TYPES OF VALENCE 397 



tion of the compound NH4F which consists of NH4* ions and F~ ions. The 

 total number of covalence bonds has not been changed, they have merely 

 been distributed dififerently. But this causes the atoms to become charged 

 and makes the compound an electrolyte. It should be noted that this theory 

 indicates definitely in what direction the change of charge occurs. Thus we 

 should not expect NH3 and HF to give a compound consisting of ions 

 NHo" and HoF"^ although under other conditions these ions might exist. 



Similarly NH3 and H2O may react to form NH4OH which will consist 

 of ions NH4* and OH". But the tendency to form a compound such as this 

 is much less than in the case we have just considered, for the charge on the 

 kernel of the oxygen atom is less than that of the fluorine atom so it has 

 less tendency to become negative. As a result NH3 is much less active 

 towards H2O than towards HF. Examples of this kind can be extended 

 almost indefinitely and can even be used to obtain quantitative relationships 

 between the heats of formation of various substances. 



Since the sheaths of atoms of atomic number less than about 25 never 

 contain more than 8 electrons, the covalence of these atoms can not exceed 

 4. With heavier atoms, however, we might expect in some cases larger 

 covalences than 4. Large covalences are improbable in most cases for they 

 imply equally large negative valences which means that the number of 

 electrons in the sheath must be very much larger than the charge of the 

 kernel. There are a few compounds, however, which suggest that large 

 covalences sometimes exist. For example the compounds Fe(CO)5 and 

 Ni (CO) 4 correspond to complete compounds in which the central atoms 

 have the covalences 10 and 8 respectively. Since e for iron is 8 and for 

 nickel is 10 and the complete sheaths for these atoms contain 18 electrons, 

 the negative valences of iron and nickel are 10 and 8, that is the same as the 

 covalences needed to account for the above compounds. Thus these com- 

 pounds are in accord with both Postulates i and 3, and are to be regarded 

 as of a type analogous to organic compounds in which the covalence of 

 every atom is equal to its negative valence. It should be noted that both of 

 these compounds are liquids of low boiling point (102° and 43°) and their 

 molecular weights have been determined. Their properties are about those 

 to be expected if they have the structure assumed above. Other compounds 

 of iron with carbon monoxide are known, but they have only been obtained 

 in the crystalline state and their molecular weights are unknown. 



Molybdenum carbonyl. Mo (CO)6,^ is a very easily volatile crystalline 

 compound. It is interesting to note that the negative valence of molybdenum 

 (^ — ^ = 18 — 6) is twelve, so that with a covalence of 12 for the molyb- 

 denum atom in this compound we again obtain a structure consistent with 

 the valence theory discussed above. 



^ Mond, Hirtz, Cowap, /. Cheni. Soc, <)7, 798 (1910). 



