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the molecule CO The formulae C=0 and O=C=0 are generally used to express these statements.

These formulae are evidently based on a meaning of atomic valency different from that we have been giving to this expression in preceding paragraphs. It is rather difficult to grasp the exact meaning of the statement, the atom of carbon is divalent in the molecule CO and tetravalent in the molecule Co,.' The statement seems to imply that an atom of carbon is capable of directly combining with either 2 or 4 hydrogen, fluorine, chlorine, bromine, or iodine, atoms, or with that number of other atoms which is equivalent to 2 or 4 atoms of chlorine, &c. The statement seems to assert that one atom of oxygen is truly equivalent sometimes to 2 atoms of hydrogen, or 2 atoms of chlorine, &c. and sometimes to 4 atoms of hydrogen, &c. But this assertion is scarcely capable of proof, because it seems impossible to define the exact meaning to be given to the expression equivalent to, as used with regard to atoms.

It has even been asserted that the atom of carbon is tetravalent in the molecule CO. If this is so, then one atom of oxygen is equivalent to 4 atoms of chlorine, &c. when oxygen and carbon combine to form the compound CO: but 2 atoms of oxygen are equivalent to 4 atoms of chlorine, &c. when oxygen and carbon combine to form the compound CO,. We see here the extreme difficulty, if not impossibility, of giving an exact and invariable meaning to the expression equivalent to, as applied to atoms.

It is generally the custom in writing structural formulae to represent each atom whose maximum valency is greater than one with as many lines proceeding from the symbol as correspond to the maximum valency of the atom. The atom of carbon, for instance, is generally represented with 4 lines proceeding from it, the atom of oxygen with 2 lines, and so

Thus the structural formulae for ethylic aldehyde and acetic acid are generally written thus ;H H

H

O 1

1 II H-C-C, and H - 0 - C.

1 ! H O

H OH

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Similarly the structural formulae of ethylene and the hypothetical isomeride ethylidene are generally written thus:-H H

H ||

II 1 C - C, and H-C - H. The structural formula of 1 H H

H the hydrocarbon acetylene, C,H,, is put into this form

H-C=C-H; whereas the formula H С C H would be used when the meaning given to valency is that explained in par. 360.

It would be out of place in an elementary book to discuss the possible meanings of these so called double bonds' and 'treble bonds. In the opinion of several chemists they have done much to hinder the advance of chemistry, by leading chemists to trust in names, and in far-fetched analogies, instead of in realities, and in well established and accurately defined points of resemblance and difference. On the other hand the employment of 'double and treble linkings' or 'bonds' has some points in its favour. It continually reminds the chemist of the maximum valency of each atom, and by doing this it suggests the possibility of reactions.

H H

H

d Thus, either of the formulae, (1) C or (2) C C,

1

1 H H

H H represents each carbon atom as directly interacting with only 3 other atoms, but formula (2) tells us that a carbon atom can directly interact with 4 other atoms, and hence suggests the possibility of adding 2 monovalent atoms to the molecule

H H

1 CH. So the formula for ethylic aldehyde H-C-C

.

H O visibly suggests the possibility of putting 2 monovalent atoms, e.g. Ci,, in place of the atom of oxygen.

H

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the same reactions as the formulae with double bonds, if we remember that the maximum valency of the carbon atom is 4, and that of the oxygen atom is 2. These formulae have the great advantage over those with double or treble bonds that they are based on a definite hypothesis regarding atomic valency.

The essential part of the hypothesis of valency is the conception of direct action and reaction between each atom in a molecule and a limited number of other atoms. As the whole molecule is held together by the mutual interactions of the atoms, there probably is what we may call indirect action and reaction between all the atoms which constitute the molecule. The hypothesis gives us a definition of the maximum valency of an atom, as the maximum number of monovalent atoms (i.e. atonis of H, F, Cl, Br, or I) with which the given atom directly interacts (i.e. in these cases combines) in any molecule; and it teaches that the specified atom never directly interacts with a greater number of other atoms, whatever be their valencies, than is expressed by the maximum valency as thus defined.

The hypothesis of valency is meaningless apart from the theory of atoms and molecules ; it is based on this theory, and all the results gained by using it are expressed in the language of the theory.

The theory of atoms and molecules is strictly applicable, at present, only to gases; therefore the hypothesis of valency, and all the terms to which it has given birth, are strictly applicable, at present, only to gases. But just as we made use of the molecular and atomic theory as a general guide in studying the chemical changes which occur among liquid and solid substances, so may we make use of the hypothesis of valency, provided we exercise sufficient caution, as general guide in attempts to learn something regarding the structure of those aggregations of atoms which form the reacting weights of solid and liquid substances. But it would be going too far afield to attempt to indicate here even the lines on which the hypothesis of valency may probably, be

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usefully employed in discussions regarding the structure of the reacting weights of solid compounds.

Our conception of chemical composition has been widened 376 by the examination of the phenomena of isomerism.

A statement of the composition of a compound should tell the percentage composition of the compound; it should also tell the composition of a reacting weight stated in numbers of combining weights of each element, and the composition of a gaseous molecule stated in numbers of atomic weights of each element; if the compound is gasifiable, it should also give such an indication of the arrangement of the parts of the molecule relatively to each other as can be gained by studying the interactions of the compound and expressing these in a structural formula based on the hypothesis of atomic valency.

The formula which best expresses the composition of a compound also tells a great deal about the properties of the compound. A satisfactory structural formula suggests many of the characteristic reactions of the compound the composition of which it expresses.

The structural formulae of many compounds of carbon 377 which are acids have been determined; we are therefore able to trace some of the connexions between the properties of this class of compounds and their composition, using composition in the widest sense we have given to the term.

In the molecules of the greater number of the carbon acids an atom of carbon probably directly interacts with an atom of oxygen and with the atomic group OH; this statement is usually expressed by saying that these molecules contain the

The following structural

H, formulae illustrate the statement concerning the composition of many carbon acids which has just been expressed in the symbolic language of the hypothesis of valency. Structural

Structural Acid.

Acid. formula.

formula. Acetic CH.CO.OH Benzoic CH.CO.OH Acrylic CH.CO.OH Phthalic CH.(CO.OH)2 Succinic CH(CO.OH), Mellitic C (CO.OH).

The basicity of these acids is connected with, and is measured by, the number of CO.OH groups in the molecule; thus acetic, acrylic, and benzoic, acids are monobasic, succinic and phthalic acids are dibasic, and mellitic acid is hexabasic.

group Co. OH

We can thus, in a great many cases, connect the group of properties connoted by the term acid with a certain arrangement of the parts of the molecules of the compounds which exhibit these properties.

In Chap. XIll. we found that a number can be assigned to each acid, called the relative affinity of the acid, which tells how much of a definite chemical change can be accomplished by that acid under defined conditions. An acid with a large affinity-constant is called a strong acid ; an acid with a small affinity-constant is called a weak acid. If definite relations can be established between the values of the affinity-constants, and the compositions, of acids, a great advance will be made in solving the essential problem of chemistry, which is to connect changes of composition with changes of properties. Investigations have been made of late years in this direction, and many results have been obtained. Thus the relative affinities of the acids HCI, HBr, HI, HF, H,S, HCN, in aqueous solutions, are approximately in the ratio 89 : 89 : 89 : 30; 0.1 : 0-2. The change of composition from HCl to HBr or HI is attended with practically no change in the strength of the acid ; but when an atom of fluorine is put in the place of an atom of chlorine, bromine, or iodine, in the molecule HX (X = Cl, Br, or I) this change is attended with a great decrease in the strength of the acid. Change of composition from HCI, &c. to HS or HCN is accompanied by a very great decrease in the strength of the acid.

The ratio of the affinities of the three acids, of similar composition, H,PO,, H POg, and H PO, is approximately 77.8 : 74.5 : 61.8. The ratio of the affinities of the four similar acids H,SO, H.SO, A_$,0g, and H.8.0 is approximately 66.5 : 150.5 : 178 : 181.5, and the ratio for the pair of acids H Seo and H Se, is 45 : 158. The basicities of the acids H PÓ,, H PO,, and H PO, are 1, 2, and 3, respectively; but all the acids of sulphur or selenion enumerated above are dibasic. In one series, H PO, to H PO,, a decrease in the value of the affinity-constant is accompanied by an increase of the basicity, and also by an increase in the number of oxygen atoms in the reacting weights, of the acids. In the other series, the basicity remains constant, and an increase in the number of oxygen atoms is accompanied by an increase in the value of the affinity-constant.

Relations have also been traced between the structural formulae of isomeric acids and the values of their affinity

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