643 



TABLES 697-712.— ATOMIC AND MOLECULAR DIMENSIONS 



TABLE 697.— EFFECTIVE ATOMIC RADII 



Goldschmidt, on the basis of reasonable though empirical assumptions, has calculated effective 

 radii of atoms in various charged conditions ; Pauling, on the basis of wave mechanics, has pre- 

 sented theoretical values for most of the elements, the two series agreeing well in many cases. The 

 latter values are printed in boldface type ; the values considered nontypical are in parentheses ; e.g., 

 for silicon we have: Si +4 (0.22—) 0.39-0.41, Si (1.12—) 1.18, Si" 4 (1.98) ; 2.71, signifying silicon, 

 carrying 4 -j- charges, has apparent radius between 0.22 and 0.41 ; but the lower values relate to 

 compounds where the atoms appear to be deformed ; so Goldschmidt gives 0.39 as most significant. 

 Wave mechanics yields 0.41. Neutral, the radius ranges from 1.2, in abnormal compounds, to 1.18 

 in those typical ; when carrying 4 — charges, the value is 1.98, according to calculations deemed 

 faulty, 2.71 according to theory. 



In applying the data to replacements, halides and oxides are usually ionized, and the values in 

 the outer columns apply. Thus in fluorite the value for Ca +2 should be added to that for F" 1 , 

 giving between 2.32 and 2.42, or 2.37 as a mean ; and the observed Ca-F distance in the crystal is 

 2.36 angstrom units. In the remaining types of compounds the atoms appear to be largely neutral 

 and the first column should be used. 



SMITHSONIAN PHYSICAL TABLES 



