In the 6th period, however, there are 24 transition elements. The heaviest nine exhibit extremely close resemblance to the heaviest nine of the two preceding transition periods, celtium (hafnium) thus being proper to group IV. like zirconium and titanium, while mercury is proper to group II. like cadmium and zinc, the triad, osmium, iridium and platinum like its two fore runners being included within group VIII. Group III. alone re mains unoccupied and 15 elements remain in this transition series. These 15 elements, the "rare earth" metals from lanthanum to lutecium, all exhibit characteristic trivalency and an extremely close resemblance to yttrium and scandium of the preceding two periods. The 15 elements are consequently all proper to group III. Of these 15, however, lanthanum alone is characteristic in yielding a strongly alkaline hydroxide and diamagnetic salts. The remain ing 14 elements, cerium to lutecium, may, therefore, be relegated to a transition sub-series, lanthanum remaining as the typical group III. element of this transition period, thereby reduced to ten elements closely resembling the ten in each of the preceding transition periods. The 14 excluded elements (see Table III.) can not be arranged to form a period as none of them exhibits valency higher than four or lower than two. They thus constitute an anomalous sub-series of this transition period.
In the last period, the 7th, the transition series includes only the radioactive elements, actinium, thorium, protoactinium, and uranium, with valency from three to six respectively. These ele ments may, therefore, be allocated to groups III., IV., V., and VI.
of the transition period. As there is no gap in atomic number be tween actinium (89) and thorium (go), similar to the gap of 14 between lanthanum (57) and celtium (72) in the preceding transi tion period, it may be regarded as certain that the transition ele ments of period 7 do not include a "rare earth" sub-series similar to that of period 6.
The transition classification, shown in Table II. consists of four similar periods comprising an octet of groups from III. to II., each period containing a triad of elements in group VIII. and a single element in each of the other groups. This classification resembles the abridged or typical classification in that basic character de creases in each period from a maximum in the lightest to a mini mum in the heaviest element, and increases in each group from a minimum in the lightest to a maximum in the heaviest element. The transition classification differs from the abridged classification in that valency differences of one unit in an element can exist, that all the elements are metals, and that nearly all can yield col oured salts at all stages of valency, whereas the valency difference for elements in the abridged classification is invariably two or four units.
Seven main periods suffice to include all the elements, a rudi mentary period of two elements, two simple or short periods each of eight elements, and four complex or long periods each consist ing of two sub-periods. Each period comprises eight groups com
mencing with I. and ending with VIII., the simple periods and the first sub-periods of the long periods terminating with inert or noble gases, whereas the second sub-periods of the long periods terminate with noble metals. To indicate the close resemblance of hydrogen in its univalency to the alkali metals and to the halo gens, it is included in both groups I. and VII., helium being in cluded in group VIII. from its close resemblance to the other noble gases, despite the fact that it differs from them in possessing only two instead of eight potential valency factors or electrons.
The transition elements are shown in thin type to distinguish them from the elements most closely allied to the typical elements of the first two octet periods. To preserve the symmetry of the octet classification, the 15 "rare earth" elements of group III., succeeding barium in atomic weight, are represented only by their most typical member, lanthanum, the other 14 being shown in detail in a footnote to the table.
Mendeleev stated that a natural law only acquires scientific im portance when it yields practical results in elucidating unex plained phenomena, in disclosing unrecognized occurrences, and in evoking verifiable predictions. The periodic law has been used in the classification of the elements, in the determination of atomic weights of elements from equivalent weights, in correcting atomic weights, in the prediction of the existence and properties of new elements and in the determination of the electronic structures of atoms. It was used by Mendeleev to determine the atomic weights of indium and beryllium, which were then in dispute and could not be settled, as only the equivalent weights were known. In the same way Mendeleev was enabled to predict the existence and properties of eka-boron, eka-aluminium, eka-silicon, eka-manga nese, dwi-manganese, and eka-tantalum, now identified with the elements scandium, gallium, germanium, masurium, rhenium and polonium respectively. Similarly the discovery of the noble gases, helium and argon (crude) led to the prediction of eka-helium, and eka-, dwi-, and tri-argon, now known as neon, krypton, xenon and radon respectively. (See ATMOSPHERE.) Interpretation of the Periodic Law.—The recognition of the existence of a definite periodicity in the chemical properties of the elements, particularly with regard to valency, soon led to many hypotheses as to the inner meaning of the periodic law. At first, attempts were made to express this periodicity in terms of mathematical relations between atomic weights, but after the periodicity of properties was found to be a function of atomic numbers, instead of atomic weights, investigations were almost solely directed to the relations between electrons in atoms. All modern interpretations of the periodic law are based on hypotheses as to the electronic structures of atoms, and all possess the com mon feature that they postulate, tacitly or explicitly, the existence of a natural law of uniform atomic plan to which all atoms con form. Essentially this law is that the electronic structural pattern of every atom is a recapitulation of the patterns of all atoms having fewer electrons.