are met, but what has already been exemplified is paralleled. In all of these new chemicals the properties of interest are variables reflecting the mode and recency of origin. As their use increases, the bottle label will rival the handbook as the vude mecum of the bench chemist. One aspect of nuclear chemistry diiers from the more familiar ones of isotope and nuclide chemistry, and that is the chemistry of mixtures of nuclear isomers. Under some conditions a mixture of nuclear isomers is self-separating, we might almost say antihylotropic. The ordinary chemist will rarely encounter this type of "hot-atom" chemistry. AGGREGATIONAL DIVERSIFICATION
79
which the concept of hounded microscopic structure cannot he extended. While some solids are composed of discrete ions and others of discrete molecules, there are a large number with fibril, sheet, or net structures of indefinite extent which have been respectively termed gigantic ions or gigantic molecules. The stoichiometry of solids often yields no structural or componential information. We are often left in a position where we cannot reason unambiguously with constitutional questions short of a complete and tedious X-ray structural determination of a number of anomalous points in a phase diagram. We frequently distinguish mixtures from compounds by comparing component properties with the properties of the aggregation. Persistence of component properties points toward mixture or solution. If we have a mixture, some component property usually varies with the amount of component present. In a compound on the other hand, we have so far believed that the constituent proportions cannot be varied; adding excess is supposed to yield a mixture. These constant proportions we attribute to valency, which possesses an integral character because discrete electrons are involved in chemical combination. In metals, electrons as well as atoms act as stmct.ura1 members. Intefmetallic compounds form at certain electron/atom ratios, e. g., 21/12, 21/13, 21/14. These are devoid of the usual valence meaning. We can sharply distinguish saltlike from metallic properties. Sodium tungstate is a typical salt. Partially reduced, it yields a "tungsten bronze," metallic in glitter and conductance. There is a variation of properties in the tungsten bronzes, but no component we can ~ e f e rit to. We can refer it t o a constituent, which is a different matter. We can note that sodium tungstite would have to be isomorphous with sodium tungstate, and that every intermediate state up to the tungstate would have to be similar to fulfill the requirements of an isomorphous series. The tungsten bronzes seem anomalous. N. Kurnako@ first presented the idea that such entities are a new type of chemical species. He called ordinary compounds "Daltonides" (the French, mindful of the great debate, prefer "Proustide"), and compounds such as the tungsten bronzes, the corresponding molybdenum blues, and the non-stoichiometric chalcogenides he referred to as 'LBerthollides.'' Kurnakow predicted that further study would discover a vast number of berthollides. E. Huttig remarked' that with the atomic theory firmly accepted, we need no longer cling to old concepts affirmed to give confidence in it, hut could now go on to accept the laws of classical stoichiometry as limiting laws.
Finding how "components" become "constituents" is a chief concern of chemistry. A change in the conceptualism of aggregation causes more upheaval to the chemist's mind than a mere diversification of components, such as the discovery of a new compound, element, nuclide, or nucleon. The bitterest struggles in the history of chemistry have been those concerning aggregational notions. One need only recall radicals, types, molecules, ions, and tetrahedral carbon- each associated with polemic and bitterness. Of late, resonance has gone-off to the cold war. ' Z . anorg. allgem. Chem., 88, 109 (1914); Acad. Sci., St. U. R., Tram. Faraday Soc., Perhaps another such struggle is brewing as a result Pelmsbu~g,1914, 321. Also, EVANS, ". 19.. 339:. Chem. of recent work in the solid state which seems to por- 19. 420 (1923). G ~ a z m o v .S.. Chem. Listv. 9,153, 155 (1934); ~oileclkmCzeehoslov. Chem. Communs., tend a modification of the law of definite proportions. 0b&, 7, 77 (1934). In the solid state we have materials whose molecHoehsehulwisser, 4, 261, 317, 365 (19133); Chem. Zentr., 1927, ular weight is either not directly determinable, or to Vol. 11, 1925.
