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knows, the valency of a combination of elements can be immediately deduced from a knowledge of the constitution of the combination, and of the valencies of its elements. The relation between the magnitude of the charge carried and the constitution is thus comparatively simple.

The sign of the charge carried is intimately connected with the nature of the ion. As has been mentioned in Chapter I., all the simple metallic ions, the hydrogen ion and certain complex ions, such as those derived from ammonium, NH,, phosphonium PH, etc., carry positive charges. On the other hand, the halogens, hydroxyl, and an extremely large number of complex ions, carry negative charges.

The transition from an anion to a cation is generally accompanied by a large change of constitution, e.g. CrO," to Cr, so that a comparison between the constitution of anions and of cations gives very little result. We can only say that an ion containing a large number of electro-negative atoms or groups-e.g. O, Cl, NO-will probably be an anion.

Among the metal-ammonia compounds the change from anion to cation or vice versa can be made to take place in stages, in each of which the change of constitution is comparatively small. In the example given on p. 136, the effect of successive introduction of negative groups into a cation is seen clearly.

§ 2. RELATION OF MOBILITY TO CONSTITUTION.

The numbers quoted in this section are not the actual velocities of the ions, but are proportional to them, being in fact the equivalent conductivities (in mhos 1) for infinite dilution at 25°. From them the following conclusions have been drawn :

The mobility of elementary ions is a periodic function of the atomic weight, and rises with it in each series of analogous

In cases where a different unit was used in the original memoirs, the numbers have been recalculated by means of the factors given by Kohlrausch and Holborn, Leitvermögen der Elektrolyte.

elements; but analogous elements with atomic weights greater than 35 have approximately equal mobilities.1

This is apparent from the curve (Fig. 29), which represents the following numbers :—

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The mobility of hydrogen-364'9 at 25°2-is much greater than that of any other ion, and, like many other properties of hydrogen, finds no expression in the periodic law.

Isomeric ions and metameric anions have the same mobility.3

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2

Noyes, Jour. Amer. Chem. Soc., 24. 944 (1902). 3 Bredig, loc. cit.

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Phenylacetic acid is metameric with the three toluic acids, and bears out the rule that metameric anions have the same mobility.

Metameric cations, on the other hand, have, in general, different mobilities. And Bredig found that among metameric cations of the ammonium type, the mobility is greater, the greater the number of carbon atoms joined directly to the nitrogen atom. That is, for ions of the same empirical formula, a quaternary substituted ammonium ion has a greater mobility than a tertiary ion, a tertiary ion a greater mobility than a secondary ion, and a secondary ion a greater mobility than a primary ion.

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Comparing anions with anions, and cations with cations, the mobility of an ion is the smaller the greater the number of atoms

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Both of these series of organic compounds are homologous,

Bredig, loc. cit.

and it is seen that the effect of substitution of CH for hydrogen is less, the greater the number of atoms in the molecule. It follows that substitution of CH, for hydrogen in an ion with a very large number of atoms, will cause a very small change in the mobility.

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There are many cases among cations to which the rule that the mobility decreases with an increase in the number of atoms, does not apply. Such are those in which substitution of an alkyl radical causes an increase in the number of carbon atoms directly attached to nitrogen, so that the increase of mobility due to this cause is greater than the decrease in mobility due to the increase in the number of atoms in the ion, eg.

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An increase of CH2 is here accompanied by an increase in mobility.

The following changes diminish the mobility:

(1) Addition of hydrogen, carbon, chlorine, or bromine. (2) The replacement of hydrogen by chlorine, bromine, or iodine, or by the methyl group (with the exceptions mentioned above), or by the amido group NH2, or by the nitro group NO2

(3) The substitution of an element in an ion by a chemically analogous element of greater atomic weight, e.g.

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The substitution of sulphur for oxygen forms an exception to this rule, since it is accompanied by an increase in mobility.

In all cases the effect on the mobility of any particular change in constitution is less the greater the number of atoms in the ion. It follows that the mobility of ions tends to a minimum value, which, according to Bredig, is reached with ions of 50 or 60 atoms, and which appears to lie between 17 and 20, both for anions and cations. Moreover, the differences in the effects of different elements soon disappear as the number of atoms in the ion increases, and it has been found that the mobility of anions of more than 12 atoms (Ostwald), and of cations of more than 40 atoms (Bredig), depends only on the number of atoms in the ion, and not on their nature.

§ 3. RELATION OF NUMBER OF IONS IN SOLUTION TO CONSTITUTION.

The number of ions produced in the solution of a compound depends on the number of ions formed in the splitting up of one molecule, and on the number of molecules which split up.

(a) Number of Ions produced from One Molecule.

The number of ions given by a molecule of a compound. on ionisation is determined by its constitution. The following cases are those which most often occur :

(1) Compounds giving primarily Two Ions.-Such are mono-basic acids, mono-acid bases, and the salts formed by their interaction; and the salts formed by the interaction of di-basic acids and di-acid bases. This is the simplest mode of ionisation possible, but the case is complicated by the power which ions have of uniting with neutral molecules to form complex ions.

As will be seen later, there are but few univalent ions

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