The first chemist of note to grasp the significance of Mendeléeff’s generalisation was Lothar Meyer, who, dealing at the outset with one of the characteristic properties of the elements—viz., their specific or atomic volumes (that is, the values obtained by dividing their specific gravities into their respective atomic weights)—greatly developed the principle of periodicity, representing it graphically in a most striking and suggestive manner, leading up to a classification almost identical with that of Mendeléeff.
Since the date of its promulgation the scheme of classification of the elements in accordance with the principle of periodicity has experienced certain minor modifications necessitated by fuller knowledge; but in its essential features it remains very much in the form devised by Mendeléeff. The discovery of the so-called inert and radio-active elements required that their relations to the periodic law should be defined. Their inclusion raises no fundamental difficulty. Indeed, the generalisation seems to adapt itself to the far-reaching considerations which spring from modern views of the nature of the atom, its electro-chemical relationships, and the orderly arrangement of the corpuscles of which it may be composed. In the 1905 edition of the English translation of his famous Principles of Chemistry, Mendeléeff has given a table which may be said to embody his final views concerning the systematic classification of the elements. This is reproduced on p. 105. In this table he postulates the existence of two hypothetical elements, x and y, the former of which he regards as identical with the physical ether; while the latter is an analogue of helium, possibly identical with the “coronium” of the solar coronal atmosphere, with a molecular weight of about 0.4.
The striking feature of Mendeléeff’s generalisation is its universality. In this respect it differs from all previous attempts at natural classifications of the elements; these were limited and partial, and therefore unsatisfactory. Nevertheless, it is easy to trace in them fundamental conceptions upon which Mendeléeff built. Mendeléeff, in fact, gave a great extension to ideas with which the chemical world of half a century ago was more or less familiar; and doubtless it was this circumstance, combined with the remarkable boldness and comprehensiveness of this extension, followed almost immediately by a most striking series of confirmations of his own previsions, as logical consequences of his generalisation, that secured for it attention, and ultimately universal adoption.
| Series. | Zero group. | Group I. | Group II. | Group III. | Group IV. | Group V. | Group VI. | Group VII. | Group VIII. | |
|---|---|---|---|---|---|---|---|---|---|---|
| 0 | x | — | — | — | — | — | — | — | — | |
| 1 | y | Hydrogen. H = 1.008 | — | — | — | — | — | — | ||
| 2 | Helium. He = 4.0 | Lithium. Li = 7.03 | Beryllium. Be = 9.1 | Boron. B = 11.0 | Carbon. C = 12.0 | Nitrogen. N = 14.04 | Oxygen. O = 16.0 | Fluorine. F = 19.0 | — | |
| 3 | Neon. Ne = 19.9 | Sodium. a = 23.05 | Magnesium. Mg = 24.1 | Aluminium. Al = 27.0 | Silicon. Si = 28.4 | Phosphorus. P = 31.0 | Sulphur. S = 32.06 | Chlorine. Cl = 35.45 | — | |
| 4 | Argon. Ar = 38 | Potassium. K = 39.1 | Calcium. Ca = 40.1 | Scandium. Sc = 44.1 | Titanium. Ti = 48.1 | Vanadium. V = 51.4 | Chromium. Cr = 52.1 | Manganese. Mn = 55.0 | { | Iron. Fe = 55.9 Cobalt. Co = 59 Nickel. Ni = 59(Cu) |
| 5 | — | Copper. Cu = 63.6 | Zinc. Zn = 65.4 | Gallium. Ga = 70.0 | Germanium. Ge = 72.3 | Arsenic. As = 75.0 | Selenium. Se = 79.0 | Bromine. Br = 79.95 | — | |
| 6 | Krypton. Kr=81.8 | Rubidium. Rb = 85.4 | Strontium. Sr = 87.6 | Yttrium. Y = 89.0 | Zirconium. Zr = 90.6 | Niobium. Nb = 94.0 | Molybdenum. Mo = 96.0 | — | { | Ruthenium. Ru = 101.7 Rhodium. Rh = 103.0 Palladium. Pd = 106.5(Ag) |
| 7 | — | Silver. Ag = 107.9 | Cadmium. Cd = 112.4 | Indium. In = 114.0 | Tin. Sn = 119.0 | Antimony. Sb = 120.0 | Tellurium. Te = 127 | Iodine. I = 127 | — | |
| 8 | Xenon. Xe = 128 | Cæsium. Cs = 132.2 | Barium. Ba = 137.4 | Lanthanum. La = 139 | Cerium. Ce = 140 | — | — | — | — | |
| 9 | — | — | — | — | — | — | — | — | — | |
| 10 | — | — | — | Ytterbium. Yb = 173 | — | Tantalum. Ta = 183.0 | Tungsten. W = 184 | — | { | Osmium. Os = 191 Iridium. Ir = 193 Platinum. Pt = 194.9(Au) |
| 11 | — | Gold. Au = 197.2 | Mercury. Hg = 200.0 | Thallium. Tl = 204.1 | Lead. Pb = 206.9 | Bismuth. Bi = 208 | — | — | — | |
| 12 | — | — | Radium. Rd = 224 | — | Thorium. Th = 232 | — | Uranium. U = 239 | — | — |
The periodic law, in the words of its author, is the “direct outcome of the stock of generalisations of established facts which had been accumulated by the end of the decade 1860–1870.” It is founded wholly on experiment, and is as much the embodiment of fact as are the laws of chemical combination. It was based upon the adoption of the definite numerical values of the atomic weights, as indicated by Cannizzaro, as a consequence of the hypothesis of Avogadro, and upon the assumption that the relations between the atomic weights of analogous elements must be governed by a general law. The application of the periodic law immediately led to the re-determination of certain atomic weights and to the correction of their assumed atomic values. At the time of its enunciation the determination of the valency of an element was purely empirical, with no apparent necessary relation to that of other elements. We find now that the valency is a matter of a priori knowledge, just as much as any other property of the element. The amended values for the atomic weight and valency of a number of elements thus demanded by the law have been confirmed by all the experimental criteria employed by chemists. The generalisation further indicated the existence of new elements; it pointed out their probable sources, and foretold their properties. Instances of this power of divination in the law are to be seen, as already mentioned, in the discovery of gallium by Lecoq de Boisbaudran, of scandium by Nilson, and of germanium by Winkler, the existence and main properties of which were severally foretold by Mendeléeff in 1871.
The promulgation of the law was heralded as a proof of the validity of the conception of a primordial matter. It was held that it can find a rational explanation only in the idea of unity in the formative material. But its author would not admit that his generalisation had any relation to the Pythagorean hypothesis:
The periodic law, based as it is on the solid and wholesome ground of experimental research, has been evolved independently of any conception as to the nature of the elements. It does not in the least originate in the idea of an unique matter, and it has no historical connection with that relic of the torments of classical thought; and therefore it affords no more indication of the unity of matter or of the compound nature of the elements than do the laws of Avogadro, and Gerhardt, or the law of specific heats, or even the conclusions of spectrum analysis. None of the advocates of a unique matter has ever tried to explain the law from the standpoint of ideas taken from a remote antiquity, when it was found convenient to admit the existence of many gods—and of a unique matter.
The reader who desires a fuller exposition of the principles of the periodic law must be referred to special treatises on the subject, or to the larger manuals on general chemistry. It must, however, be stated that, while many facts discovered since the original promulgation of the principle and since its development by Lothar Meyer, Carnelley, Thomsen, and others, are consistent with the law, other facts, some of which were known before 1870, are apparently out of harmony with it, or at all events await a fuller interpretation. For example, tellurium is not in its proper place in the scheme if its atomic weight, 127.5, has been correctly ascertained. Cobalt (58.97) and nickel (58.68) have atomic weights so closely accordant that their properties and those of their corresponding compounds should be very similar, and, in fact, almost identical; but such is not the case. Indeed, it has been said, no prevision of the periodic law would have led to the discovery of nickel. Similar considerations apply to manganese, chromium, and iron; the atomic weights of these elements are less widely different than the differences in their properties and the divergence in their chemical relationships would seem to require. The relative positions of argon and potassium are also not consistent with the law. There are difficulties, too, connected with what we know at present concerning the atomic weights of the so-called rare earth metals. In spite, however, of these seeming anomalies, it can hardly be doubted that the periodic law is as much the expression of a natural law as is the law of gravitation; although it is possible, and indeed probable, that, as we now define it, it is only the first approximation to the truth, and that, as our knowledge becomes more precise, Mendeléeff’s classification, in its present form, will require modification and extension, just as Mendeléeff’s own scheme may be said to be a modification and extension of the attempts at the rational classification of the chemical elements made by his predecessors.
Dmitri Ivanowitsch Mendeléeff.