represent the number of mono-, di-, tri- and tetra-valent atoms, then the maximum number of monovalent atoms will be n1 = 2n2 + 3n ̧ + 4n4 + 5n5 + 6n6 + 7n, + 8n ̧ − 2 (n2+ nz + nj + nz + n + n + n − 1) In organic compounds, which usually contain only mono-, di-, tri-, and tetra-valent atoms, n1 =2+1n2+2n1. The number of divalent atoms has no influence on the number of monovalent atoms. Each trivalent atom increases the number of possible monovalent atoms by one, and each tetravalent atom by two. As no compounds are known in which the number Պլ of monovalent atoms is greater than is here indicated, this is regarded as a confirmation of the theories of valency and of atomic linking. If there is only one polyvalent atom in the molecule then only one constitution is possible, even when the monovalent atoms differ in their nature, because the valencies of one and the same atom must be held to be equivalent (§ 43), and it is, consequently, immaterial which atom is united to a given affinity. The following formulæ admit of only one interpretation: 4 CH2Cl CH,Br2 CH CHBг3 CH,ClBr Methane Methylchloride Methylene bromide Bromoform Chlorobromomethane It is an open question (§ 54) whether the formulæ only admit of one signification when all four monovalent elements are different. When two polyvalent atoms combine, the formula has only one meaning when all the affinities are satisfied by monovalent atoms of the same element. For example : Three similar polyvalent atoms can unite with one kind of monovalent atom to form saturated compounds, which can only exhibit one form of structure, e.g. propane : Again, one structure alone is possible in a compound containing INVESTIGATION OF ATOMIC LINKING 79 two polyvalent atoms where one of the monovalent atoms differs But this is no longer the case when a second monovalent atom of another element enters the compound. The entrance of the first of these two atoms, puts an end to the equality between the two polyvalent atoms, and consequently the particular polyvalent atom with which the second monovalent atom unites is now of material importance. Isomeric compounds are now possible, and the number of such compounds theoretically possible are actually known. When there are only two atoms of the new kind present, it does not matter whether they are alike or dissimilar. Only two forms are possible for the com H,C--CHCIBr and H,CIC--CH,Br Ethylidene chlorobromide Ethylene chlorobromide But if the number of different monovalent elements increases the number of isomerides will increase. If all six monovalent atoms are different, then there will be 10 isomerides, as can be calculated by permutation. If the six monovalent atoms are numbered 1, 2, 3, 4, 5, 6, then we have the following combinations: In the case of three similar or two dissimilar polyvalent atoms, isomerism occurs as soon as a single atom of a second monovalent element enters the compound : CNH,CI CH,Cl - - NH, or CH, - - NHCI = CH, - - CH2 - CH2Cl or CH, – – CHCI – – CH, Propylchloride 2 Isopropylchloride The number of isomeric compounds increases rapidly as the number of polyvalent atoms and the variety of monovalent elements increase. Three carbon atoms and eight similar monovalent atoms can only be arranged in one manner, but if the 8 monovalent atoms are all different, then no less than 280 forms of combination are possible. It is clear that the atomic linking in a given compound can only be determined by calculation in the very simplest cases. $ 46. Determination of the Linking by Synthesis and Analysis. The decomposition and building up of compounds afford a valuable means of determining the atomic linking. The conclusions based on these methods depend on the assumption that those atoms which are united together before the union of two compounds remain united together after the act of combination, and that, on the other hand, those atoms which remain joined together after the decomposition of a compound were previously united in the said compound. This deduction was made use of long before the doctrine of atomic linking was known; but, strange to say, the conclusions arrived at in this way were proved to be untenable by the knowledge of atomic linking and were abandoned after a prolonged discussion. The very ancient observation that a salt is formed from an acid and a base, and can again be decomposed into these constituents, led to the view that the acid and base are present as such in the salt. Calcium carbonate decomposes into lime and carbon dioxide, CaCO3 = CaO+CO,; from this it was inferred that calcium carbonate contained the proximate constituents CaO and CO2, and that its formula was CaO,CO2. Analogous formula were given to other salts, e.g.— RADICALS And similar formulæ were used for the acids : H2O.SO3 and so on. 2 2 3H,O,P2O Phosphoric Acid 81 It is evident that formula of this description are not permissible, for the compounds are represented as composed of groups of atoms: these groups are already saturated and therefore have no free affinities available for mutual combination. H--O‒‒H, Ca== 0, K--0--K, 0==C==0 These formulæ are called dualistic, on account of their separation into two parts. They have been replaced by others in which the groups are united together by means of their oxygen assump The principle underlying these old formula, viz. the tion that those atoms which were united in a compound, must be regarded as remaining combined, still remains in force, but in its application due care is taken to comply with the law of atomic linking. For example, the action of hydrochloric acid on alcohol is represented by the equation In this case two of the carbon atoms and at least four of the hydrogen atoms remain together; we may assume, therefore, that these atoms are contained in alcohol and in ethyl chloride also The two groups H,O, water, and HCl, hydrochloric acid, are closed groups, in which all the affinities are saturated. We must therefore assume, that not more than one hydrogen atom in alcohol is united to oxygen; that is to say, that alcohol contains the monovalent 'hydroxyl' group, -0 - H, and that this group and the five hydrogen atoms, must be directly attached to the carbon thus: C,H,- 0- – H. - By the action of hydrochloric acid, oxygen is split off; there is no doubt about this taking place, as the oxygen ceases to be united to carbon. The oxygen atom takes away with it the atom of hydrogen to which it was previously united. Where does the second atom of hydrogen come from? Is it taken away from the carbon or the chlorine? Since the monovalent chlorine must detach itself from the hydrogen, before it can unite with the carbon, it is more than probable that the hydrogen atom from the hydrochloric acid, combines with the hydroxyl. As there is only one interpretation for a group containing two carbon atoms, the formula must be CH¿—CH2—O—H+H—-Cl=CH‚—-CH2Cl + H—0—H Alcohol 2 Ethyl Chloride The groups of atoms which remain united together in these reactions are termed 'radicals,' as they were considered to be the roots from which the peculiarities of the compounds arose: this expression dates back to the time of Lavoisier. Alcohol and ethyl chloride have the same root, the radical ethyl, C2H ̧. Further conclusions as to atomic linking may be deduced from the decomposition and the mode of formation of substances. If an atom or a radical replaces another in a compound, we assume that it takes the place previously occupied by the latter. The chlorine, from the hydrochloric acid, takes the place of the hydroxyl and the hydroxyl takes the place of the chlorine and unites with the hydrogen atom. This replacement of one atom or radical by another is termed 'substitution,' and is distinguished from 'addition,' i.e. the simple union or combination of two radicals or atoms; e.g. |