1920] on Crystal Structure 201 



atomic structure, which has been developed by the work of Kossel, 

 Lewis, Born, Lande and many others, and which has been so strikingly 

 summarized and extended recently by Langinuir, affords a ready explana- 

 tion of the relationships. An essential feature of this theory is the sup- 

 position that the electrons, which surround the positive atomic nucleus, 

 are fixed at, or oscillate about, certain definite positions in the atomic 

 structure. This may be contrasted with the type of atomic structure 

 represented by the Bohr atom, when the electronic orbits are supposed 

 to enclose the atomic nucleus. In the "fixed electron" atom the 

 electrons are placed in certain definite positions in a series of con- 

 centric spherical shells surrounding the nucleus. The commencement 

 of each period of the periodic table marks the commencement of a 

 new shell, and the inert gases which separate the periods are the 

 atomic systems in which the outer shell has in each case its full 

 complement of electrons. 



The arrangement of electrons in an inert gas is a very stable 

 one, corresponding to the chemical inactivity of the elements. The 

 chemical properties of the other elements are due to the tendency of 

 the system to revert to a more stable arrangement, such as that of 

 one of the inert gases. Argon, for example, with atomic number 18 

 has a stable system of 18 electrons surrounding the nucleus. 

 Potassium has an atomic number 19, and the nucleus is surrounded 

 by 19 electrons in a neutral atom. There is, therefore, a tendency 

 for the potassium atom to lose an electron and revert to the stable 

 argon arrangement of 18 electrons, so that potassium behaves 

 as a monovalent electropositive element. Chlorine, on the other 

 hand, with atomic number 17, tends to gain an electron, and is a 

 monovalent electronegative element. The atoms which have thus 

 lost or gained an electron are positively and negatively charged ; they 

 are the kations and anions of potassium and chlorine. 



The atomic numbers of the inert gases tell us the number of 

 electrons in the outer shell which each atom must have for complete 

 stability. The atomic numbers of helium, neon, argon, krypton, 

 xenon and niton are 2, 10, 18, 36, 51 and 86, so that the number 

 of electrons which must be added to complete each successive shell 

 must be 2, 8, 8, 18, 18 and 32. 



9. On this theory, two different types of chemical combination 

 may be distinguished. The first is that represented by such a 

 compound as potassium chloride. Here the potassium atom has 

 lost an electron in reverting to the argon arrangement of eighteen 

 electrons, and the chlorine atom has gained one. The oppositely 

 charged ions are held together by the electrostatic attraction of their 

 resultant charges. In the crystal of potassium chloride the positive 

 potassium ion tends to surround itself with as many negative ions as 

 possible. This is realized in the crystal structure (Plate I, fig. b), 

 where each ion is surrounded by six of the opposite sign. The 

 fact that the " molecule " of KC1 has no apparent existence in the 



