Page:EB1922 - Volume 30.djvu/671

80

Atomic

Specific Heat "+' at 50A

60

40

20

U WC HO F Atomic

NdUttSi Weight

A1S,PS d KCa Ti VCrfcF.KCo 0/Zn Ca A. 5* lib Sr Zr bo Kn RhMAg U In SoSb I Tt CiSjI.OD,

Ta W 0> IrftAuNj 71 PbBi

20

40

60

80

100

120

140

160

ISO

200

220

240

FIG. i. Diagram showing periodic variations of heat capacity and atomic volume.

absorbing capacity is dependent upon temperature. The amount of heat absorbed, by atomic proportions of a majority of the primaries, when their temperature is raised through a given interval, it is well known, is nearly a constant quantity over a wide range of tempera- ture; only in the specifically non-metallic elements, silicon, beryllium, boron and carbon, is the departure from this " rule " at all con- siderable; and in the case of these, as the temperature is raised, the heat capacity increases, until towards 1, 000 their behaviour approximates to that of the metals. It was long supposed, in fact, that there was a general tendency for the atomic heats to converge towards a constant value as the temperature was raised and to diverge as the temperature was lowered. Taking into account the fact that metals generally appear to be of simple molecular com- position compared with the non-metals, it was not improbable that the differences were, in the main, differences due to molecular com- plexity; recent determinations of specific heat at the very low tem- perature of liquid hydrogen (50 absolute), by Sir James Dewar, nave brought to light, however, the surprising fact that heat capacity is subject to periodic variation, much as the volume occupied by atomic proportions varies even at ordinary temperatures. The two properties are contrasted in the accompanying diagram.

The striking fact is brought out in this diagram that whilst the chemically most active metallic elements (the alkali metals) are but little affected, the best defined metals diminish in heat- absorbing power to a very marked extent.

The values deduced with the aid of ordinary materials, especially in the solid state, cannot be regarded as " atomic " in any proper sense of the term, as they are of different degrees of molecular com- plexity and the molecular complexity varies considerably with temperature. Thus a large number of the metals appear to have monatomic molecules, whilst there is reason to believe that those of the non-metallic elements, carbon especially, are of considerable complexity; but even in the case of the elements having monatomic molecules, intermolecular affinity is subject to great variation, being slight for example in mercury but considerable in the case of metals such as gold, silver and copper. In a complete theory of atomic structure, all these variations must be taken into account.

The correlation of molecular structure with function, in the carbon compounds, has been carried so far that the chemist has entire confidence in his conclusions because of the large number of instances in which a comparison of fact with hypothesis can be made. The assumptions involved are few and it is more than re- markable that it should have been possible to erect so vast and complex a system upon so simple a foundation. The structural formulae of organic chemistry are to be regarded, however, main- ly as condensed symbolic expressions, indicative of the general arrangement of the constituent radicles and of the functional behaviour of the compounds represented, not as absolute expres- sions of structure; indeed, it is becoming clear that the conven- tions which have hitherto sufficed should be modified in certain particulars to give fuller symbolic expression to the ascertained facts and to render the formulae more nearly a representation of the molecular architecture. In the case of the compounds of elements other than carbon, valid methods of determining struc- ture are yet to be devised. It is surprising, to take an example, that we have no clear conception of the atomic arrangement of so simple and important a substance as sulphuric acid, H 2 SOi.

New methods of promise are coming into use, and it is to be expected that much will be learnt, especially by the study of the internal structure of crystalline solids, by crystallographic (geo- metric) methods and by means of X-rays a field of inquiry opened up by Laue and then by the Braggs and others.

Frankland's original conception that the carbon atom has four affinities still holds the field. In modern times, it has been am- plified by the introduction of space conceptions and the use of the tetrahedron as a model of the atom; in this way greater precision has been arrived at because of the limitations which are introduced. Perhaps the most important outcome of the hy- pothesis is, that whenever but in no other case a system is formed in which a single carbon atom is associated with four dif- ferent unit systems, the complex may exist in two like asymme- tric forms (of opposite character), distinguished by their power of influencing plane polarized light in opposite directions.

This conception of the carbon atom is entirely justified by the results of the analysis of the internal structure of the diamond by means of X-rays, carried out by Sir William Bragg and his son W. L. Bragg. The arrangement of the carbon atoms is such that every atom is the centre of gravity of four others arranged around it in tetrahedral fashion. Apparently there are definite sub-cen- tres of force on the outskirts of an atom; in the carbon atom of which the diamond is composed, there is evidence of four such sub-centres arranged symmetrically that is to say tetrahe- drally. The atoms in the diamond form two sets; in each set the individual atoms present the same orientation and constitute a cubic space-lattice, but the orientations of the two sets are oppo- site. The effect of this difference is mirrored in the X-ray spec- trum. This conclusion of the physicist is a complete justification of the views long held by chemists that the carbon atom has di- rected valencies and may give rise to asymmetric structures.

In the Bragg model of the diamond, although they are united similarly and symmetrically, in all four directions of trigonal axes, the carbon atoms can be allotted to similar sets of six, in each of which the individual atoms are united in the manner pic- tured by the chemist in the symbol of hexamethylene.

CH,

" C CH

^ H,C^SCH,

CH,

Although, in the diamond, the carbon atom is the physical unit or molecule, the molecule may equally well be regarded as inde- terminate, indeed as coterminate with the mass, as the constitu- ent units are uniformly related. As influencing our views as to the manner in which solids act in solutions and attract molecules of their own kind, it should be noted that each carbon atom at the exposed surface of the diamond mass has an affinity free.