30 SUNDAMENTALS OF SUBMICROSCOPIC MORPHOLOGY I 



constancy, not only in simple compounds, but also in very complicated 

 ones. In the heteropolar ion lattices a disturbing effect occurs because 

 of the opposite charges of the two partners. The ions have a polarizing 

 effect upon each other, which may lead to deformations of the electron 

 orbits (Fajans, 1923, 1925) in those cases where the symmetry of the 

 lattice does not exclude such effects, as, for instance, in the lattice of 

 NaCl (Fig. 26). The ions can then no longer be represented by spheres; 

 they represent dipoles similar to the water molecules in Fig. 17 (p. 19). 

 The result is that ion lattices often possess Httle symmetry and that 

 the atomic distances between given partners are subject to certain 

 fluctuations, depending on the circumstances. 



Fortunately this does not apply to the molecular structures of 

 organic compounds which always have a homopolar character; the 

 distances found in certain compounds can therefore be transferred 

 with perfect confidence to other ones, so that one can speak of 

 distance rules. In Table III a number of atomic distances are given as 

 determined in organic crystals by means of X-rays. In these considera- 

 tions the hydrogen atoms must be neglected, as they do not scatter 

 X-rays ; nor do they seem to have a perceptible influence on the dis- 

 tances between the atoms. Table III, for example, shows that in single 

 bonds the atom radius of carbon, r,;, amounts to 0.77 A and that of 

 nitrogen, r^^, to 0.71 A. In spite of the larger atomic weight of nitrogen, 

 its sphere of action is smaller than that of carbon. It is also seen that 

 the sphere of influence of the carbon atoms is decreased by double 

 bonds. 



Each valency in an organic molecule corresponds to a definite 

 amount of energy (Meyer and Mark, 1930). In the combustion of the 

 homologous paraffins, for instance, the heat of combustion per mole 

 increases by a definite amount for each new C-atom introduced; this 

 value amounts to about 70 kcal. The energy equivalents for the other 

 compounds mentioned in Table III have been determined in a similar 

 way. It will be apparent that with decreasing distance between the 

 C-atoms the energy content of the different bonds increases. 



To sum up, it can be said that in the main valency bonds which play 

 a part in the structure of protoplasm, distances of 1-1.5 A and bond 

 energies of the order of 100-200 kcal occur. 



Molecule lattice. In addition to homopolar main valency lattices and 

 heteropolar ion lattices we must consider molecule lattices. If the 



