The Principles of Chemistry, Volume II

by Dmitry Ivanovich Mendeleyev

R₂O₈ or RO₄. A very rare type, and only known in OsO₄ and RuO₄.

It is evident from the circumstance that in all the higher types the acid hydroxides (for example, HClO₄, H₂SO₄, H₃PO₄) and salts with a single atom of one element contain, like the higher saline type RO₄, not more than four atoms of oxygen; that the formation of the saline oxides is governed by a certain common principle which is best looked for in the fundamental properties of oxygen, and in general of the most simple compounds. The hydrate of the oxide RO₂ is of the higher type RO₂2H₂O = RH₄O₄ = R(HO)₄. Such, for example, is the hydrate of silica and the salts (orthosilicates) corresponding with it, Si(MO)₄. The oxide R₂O₅, corresponds with the hydrate R₂O₅3H₂O = 2RH₃O₄ = 2RO(OH)₃. Such is orthophosphoric acid, PH₃O₃. The hydrate of the oxide RO₃ is RO₃H₂O = RH₂O₄ = RO₂(OH)₂—for instance, sulphuric acid. The hydrate corresponding to R₂O₇ is evidently RHO = RO₃(OH)—for example, perchloric acid. Here, besides containing O₄, it must further be remarked that the amount of hydrogen in the hydrate is equal to the amount of hydrogen in the hydrogen compound. Thus silicon gives SiH₄ and SiH₄O₄, phosphorus PH₃ and PH₃O₄, sulphur SH₂ and SH₂O₄, chlorine ClH and ClHO₄. This, if it does not explain, at least connects in a harmonious and general system the fact that the elements are capable of combining with a greater amount of oxygen, the less the amount of hydrogen which they are able to retain. In this the key to the comprehension of all further deductions must be looked for, and we will therefore formulate this rule in general terms. An element R gives a hydrogen compound RH_n, the hydrate of its higher oxide will be RH_nO₄, and therefore the higher oxide will contain 2RH_nO₄ - nH₂O = R₂O_{8 - n}. For example, chlorine gives ClH, hydrate ClHO₄, and the higher oxide Cl₂O₇. Carbon gives CH₄ and CO₂. So also, SiO₂ and SiH₄ are the higher compounds of silicon with hydrogen and oxygen, like CO₂ and CH₄. Here the amounts of oxygen and hydrogen are equivalent. Nitrogen combines with a large amount of oxygen, forming N₂O₅, but, on the other hand, with a small quantity of hydrogen in NH₃. The sum of the equivalents of hydrogen and oxygen, occurring in combination with an atom of nitrogen, is, as always in the higher types, equal to eight. It is the same with the other elements which combine with hydrogen and oxygen. Thus sulphur gives SO₃; consequently, six equivalents of oxygen fall to an atom of sulphur, and in SH₂ two equivalents of hydrogen. The sum is again equal to eight. The relation between Cl₂O₇ and ClH is the same. This shows that the property of elements of combining with such different elements as oxygen and hydrogen is subject to one common law, which is also formulated in the system of the elements presently to be described.[7]

In the preceding we see not only the regularity and simplicity which govern the formation and properties of the oxides and of all the compounds of the elements, but also a fresh and exact means for recognising the analogy of elements. Analogous elements give compounds of analogous types, both higher and lower. If CO₂ and SO₂ are two gases which closely resemble each other both in their physical and chemical properties, the reason of this must be looked for not in an analogy of sulphur and carbon, but in that identity of the type of combination, RX₄, which both oxides assume, and in that influence which a large mass of oxygen always exerts on the properties of its compounds. In fact, there is little resemblance between carbon and sulphur, as is seen not only from the fact that CO₂ is the higher form of oxidation, whilst SO₂ is able to further oxidise into SO₃, but also from the fact that all the other compounds—for example, SH₂ and CH₄, SCl₂ and CCl₄, &c.—are entirely unlike both in type and in chemical properties. This absence of analogy in carbon and sulphur is especially clearly seen in the fact that the highest saline oxides are of different composition, CO₂ for carbon, and SO₃ for sulphur. In Chapter [VIII.] we considered the limit to which carbon tends in its compounds, and in a similar manner there is for every element in its compounds a tendency to attain a certain highest limit RX_n. This view was particularly developed in the middle of the present century by Frankland in studying the metallo-organic compounds, i.e. those in which X is wholly or partially a hydrocarbon radicle; for instance, X = CH₃ or C₂H₅ &c. Thus, for example, antimony, Sb (Chapter [XIX.]) gives, with chlorine, compounds SbCl₃ and SbCl₅ and corresponding oxygen compounds Sb₂O₃ and Sb₂O₅, whilst under the action of CH₃I, C₂H₅I, or in general EI (where E is a hydrocarbon radicle of the paraffin series), upon antimony or its alloy with sodium there are formed SbE₃ (for example, Sb(CH₃)₃, boiling at about 81°), which, corresponding to the lower form of combination SbX₃, are able to combine further with EI, or Cl₂, or O, and to form compounds of the limiting type SbX₅; for example, SbE₄Cl corresponding to NH₄Cl with the substitution of nitrogen by antimony, and of hydrogen by the hydrocarbon radicle. The elements which are most chemically analogous are characterised by the fact of their giving compounds of similar form RX_n. The halogens which are analogous give both higher and lower compounds. So also do the metals of the alkalis and of the alkaline earths. And we saw that this analogy extends to the composition and properties of the nitrogen and hydrogen compounds of these metals, which is best seen in the salts. Many such groups of analogous elements have long been known. Thus there are analogues of oxygen, nitrogen, and carbon, and we shall meet with many such groups. But an acquaintance with them inevitably leads to the questions, what is the cause of analogy and what is the relation of one group to another? If these questions remain unanswered, it is easy to fall into error in the formation of the groups, because the notions of the degree of analogy will always be relative, and will not present any accuracy or distinctness Thus lithium is analogous in some respects to potassium and in others to magnesium; beryllium is analogous to both aluminium and magnesium. Thallium, as we shall afterwards see and as was observed on its discovery, has much kinship with lead and mercury, but some of its properties appertain to lithium and potassium. Naturally, where it is impossible to make measurements one is reluctantly obliged to limit oneself to approximate comparisons, founded on apparent signs which are not distinct and are wanting in exactitude. But in the elements there is one accurately measurable property, which is subject to no doubt—namely, that property which is expressed in their atomic weights. Its magnitude indicates the relative mass of the atom, or, if we avoid the conception of the atom, its magnitude shows the relation between the masses forming the chemical and independent individuals or elements. And according to the teaching of all exact data about the phenomena of nature, the mass of a substance is that property on which all its remaining properties must be dependent, because they are all determined by similar conditions or by those forces which act in the weight of a substance, and this is directly proportional to its mass. Therefore it is most natural to seek for a dependence between the properties and analogies of the elements on the one hand and their atomic weights on the other.

This is the fundamental idea which leads to arranging all the elements according to their atomic weights. A periodic repetition of properties is then immediately observed in the elements. We are already familiar with examples of this:—

F	=	19,	Cl	=	35·5,	Br	=	80,	I	=	127,
Na	=	23,	K	=	39,	Rb	=	85,	Cs	=	133,
Mg	=	24,	Ca	=	340,	Sr	=	87,	Ba	=	137.

The essence of the matter is seen in these groups. The halogens have smaller atomic weights than the alkali metals, and the latter than the metals of the alkaline earths. Therefore, if all the elements be arranged in the order of their atomic weights, a periodic repetition of properties is obtained. This is expressed by the law of periodicity, the properties of the elements, as well as the forms and properties of their compounds, are in periodic dependence or (expressing ourselves algebraically) form a periodic function of the atomic weights of the elements.[8] Table I. of the periodic system of the elements, which is placed at the very beginning of this book, is designed to illustrate this law. It is arranged in conformity with the eight types of oxides described in the preceding pages, and those elements which give the oxides, R₂O and consequently salts RX, form the 1st group; the elements giving R₂O₂ or RO as their highest grade of oxidation belong to the 2nd group; those giving R₂O₃ as their highest oxides form the 3rd group, and so on; whilst the elements of all the groups which are nearest in their atomic weights are arranged in series from 1 to 12. The even and uneven series of the same groups present the same forms and limits, but differ in their properties, and therefore two contiguous series, one even and the other uneven—for instance, the 4th and 5th—form a period. Hence the elements of the 4th, 6th, 8th, 10th, and 12th, or of the 3rd, 5th, 7th, 9th, and 11th, series form analogues, like the halogens, the alkali metals, &c. The conjunction of two series, one even and one contiguous uneven series, thus forms one large period. These periods, beginning with the alkali metals, end with the halogens. The elements of the first two series have the lowest atomic weights, and in consequence of this very circumstance, although they bear the general properties of a group, they still show many peculiar and independent properties.[9] Thus fluorine, as we know, differs in many points from the other halogens, and lithium from the other alkali metals, and so on. These lightest elements may be termed typical elements. They include—

H.

Li, Be, B, C, N, O, F.

Na, Mg....