646 



TABLE 705.— CROSS SECTION AND LENGTHS OF SOME 

 ORGANIC MOLECULES 



According to Langmuir, 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 compounds 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 mole- 

 cules are not spheres. Were they spheres and an attraction existed between 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 until all the — COOH 

 groups are in contact if possible. Pure hydrocarbon oils will not spread over water. Ben- 

 zene 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 w of oil 

 spreads over water surface A, the area covered by each molecule is AM/wN where M 

 is the molecular weight of the oil (O = 16), N, Avogadro's constant. The vertical length 

 of a molecule / = M/apN = W IpA where p is the oil density and a the horizontal area of 

 the molecule. 



Cross section in / in cm 



Substance cm 2 X 10 16 (length) X 10 8 



Palmitic acid G-,H 3 iCOOH 24 19.6 



Stearic acid G,H 35 COOH 24 21.8 



Cerotic acid G 3 H 51 COOH 25 29.0 



Oleic acid G:H 33 COOH 48 10.8 



Linoleic acid G,H 31 COOH 47 10.7 



Linolenic acid G7H20COOH 66 7.6 



Ricinoleic acid G;H 32 (OH)COOH 90 5.8 



Cetyl alcohol G.H»OH 21 21.9 



Myricyl alcohol GoH ai OH 29 35.2 



Cetyl palmitate G 5 H 31 COOG,H 33 21 44.0 



Tristearin (G s H 35 2 ) 3 C 3 H 5 69 23.7 



Trielaidin (C.,H»Oi)»C»H, 137 11.9 



Triolein (C,.H»0,),GH. 145 11.2 



Castor oil (C„H»(OH)COO),GH. 280 5.7 



Linseed oil (CitH m COO),C,H s 143 11.0 



TABLE 706.— VOLUMES OF INERT GAS ATOMS* 



Vo'ume 

 from 

 ionic 

 Gas radius 



Neon 3.33 



Argon 8.6 



Krypton 12.5 



Xenon 18.8 



* For reference, see footnote 203, p. 624. 



SMITHSONIAN PHYSICAL TABLES 



