248 PHENOMENA, ATOMS, AND MOLECULES 



tween substances which according to the ordinary valence theory are 

 not closely related. Among these he mentions as types KI04-CaW04; 

 KC104-BaS04-KBF4 ; K2S04-KoBeF4 ; and NaNOs-CaCOs. He shows 

 how these cases of isomorphism are inconsistent with the ordinary valence 

 theory, but are in a general way in agreement with Werner's coordination 

 theory. 



By examining Table I it is evident that we should expect the following 

 cases of isomorphism of the kind just considered : 



TABLE II 



Typical Cases of Isomorphism Based on Isosterism 



Type. Table L 



2 (a) NaF-MgO; (b) MgFr-NajO 



3 • (a) KCI-CaS; (b) CaClr-KsS 



5 (a) RbBr-SrSe; (c) SrBrr-RbsSe 



7 (o) CsI-BaTe; (c) Batj-CsjTe 



8 Nj-CO 



i6 KNCO-KNa, etc. 



11,..^ (a) NaNOr-CaCOs; (b) KNOi-SrCOj 



J4., , (a) KC10«-SrS04; (b) NaHS0«-CaHP04 



(c) KHS04-SrHP0« 



15 (a) NaClOa-CaSOs; {b) KHSOi-SrHPO, 



17, NajSzOs-Ca^PaOe 



18 NazSzOr-CazPzOi 



20 RbMnOi-BaCrO* 



21 MnSeO4.2H2O-FeAsO4.2H.2O 



Sodium nitrate crystals resemble those of calcium carbonate a little 

 more closely than those of magnesium carbonate, although the sodium 

 ion is isosteric with the magnesium ion but not with the calcium ion. 

 The molecular volume of sodium nitrate is nearer to that of calcium car- 

 bonate than that of the magnesium compound. The much larger forces 

 in the compounds containing the divalent ions must tend to draw the atoms 

 closer together. The substitution of the larger calcium ion in place of the 

 magnesium ion offsets this difference, and this probably accounts for the 

 closer resemblance between the sodium and the calcium compounds. This 

 same relationship is noticed in other pairs of compounds. Since in these 

 replacements calcium corresponds most nearly to sodium we should expect 

 strontium to correspond to potassium and barium to rubidium. This is well 

 borne out in every case. The following tables illustrate the cases of iso- 

 morphism corresponding to Types 11 to 21, shown in Table II, for which 

 data are given by Groth. The pairs of substances at the head of each table 

 are those given in Table II. The other substances are generally recognized 

 as being isomorphous with one or the other members of these pairs. 



