II. MOLECULAR STRUCTURE 



(Isomerism, Tautomerism) 



WHILST so far we have been concerned with the size of 

 the molecules, we have now to consider their internal 

 structure. The necessity for going into this point lies in 

 the existence of isomerism. From the moment that bodies 

 of the same molecular magnitude and composition with dif- 

 ferent properties are known, such as ethyl- and dimethyl- 

 amine, which, it is well known, both have the formula 

 C 2 H 7 N, the possibility of different arrangements suggests 

 itself as an explanation. For that reason the development 

 of the theory of molecular structure has taken place in 

 organic chemistry. Isomerism among inorganic compounds, 

 such as between ammonium phosphite and hydroxylamine 

 hypophosphite, both NH 6 PO 3 , is rare l . 



In the methods now to be described for arriving at the 

 molecular structure, the molecular formula, which gives 

 the number and nature of the atoms in the molecule, serves 

 as a starting-point. It may be obtained, as mentioned 

 on p. 14, from the qualitative and quantitative composition 

 and the molecular weight. 



The further step towards molecular structure rendered 

 necessary by isomerism is to find the mode of connexion 

 of the atoms, a problem known as that of determining the 

 structure or constitution. Then, the information derived 



1 Sabanejeff, Berl. Ber. 30. 285. See also the comprehensive investigations 

 of Werner on metallic ammonium derivatives (Zeitschr. f. anorg. Chem.') ; 

 also Kurnakow, Berl. Ber. 1898, p. 207. 



