Tables 676 and 677 551 



TABLE 676. — Cross-Sections and Lengths of Some Organic Molecules 



According to Langmuir (J. Am. Ch. Soc. 38, 2221, igi6) in solids and liquids every atom is chemically combined 

 to adjacent atoms. In most inorganic substances the identity of the molecule is generally lost, but in organic com- 

 pounds a more permanent existence of the molecule probably occurs. When oil spreads over water evidence points 

 to a layer a molecule thick and that the molecules are not spheres. Were they spheres and an attraction existed be- 

 tween them and the water, they would be dissolved instead of spreading over the surface. The presence of the — COOH, 

 — CO or —OH groups generally renders an organic substance soluble in water, whereas the hydrocarbon chain decreases 

 the solubility. When an oil is placed on water the -COOH groups are attracted to the water and the hydrocarbon 

 chains repelled but attracted to each other. The process leads the oil over the surface antil all the -COOH groups 

 are in contact if possible. Pure hydrocarbon oils will not spread over water. Benzene will not mix with water. When 

 a limited amount of oil is present the spreading ceases when all the water-attracted groups are in contact with water. 

 If weight vi of oil spreads over water surface A . the area covered by each molecule is AM/wN where M is the molec- 

 ular weight of the oil (O = 16), N, Avogadro's constant. The vertical length of a molecule I = M/apN = W/pA 

 where p is the oil density and a the horizontal area of the molecule. 



TABLE 677.— Size of Diffracting Units in Crystals fl 



The use of crystals for the analysis of X-rays leads to estimates of the relative sizes of molecular magnitudes. The 

 diffraction phenomenon is here not a surface one, as with gratings, but one of interference of radiations reflected from 

 the regularly spaced atomic units in the crystals, the units fitting into the lattice framework of the crystal. In cubical 

 crystals jiool this framework is built of three mutually perpendicular equidistant planes whose distance apart in 

 crystaltographic parlance is d\oo. This method of analysis from the nature of the diffraction pattern leads also to a 

 knowledge of the structure of the various atoms of the crystal. See Bragg and Bragg, X-rays and Crystal Structure, 

 1918. 



* Each atom is so nearly equal in diffracting power (atomic weight) in KC1 that the apparent unit diffracting element 

 is a cube (simple) of j this size. Elementary body-centered cube, — atom at each corner, one in center; e.g., Fe, Ni (in 

 part), Na, Li? Elementary face-centered cube, — atom at each corner, one in center of each face; e.g., Cu, Ag, Au, 

 Pb, Al, Ni (in part), etc. Simple cubic lattice, — atom in each corner. Double face-centered cubic or diamond lattice 

 — C (diamond); Si, Sb, Bi, As?, Te?. 



t Diamond lattice. t Cubic-holohedral. § Cubic-pyritohedral. 



Metals taken from Hull, Phys. Rev. 10, p. 661, 1917 



% See page 543 for best values of calcite and rock-salt grating spaces. 



Note: —(Hull, Science 52, 227, 1920). Ca, face-centered cube, side 5.56 A, each atom 12 neighbors 3.93 A distant. 

 Ti, centered cube, cf. Fe, side 3.14 A, 8 neighbors 2.72 A. Zn, 6 nearest neighbors in own plane. 2.67 A, 3 above, 3 

 below, 2.92 A. Cd, cf. Zn, 2.98 A, 3.30 A. In, face-centered tetragonal, 4 nearest 3.24 A, 4 above, 4 below, 3.33 A. 

 Ru, cf. Zn, 2.1-q A, 2.64 A. Pd, face-centered cube, side 3.92 A, 12 neighbors. 2.77 A. Ta, centered cube, side 

 3.27 A, 8 neighbors 2.83 A. Ir, face-centered cube, side 3.80 A, 12 neighbors, 2.69 A (A =io"8 cm). 



Note : — (Bragg, Phil. Mag. 40, 169, 1920). Crystals empirically considered as tangent spheres of diameter in table, 

 atom at center of sphere. When lattice known allows estimation of dimensions of crystal unit. Table foot of page 548 

 (atomic numbers, elements, diameter in Angstroms, io" 8 cm). 



Smithsonian Tables. 



