DIMENSIONS OF ATOMS AND MOLECULES 49 



inert gases in the periodic series. The atom tends to lose or 

 gain electrons until the total number is equal to that for one of 

 the inert gas atoms, a relation which has been pointed out very 

 clearly by Kossel {Ann. der Physik, 49, 229, 19 16). With- 

 out any knowledge of the reason for the stability of those par- 

 ticular arrangements of electrons which constitute the inert 

 gases, the chemical behaviour of the elements may be explained 

 as a tendency of the electrons to take up a more stable arrange- 

 ment. 



It is supposed that this can come about in two ways. A 

 molecule such as that of potassium chloride is held together 

 on account of the electrostatic attraction between the two 

 oppositely charged ions of which it is formed. The more stable 

 arrangement of electrons is realised by the potassium atom part- 

 ing with an electron and the chlorine atom absorbing one, so 

 that both atoms have the same number of electrons as argon. 

 We thus have two complete stable electron systems surrounding 

 each nucleus, the two being bound together by the residual 

 charge on each. 



Another mechanism must be assumed for the combination of 

 two electro-negative atoms, for instance, two atoms of chlorine. 

 In this case both atoms have one electron less than the argon 

 atom, and it is supposed that the more stable arrangement is 

 realised by the two atoms holding a pair of electrons in common, 

 this pair in some way fulfilling the same role in giving rise to a 

 stable structure for both of the atoms. The justification of this 

 view, which has been largely developed by Lewis {Journ. Amer. 

 Chem. Soc, vol. xxxviii, p. 762, 1916) and Langmuir {Journ. 

 Amer. Chem. Soc, vol. xli, p. 868, 19 19), lies in the simplification 

 which it introduces into the conception of chemical valency in 

 the case of many compounds of electro-negative elements. One, 

 two, and perhaps three, pairs of electrons may be thus held in 

 common by two electro-negative elements. 



Crystalline structures show very clearly that there is a 

 difference in atomic arrangement for these two types of chemical 

 combination which have been postulated. In a crystal of 

 potassium chloride, the most accurate analysis which has so far 

 been possible fails to reveal the existence of anything corre- 

 sponding to the KCl molecule. The structure consists of an 

 alternate arrangement of potassium and chlorine atoms situated 

 at the corners of a series of cubes which form the basis of the 

 structure. Each potassium atom is surrounded by six chlorine 

 atoms at equal distances from it, and vice versa. This is diffi- 

 cult to understand if the potassium and chlorine atoms are 

 bound in pairs by a valency bond, but becomes comprehensible 

 when they are regarded as oppositely charged ions, since in that 

 case the electrostatic attraction of the potassium kation may be 



