so 



SCIENCE PROGRESS 



exerted equally on the surrounding chlorine anions. On the 

 other hand, in a crystal such as calcite (CaCOs) the four 



electro-ne^^ative atoms 



ATOMIC DIAMETER X 10 CM. 



en 



o 



—• >J CO 4^ 



,11 I I II I I I 



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1 r 





forming the CO3 group 

 are bound together in 

 a unit with a special 

 relationship to each 

 other, and with a 

 relatively small dis- 

 tance between the 

 atomic centres, a fact 

 which may be ex- 

 plained by the con- 

 ception of certain 

 electrons being com- 

 mon to pairs of atoms 

 so that their struc- 

 tures have become 

 closely linked together. 

 The table on the 

 following page will 

 show the relationships 

 between the distances 

 separating atoms of 

 metal and halogen for 

 a series of crystals 

 which have the same 

 structure as potassium 

 and sodium chlorides. 

 This distance is that 

 of the side of the 

 elementary cubes on 

 which the atoms are 

 arranged. All dis- 

 tances are expressed 

 in Angstrom units 

 (io~* cm.). 



The substitution of 

 chlorine by bromine, 

 or bromine by iodine, 

 increases the distance 

 between atomic 

 centres by an amount 

 which is approxi- 

 mately the same 

 throughout the series of compounds, as the figures in italics 

 show. The same is true when potassium replaces sodium, or 



XENON 



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 8 



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