220 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



oxygen atoms, while each oxygen atom has about it four copper 

 atoms. 37 Magnetite, as a typical spinel, has been studied with great 

 care, and in spite of its rather complicated chemical composition its 

 structure is in all probability that given in figure 26. 38 In this case 

 each divalent metal is surrounded by four and each trivalent metal 

 by six equally distant atoms. In each of these cases it is to be noted 

 that the number of the nearest atoms to any atom is always either 

 equal to its chemical valence or is twice that number. The constant 

 recurrence of this relation suggests that it may be other than purely 

 an affair of chance. Furthermore, a consideration of the sorts of 

 chemical bonding which are conceivable on the basis of our present 



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Pig. 26. — The spinel arrangement. The divalent metal atoms of a spinel occupy all 

 of the positions of Fig. 24. The arrangement of the other atoms is here shown for 

 the two typical divisions of this unit. Each divalent metal atom (R")is equally dis- 

 tant (from four atoms each trivalent metal atom (R'") from six atoms ; and each 

 divalent oxygen atom (0) from four atoms. 



knowledge of the structure of the atom points to the possible ex- 

 istence of just such a relation as this. 



Information to be derived from the structure of the atom. 59 — A 

 neutral atom appears to be made up of a minute positively charged 

 nucleus surrounded by enough electrons to neutralize the nuclear 

 charge. The chemical inertness of certain of the elements — the rare 

 gases of the atmosphere — and the fact that atoms having one, two, 

 three, and in some cases four more electrons than a rare gas exhibit 

 a marked tendency to lose just enough electrons in becoming posi- 

 tively charged to revert to the configuration of the inert gas, seem to 



87 W. H. and W. L. Bragg, X-rays and Crystal Structure, p. 155. 



» 8 S. Nishikawa, op. cit. W. H. Bragg, Phil. Mag. (6), 30, 305, 1915. 



88 J. J. Thomson, Phil. Mag. (6), 27, 757, 1914. G. N. Lewis, J. Am. Chem. Soc, 38, 762, 

 1916. Ralph W. G. Wyckoff, J. Wash. Acad. Sci„ 9, 565, 1919. 



