1920] on Crystal Structure 203 



electronegative elements of the same period ; it expresses the fact that 

 the electropositive atom never shares electrons with the neighbouring 

 atoms, and is therefore always found to be at a distance from the 

 other atomic centres as if it occupied a large domain in the crystal 

 structure. 



12. In each period the divalent ion is assigned a smaller diameter 

 than the preceding monovalent ion. Sodium fluoride and magnesimn 

 oxide, for example, have exactly the same atomic arrangement, that 

 of potassium chloride. The dimensions of the magnesium oxide 

 structure are, however, smaller than those of sodium fluoride: the 

 side of the unit cube is 4*22 A, as compared with 4*78 A. Since 

 other data assign practically identical dimensions to oxygen and 

 fluorine, the difference in the dimensions is accounted for by taking 

 a smaller diameter for magnesium than for sodium. A comparison 

 of the identical stuctures, magnesium carbonate and sodium nitrate, 

 leads to the same result. AVe may ascribe this closer grouping of 

 the atoms in the magnesium oxide or carbonate to the fact that the 

 charges on the ions are double those in sodium fluoride or nitrate, 

 so that the forces of attraction are more than four times as great. 

 For the same reason the trivalent elements appear to occupy a still 

 smaller domain in the crystal structure. 



13. At the end of each period the diameters approach a lower 

 limit. Since the electronegative elements are holding electrons in 

 common, an estimate of the size of the outer electron shell can be 

 formed from the distances which separate the atoms. The limiting 

 values for the periods are, approximately : — 



Neon 1"30A 



Argon . . . . . . 2 • 05 A 



Ivrvpton . . . . . 2 • 35 A 



Xenon 2*70 A 



The diameters of the outer electron shells would appear to be equal 

 to, or slightly less than, these values. 



14. AVe can now review the types of crystalline structure. In 

 what follows the various points have either been made by Langmuir 

 or are direct consequences of the atomic theory which has been 

 described above. 



The crystals of a salt have been discussed above. Crystals of 

 sodium chloride, or calcium carbonate, consist of ions of opposite 

 signs. The ions are surrounded each by its own electron cloud ; they 

 are held together by their electric charges, and kept apart by 

 repulsive forces which must be supposed to exist between the outer 

 electron shells. Diamond represents yet another type of crystal. 

 The carbon atoms have each four electrons in the outer shell, and 

 the complement of eight is attained by each carbon atom holding in 

 common a pair of electrons with the four other atoms surrounding it 

 symmetrically. The whole crystal is one molecule, all the atoms 



