ARTICLES 55 



nuclei are held by electrostatic attraction to two pairs of the 

 electrons forming the octet. The water molecule may, 

 therefore, be pictured as a cube with two hydrogen nuclei 

 hanging on to opposite edges. This structure indicates that 

 water forms molecules in which the electrostatic forces are 

 almost completely compensated internally. All the electrons 

 form an octet, and hence the molecule should have a rather 

 weak external field of force. Water should, therefore, be easily 

 volatile and should not be a good conductor of electricity. 

 But the less symmetry of the molecule, as compared with the 

 neon atom, shows that the boiling-point should be much higher. 



For carbon dioxide, CO2, we expect each atom to form an 

 octet. Supposing n = 3, we have e =4+2x6 = 16, which 

 gives p = 4. This means that four pairs of electrons are 

 shared between three octets, leading to a structure resembling 

 three cubes arranged side by side in a row with the carbon 

 cube between two oxygen cubes. Consequently carbon dioxide 

 is a thoroughly saturated non-polar substance, a non-conductor 

 of electricity, easily volatile and chemically rather inert. 



This type of union, in which two atoms are joined by sharing 

 pairs of electrons, is regarded as the normal mode of chemical 

 combination, as distinguished from that which results in the 

 formation of salts, such as lithium fluoride, when one or more 

 free electrons are " given up." It will be noted that the forces 

 holding the atoms together are electrostatic in both cases ; 

 but, while the metallic ion has no definite point of attachment, 

 in normal compounds the relative positions of the atoms are 

 fixed. 



The characteristic distinction between a neutral compound 

 such as CO2 and a salt like LiF Hes in the fact that, while the 

 molecule of the former is electrically non-polar, with little 

 external field of force, the molecule of the latter consists of 

 two distinct parts or ions, which are oppositely charged, and, 

 though bound together by a strong internal field, are capable 

 of separation in solution, or in the fused state, by the action 

 of an electromotive force. There is also considerable external 

 field. 



It will be found that equation (ii) leads to results identical 

 with the ordinary theory of valency when applied to most 

 inorganic and almost all organic compounds. In addition, 

 however, it explains the structures of many substances which 

 have hitherto been difficult to account for. 



There is hardly a case where the ordinary theory of valency 

 fails so completely as for the compounds of nitrogen. Let us 

 see how the octet theory applies to these bodies. For the 

 compounds with hydrogen the theory postulates the occurrence 

 of NH3 and H2N-NH2 with the properties they possess, while 



