242 PHENOMENA, ATOMS, AND MOLECULES 



Thus nitrous oxide at any given temperature has properties practically 

 identical with those of carbon dioxide at a temperature 3° lower. 



NjO. COi. 



Critical pressure, atm 75 77 



Critical temp 35-4° 31 -9' 



Viscosity at 20°. 148 X lo"* 148 X io-< 



Heat conductivity at 100" o. 0506 o . 0506 



Density of liquid at — 20° o. 996 i . 031 



Density of liquid at +10'' 0.856 0.858 



Refractive index of liquid, D line, 16° i . 193 i . 190 



Dielectric constant of liquid at 0° i . 598 i . 582 



Magnetic susceptibility of gas at 40 atm., 16° o. 12 X lo~* o. 12 X io~* 



Solubility in water o" i-305 1.780 



Solubility in alcohol at 15° 325 3.13. 



There is one property, however, which is in marked contrast to those 

 given above. The freezing point of nitrous oxide is —102°, while that of 

 carbon dioxide is —56°. This fact may be taken as an indication that the 

 freezing point is a property which is abnormally sensitive to even slight 

 dififerences in structure. The evidences seem to indicate that the molecule 

 of carbon dioxide is more symmetrical, and has a slightly weaker external 

 field of force than that of nitrous oxide. Such dififerences could easily be 

 produced by the difference in the charges on the kernels, and may also be 

 taken as evidence that the structure of nitrous oxide is represented by 

 N = N = O rather than N = O = N. 



Compounds showing a relationship to one another like that between 

 carbon dioxide and nitrous oxide will be called isosteric compounds, or 

 isosteres. These terms, however, are not to be restricted to chemical com- 

 pounds but are applicable to chemical radicals or to groups of atoms which 

 hold pairs of electrons in common. A comolecule is defined as a group of 

 atoms held together by pairs of electrons shared by adjacent atoms. 

 Comolecules are thus isosteric if they contain the same number and arrange- 

 ment of electrons. The comolecules of isosteres must, therefore, contain 

 the same number of atoms. The essential differences between isosteres are 

 confined to the charges on the nuclei of the constituent atoms. Thus in 

 carbon dioxide the charges on the nuclei of the carbon and oxygen atoms 

 are 6 and 8, respectively, and there are 2 X 8 -)- 6 = 22 electrons in the 

 molecule. In nitrous oxide the number of charges on the nitrogen nuclei 

 is 7, but the total number of electrons in the molecule is again 2X7 + 8 = 

 22. The remarkable similarity of the physical properties of these two sub- 

 stances proves that their electrons are arranged in the same manner. 



According to the object theory we may expect the following types of 

 isosteres : 



