PHYSICAL CHEMISTRY ii 



consists of an outer cell of eight electrons, surrounding two 

 nitrogen nuclei and two electrons. Since the number of the 

 outer electrons is the same as that in the rare gases, certain 

 points of resemblance should exist between these gases and 

 nitrogen. The nitrogen molecule should behave as an elastic 

 body approximately spherical in shape. Rankin {Proc. Roy. 

 Soc, 192 1, [A], 98, 360) from measurements of viscosity comes 

 to the conclusion that whereas the kinetic behaviour of oxygen, 

 chlorine, bromine, and iodine is in accord with a two-celled 

 structure for these gases, this is not the case for nitrogen, 

 which is abnormal. J. J. Thomson {Phil. Mag., 1921, 41, 542), in 

 a theory of the structure of the molecule and chemical constitu- 

 tion, points out that the results of Rayleigh {Proc. Roy. Soc, 

 1920, [A], 98, 57) on the scattering of polarised light by gaseous 

 molecules, show a marked difference between the shape of the 

 nitrogen and oxygen molecules. The oxygen molecule deviates 

 to a greater extent than nitrogen from sphericity. The 

 resemblance between the rare gases and nitrogen is shown by 

 the remarkable fact that whereas hydrogen, chlorine, carbon, 

 and oxygen readily take up negative charges, neither neon 

 nor nitrogen has ever been observed with a negative charge. 

 The work of Franck and Herz supplies another point of resem- 

 blance. The mobility of electrons in gases is determined by 

 the nature of the outer cell of electrons, and in neon and nitrogen, 

 where it is assumed that there is a complete octet, the electron 

 is rarely retained by the molecular system. The mobility of 

 the electron in these gases is therefore considerably greater 

 than in oxygen and chlorine, where the bicellular arrangement 

 is readily broken down by the addition of electrons. 



A three-cell structure is ascribed to carbon dioxide and 

 nitrous oxide by Langmuir, the three cells being produced by the 

 sharing of electrons in pairs between the three atoms. Accord- 

 ing to Rankin {Proc. Roy Soc, 192 1, [A], 98, 369) the molecules 

 of CO2 and NgO behave not merely as If they had the same size 

 and shape, but also as if each had an external electronic arrange- 

 ment like that of three neon atoms with their centres in a line. 

 Such a hypothetical molecule composed of three neon atoms 

 would have a mean target area of 0-895 x io~^^ cm*. On the 

 basis of viscosity results Rankin obtains the values 0-870 x io~^^ 

 cm* for CO2 and 0-867 ^ io~^^ cm* for NgO. These data are a 

 striking confirmation of the Langmuir theory. 



Metallic Hydrides. — Within the last few years the number of 

 the known gaseous hydrides of the elements has been materially 

 increased. Polonium, bismuth, tin, and lead all yield hydrides 

 if brought into contact with hydrogen in an active form (Paneth, 

 Zeit. Elektrochemie, 1920, 26, 452, and Ber., 1920, 53b, 1693). 

 Paneth found that thorium C and thorium B when deposited on 



