bonds generally is just, it follows, I think, that the question alluded to is meaningless; but as it has been hotly disputed about, it may be well briefly to consider it here. It is assumed in the bond-hypothesis that the so-called affinities of atoms attract or satisfy one another, and hence those affinities of one atom which are not satisfied by affinities of another, must be satisfied by other affinities of the atom itself. No molecule, it is sometimes said, can contain an odd number of atoms of uneven valency. This outcome1 of Gerhardt's 'law of even numbers' (see ante, chap. I, par. 36) is however contradicted by the existence of the molecules I, NO, NO,, CIO,, WC,, VCI, or VOC,, and cannot therefore be accepted as a statement of facts, unless indeed the valency of an atom is a number susceptible of arbitrary variation. That the maximum valency of each atom is fixed is generally admitted. One school however holds that (e.g.) a tetrad atom is always tetrad, another school that a tetrad may function as a diad atom; in the molecule CO, for instance, the carbon atom, it is said, is tetrad, but two of its affinities are mutually satisfied. The opponents of this view would say that in CO the carbon atom is divalent, the other pair of bonds being latent. The dispute has been wholly a battle about words. Whether the bonds are latent, or are mutually satisfied, they are equally existent: as Lossen remarks, ‘zwei und zwei geben doch immer vier.' But if always existent, are the bonds always of equal value? Are the two pairs of bonds which hold the two oxygen atoms to the carbon in CO, equal in value to twice the pair of bonds by which one oxygen atom is held to a carbon atom in the molecule CO? Now if we wish to compare things we must have a standard; but I think sufficient facts have been enumerated to shew that no standard exists in terms of which the expression value of a bond' may be stated. Even if the valency of an atom is regarded as expressing the total number of parts into which the chemical energy of that atom is divisible, this 1 The statement is sometimes put in this form; "the sum of the valencies, or affinities, of the atoms in any molecule is always an even number.” must mean, that the energy is divisible when there is mutual action between the given atom and other atoms in a molecule. Thus, assume for a moment that the chemical energy of an atom of carbon is divisible into four parts, it does not follow that each part represents a fourth of the whole energy or always represents the same portion of that energy. To take an illustration, in the stable molecule CO we must suppose, on this hypothesis, that the whole of the chemical energy of the carbon atom is employed in the transaction symbolised by the formula C-O; again, in O-C-S the whole of the energy of the carbon atom is employed, but the energy represented by O-C is probably different from that represented by CS, and the sum of these is probably different from that represented by the expression O-C-O. The number of possible ways in which the energy is distributed is, on this hypothesis, measured by the valency of the atom, the amount of the energy employed in any atomic transaction depends on the nature of the atom or atoms between which and the given atom there is mutual intramolecular action1. Even if we adopt this, the most dynamical view of valency that can be adopted with any safety, the controversy concerning equal and unequal bonds is seen to be a mere logomachy2. 1 For a fuller working out of this way of regarding valency see Claus, Ber. 14. 432. 2 It is sometimes said that the hydrogen atoms in the molecule of benzene are of equal value, but when one of these atoms is replaced by a radicle, the remaining five are of different values relatively to the radicle introduced into the molecule. To make such a statement as this, it seems to me, is to employ the term value in too loose and vague a way. All the hydrogen atoms in a molecule of a monoderivative of benzene are monovalent, and therefore of equal value so far as 'proportion in exchange' for chlorine, bromine &c. goes. What appears to be meant by the statement in question is, that more than one mono-derivative (chloro-bromo-......or generally X-derivative) can be obtained from the molecule CHX; but this is simply a special illustration of the general proposition that the properties of compounds are not wholly dependent on the valencies of their constituent atoms. SECTION V. Molecular Compounds. 99. The adjectives molecular and atomic have been employed by Kekulé1 and others to distinguish those compounds which separate into two or more other substances when heated, from those which can be vaporised without decomposition. Ammonium chloride, which when heated yields a vapour containing ammonia and hydrochloric acid, may be taken as a typical molecular compound, and water, the vapour of which contains only molecules of water-gas, as a typical atomic compound. This division of compounds has played an important part in the development of the theory of valency. Kekulé has always insisted that facts regarding atomic compounds can alone be employed as data for finding the valencies of elementary atoms; his opponents have retorted by demanding a definition of molecular as opposed to atomic compounds, and by shewing that every proposed definition fails when applied to actual phenomena. But it is not so much as concerns the theory of valency that the distinction implied in the words atomic and molecular compounds ought, I think, to be insisted on; if the arguments put forward in the preceding section are of any value, we must agree to confine the theory of valency, at present, to gaseous compounds. There are however many and varied phenomena, all more or less belonging to the borderlands between chemistry and physics, which may be conveniently considered under the heading of molecular compounds. 100. And I would begin by admitting that no strict definition of molecular, as opposed to atomic, compounds can be given, which shall enable us to assign every disputed case to its proper class. A substance may yield a vapour which is chemically homogeneous below a certain temperature but heterogeneous above this temperature: we cannot fix a limit 1 See his Lehrbuch, Vol. 1. pp. 142, 145, 443, &c.: also Compt. rend. 58. 510. for each group of compounds and say, that those which yield vapours homogeneous below this temperature are atomic, while those in the vapour of which dissociation begins below the temperature-limit are molecular. I would again urge the importance of remembering that when we say that a gas consists of molecules of this or that composition, we refer and can refer only to the average composition of the gas; many molecules may be dissociated into two or more chemically different kinds of matter, other molecules may be aggregated into complex groups. Even in an elementary gas at moderate temperatures some atoms and many groups of molecules are probably present at any moment: the values obtained for the specific gravities of gaseous bromine and iodine, and for gaseous nitrogen dioxide, stannous chloride, and acetic acid well illustrate the gradual nature of the passage from one average molecular state to another1. IOI. Some chemists would recognise in mixtures of two or more liquids, in solutions of salts, and of gases (e.g. CO2) in water, the existence of molecular compounds. In such cases the proportions in which the substances are supposed to be combined are very variable. It cannot be correct to speak of a molecule of the mixture of alcohol and water, or of the solution of salt in water, although it may be permissible to regard these liquids as containing groups of molecules of alcohol and water, or of salt and water. There are other actions wherein small changes in physical conditions suffice to cause changes in the relative quantities of substances combined in definite proportions: for instance, when the substance containing water and sodium phosphate in the proportions Na,HPO. 12H,O is heated, it very readily loses water and becomes Na,HPO,. 7H,O. If by molecular compound is meant, a loose combination in definite proportions of two or more chemically different kinds of matter so as to produce another kind of matter characterised by fairly definite properties, but readily undergoing change, then 1 See par. 101, pp. 205, 208–9. we may certainly say that Na,HPO,. 12H,O is a molecular compound. Once more, compounds exist which are characterised by very definite properties, but which, when heated, undergo gradual change into two or more substances, the original compound being gradually re-formed as the vapours cool. Thus the formula PCl, expresses the elementary composition of an undoubted chemical compound; when this solid substance is heated it vaporises, but the vapour can be proved by experiment to contain molecules of PC1, and Cl, along with undecomposed PC15. The following numbers. shew the gradual progress of the change which occurs: Calculated sp. gr. of gaseous PC1;=7'2 gas consisting of PCl2+Cl2=3⋅6 ̧ Sp. gr. of vapour. [air=1] Number of molecules Temperature. The following numbers representing the specific gravities of nitrogen tetroxide at various temperatures exhibit the gradual dissociation of molecules of NO, into molecules of NO2": 100 (d-D) D 1 Calculated by means of the formula p= where number of molecules decomposed, D= observed density of gas, d=theoretical density of vapour supposing no dissociation to occur. This formula assumes that each molecule dissociates into two parts: if each molecule separates into a parts, the formula is p= chemie, pp. 114, 115. 2 Naumann, loc. cit. p. 117. |