ISOMORPHISM, ISOSTENISM AND COVALENCE 243 



TABLE I 

 List of Isosteres' 



Type. 



I H- He, Li + 



2 O— , F- Ne, Na+, Mg + +, A1++ + 



3 S~, CI- A. K+ Ca++ 



4 Cu+, Zn + + 



5 Br- Kr, Rb+, Sr + + 



6 Ag+ Cd++ 



7 I- Xe, Cs+. Ba + + 



8 N:, CO. CN- 



9 CH4,NH4+ 



lo CO2, NA N3-, CNO- 



II NO3-, C0.~ 



12 NO2- O3 



13 HF, OH- 



14 CIO4-, S04~. po«— 



T5 CIO,--, SOj— . PO,— 



16 SO3, POs- 



17 SjOfi— , P20» 



18 S2O7--P2O7— - 



19 SiH4, PH*-^ 



20 Mn04-, Cr04 — 



21 SeO*-- AsO* 



All the comolecules given under any one type are isosteric with one 

 another. 



When isosteric comolecules are also isoelectric, that is when they have 

 the same total charge on the comolecules, all their physical properties 

 should be closely similar. In Table I the only pairs of comolecules which 

 are also isoelectric are: (8) N2 and CO, (loa) CO2 and N2O and (lob) 

 N3- and NCO-. 



I have already pointed out that the physical properties of the first two 

 pairs of substances (8 and loa) furnish proof of the similarity of structure 

 predicted by the octet theory, and show the usefulness of the conception 

 of isosterism. 



The isosterism of the cyanate and trinitride ions applies of course also 

 to compounds derived from them. Thus we should expect HNCO and 

 HN.3 to be isosteric as well as pairs of compounds such as KNCO-KN3 

 Ba(NCO)2-Ba(N3)2, etc. 



There are, unfortunately, very few data on the physical properties of 

 cyanates and trinitrides. Both cyanic and hydronitric acids are liquids at 0° 

 and both explode on heating. Apparently the freezing point of cyanic acid 

 has not been determined. In a general way the solubilities of the salts of 

 these two acids are known to be similar; the potassium and the barium 



