Arrangement of Atoms in Crystals. 183 



no place in the stable argon arrangement. Chlorine has a 

 nuclear charge of 17, and is surrounded by 17 electrons, 

 one less than the number required to form the stable argon 

 arrangement. When an atom of potassium combines with 

 one of chlorine, the chlorine atom absorbs into its system the 

 additional electron from the potassium atom. Both atoms 

 are now surrounded hy the argon shells, but as the nuclear 

 charges are 19 and 17, and each atom is surrounded by 

 18 electrons, there will be a resultant positive charge of one 

 unit on the potassium atom and a negative charge of one 

 unit on the chlorine atom. The electrostatic attraction of 

 these charges holds the molecule together. The nuclei, 

 surrounded by the stable argon shells, compose the mono- 

 valent kations and anions of potassium and chlorine. 



The other type of combination is represented by a com- 

 pound of two electronegative elements. In this case both 

 atoms have fewer electrons than correspond to a stable 

 system. They complete the required number of electrons in 

 their outer shells by holding one or more pairs of electrons 

 in common. Tn such compounds as S0 2 , C0 2 , the atoms 

 complete their outer shells of eight electrons by sharing them 

 with their neighbours. 



The crystalline structure of a compound such as KC1 is 

 very simply explained by this theory. As has been pointed 

 out by Langmuir, the crystal is to be regarded as an assem- 

 blage of potassium and chlorine ions arranged on a cubic 

 lattice*. The ions consist of the stable argon shells, but as 

 the nuclear charges are 19 and 17 the ions have resultant 

 unit positive and negative charges. Each ion tends to sur- 

 round itself- with as many ions of the opposite sign as 

 possible. This is realised in the KC1 structure, where each 

 ion is surrounded by six ions of the opposite sign. There 

 are no individual molecules in the crystal structure, the 

 potassium ion has exactly the same relation to the six 

 chlorine ions surrounding it, and vice versa. Some repulsive 

 force must exist between the outer shells, which holds the 

 atoms apart against the electrostatic attraction. 



In the structure of calcium sulphide,- the calcium atom 

 loses two electrons, and the sulphur atom gains two, in 

 reverting to the argon form. The resultant charges on the 

 ions are twice as great as those in the KC1 structure, and as 

 a result of the increased attraction between the ions the 

 structure, while being similar to that of KC1, has all its 

 dimensions reduced. 



* Cp. Debye and Scherrer, Phys. Zeit. xix. (1918), where evidence is 

 given that an electron has passed from the Li atom to the F atom 

 in LiF. 



