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atoms in the molecule, we find that there are two possible arrangements of the two carbon, six hydrogen, and one oxygen atoms (assuming the valency of the carbon, hydrogen, and oxygen atom to be 4, 1, and 2 respectively), viz. (a) Η Η
(6) H H
1 H H
H hence, two compounds, each having the composition expressed by the empirical formula C,H,O, may exist.
But if we make the second assumption, viz. that variation of properties is to be correlated with variations in the distances between the atoms in the molecule, the relative positions of these atoms remaining unchanged, we may have an apparently unlimited number of compounds of the formula C,H,O; such compounds might perhaps be represented in this way, (a) H H
(6) H H
H and so on.
Now as only two compounds, C,H,O, are know to exist, we have a presumption in favour of the first supposition : much stress cannot however be laid on this argument. Moreover if the second of the two suppositions is correct, then any molecule containing two atoms should be capable of existing in more than one modification; in other words, every diatomic molecule should be capable of shewing isomerism. But there is no certainly-established instance of isomerism exhibited by any molecule containing less than three atoms; therefore, as the assumption that variations of properties exhibited by compounds having the same composition and same molecular weight are connected with variations in the relative positions of the atoms composing the molecules of these compounds, suffices to explain the vast majority of well-authenticated cases of isomerism anong gaseous molecules, we conclude that it is better, at any rate at present, to build the general theory of isomerism on this hypothesis.
67. But before more fully considering this subject, it will be well to glance at the allied phenomena of allotropy and polymerism.
The table on p. 42 shews that of the thirteen elements whose molecular weights have been determined by the help of Avogadro's law, four, viz. oxygen, sulphur, selenion and iodine (probably bromine also) possess a smaller molecular weight at high than at lower temperatures ;—the number of atoms in the molecule of oxygen at temperatures below about 300°, and under special conditions is 3, at temperatures above 300° it is 2; the molecule of sulphur at temperatures not much higher than the boiling point of that element contains 6 atoms, and at somewhat higher temperatures 2 atoms; the number of atoms in the molecule of selenion varies from 3 to 2, and in the molecule of iodine (and probably also in that of bromine) from 2 to 1, according to temperature. We know that the properties correlated with the existence of the triatomic molecule O, differ much from those which characterise the diatomic molecule Oz: no experiments have been made to compare the properties of the hexatomic with those of the diatomic molecules of sulphur, of the triatomic with the diatomic molecules of selenion, and of the diatomic with the monatomic molecules of iodine.
Of the 15 or 16 nonmetallic elements, phosphorus and
1 The supposition that isomerism may be due to variations in the distances between atoms, the relative positions of which remain unchanged, appears to be opposed to the results of physical experiments which are in agreement with deductions made from the kinetic theory of gases. See Lossen, loc. cit. p. 269.
arsenic, boron, carbon and silicon-besides sulphur and selenion-exhibit marked variations in physical and chemical properties when in the solid state. We certainly are not justified in unconditionally asserting that these variations of properties accompany differences in the atomic configurations of the molecules, or differences in the numbers of atoms in the molecules, of red and yellow phosphorus, or of octahedral and prismatic sulphur, &c. When the differences in properties are chiefly physical (e.g. differences in crystalline form, in specific gravity, in melting points, &c.), they may very probably be correlated with differences in the molecular, rather than in the atomic, configurations of the various modi· fications of the element in question'.
Be this however as it may, the differences experimentally shewn to exist between the properties of the molecules of gaseous oxygen and ozone are explicable in terms of the molecular theory only by admitting that the properties of a molecule are dependent not only on the nature but also on the number of the atoms which compose it”.
The marked chemical differences between red and yellow phosphorus would lead us to expect that the molecular weight of gaseous phosphorus would be found to vary with variations of temperature: such variations have not however as yet been observed.
1 See section B. of present chapter.
* It ought to be noted that change from one allotropic form to another is accompanied by evolution or absorption of heat; see post, chap. IV., par. 125.
3 V. Meyer states (Ber. 14. 1455; see also do. 13. 1116 note] that the vapour densities of phosphorus and arsenic at very high temperatures point to the existence of molecules weighing less than P, and As, respectively.
There are some interesting observations bearing on the subject of allotropy by W. Spring in the Berichte (see especially 16. 1002—3). Spring finds that when an element which exhibits allotropy is subjected to great pressure, that modification which has the greatest specific gravity is produced. Yellow phosphorus is changed into red by compression : red phosphorus and sulphur do not combine until heated to 260°, i.e. to the temperature at which red is changed to yellow phosphorus; red phosphorus does not combine with sulphur when the two are subjected to a pressure of 6500 atmospheres, at which pressure many metallic sulphides are produced. Hence Spring concludes that red phosphorus is less chemically energetic than yellow; and generally that the more a solid substance is rendered dense, the
68. The names allotropy and polymerism are applied to analogous phenomena exhibited by elements and compounds respectively.
If two molecules exist, consisting of the same elementary atoms, but one heavier than the other, the heavier molecule is said to be a 'polymeric modification,' or a 'polymeride' of the other :—thus C10H20 is a polymeride of CzH10, C15H,4 is a polymeride of CH6, H,C,N,O, is a polymeride of HCNO, CH,,0, is a polymeride of C,H,0. Glucose, xC,H,O, is not however regarded as a polymeride of ethylene oxide, C,H,O: the name is restricted to those molecules whose weight is a multiple of that of other molecules, and which are obtained by simple reactions, generally by the action of heat, from these other molecules. Thus, ethaldehyde, C,H,O, is easily polymerised, e.g. by the action of a very little hydrochloric or sulphuric acid, with formation of parethaldehyde, C&H 203; but the latter body is not directly obtainable from ethylene oxide, although the molecule of this compound, like that of ethaldehyde, contains 2 atoms of carbon, 4 of hydrogen, and i of oxygen.
But few examples of undoubted polymerism are furnished by compounds of the elements other than carbon; one of the most marked cases is the molecule N,O,, which is a polymeride of NO,, another is furnished by the molecules Sn, Cl, and SnCl.
69. The phenomena summarised in the term isomerism, i.e. the existence of molecules characterised by different properties but containing the same number of the same atoms, must now be examined in some detail.
Isomeric compounds are generally said to be metameric' when they belong to different chemical types. This statement does not of course furnish a definition of metameric compounds; but it is sufficient. Various hydrocarbons, all
. possessed of the general properties of paraffins, but each differing in some properties-chemical and physical—from the others, are represented by the formula CoH,: various hydromore is its chemical activity decreased. Red phosphorus he regards as a polymeride of yellow phosphorus.
carbons, all benzenes, but each characterised by its own special properties, are represented by the formula C Ho: the different paraffins—C6H, or the different benzenes, C3H1.—are said to be isomerides one of the other. But although two molecules are represented by the formula C,H,O, yet these belong to very different types, or groups of compounds; one is a primary alcohol, the other an ether : so again allylic alcohol and dimethyl ketone have both the formula C3H60, but these bodies are altogether distinct in their chemical propertiessuch compounds are said to be metameric. Metamerides are thus seen to be a sub-class included in the larger class of isomeric compounds.
A few inorganic compounds exhibit phenomena which may be explained by supposing the existence of isomeric molecules, but it is only when we study the compounds of carbon that we are obliged to admit that molecules may contain the same numbers of the same atoms but differ in chemical and physical properties.
70. The theory of valency having led to the recognition of the molecule as a structure, may be carried further; it may guide us in determining the probable relative structures of isomeric molecules (see note to p. 133).
If it be granted that isomerism is correlated with different relative positions of atoms, but not with different distances between atoms in the same relative positions in the molecule', (see p. 134), it follows, that, a molecule containing not more than two atoms cannot exhibit isomerism. The maximum number of monovalent atoms which can be combined with polyvalent atoms in a molecule is found by the formula?
where n,, 13, 1,, &c. represent the numbers of monovalent, trivalent, tetravalent, &c. atoms in the molecule. Any molecule in which the value of n, agrees with that deduced from
1 Such formulæ as (=N- and =N-O- are really, at present, the same.
? See Lothar Meyer, Die Modernen Theorien der Chemie (4th Ed.), pp. 218 et seq., of which pages free use has been made in these paragraphs.