CHAIN THEORY 51 



themselves. Towards electro-positive bodies their 

 polarity or valency is invariable. Hydrogen is 

 always a monad ; Cl uniting with one ion-atom of 

 H, S with two, N with three, C with four, providing 

 that only one ion-atom of these elements enters into 

 the combination. 



The bond between two different ions is not 

 always a stable one. HC1 is formed by the union of 

 two monovalent ions, of which H is positive and 

 Cl strongly negative. But the H is easily displaced 

 by any other positive ion for which the Cl ion has 

 a greater affinity, as the dyad calcium, or the triad 

 iron. Such displacement may be due to the attack- 

 ing ion carrying a greater charge of electricity. The 

 same displacement is observed in many organic and 

 inorganic molecules. The substitution of other ions 

 for H or OH ions takes place with ease and rapidity, 

 and OH ions are themselves greatly provocative 

 of such changes, whereby the character of the 

 molecule may be very greatly altered. 



This linking together of atoms or ions by polar 

 attraction and according to their valency has given 

 rise to what is known as the chain theory of molecular 

 constitution. In the hydrocarbons, for instance, 

 methane, CH 4 , is a complete molecule ; but it may be 

 robbed of an atom of H by a wandering OH ion, 

 leaving methyl, CH 3 , which is a monad, a rest, radical, 

 or link, having one unsatisfied valency. CH 4 might 

 be robbed of two H atoms, leaving the rest, CH 2 , and 

 a divalent ion. Such rests or links do not exist in 

 a free state, they are ions demanding companionship. 

 They attract other ions, rests or links, or even 

 displace an atom or a link from some other molecule 



42 



