OXIDATION AND REDUCTION 



electrons in the atoms. The electrons tend to group themselves in stable arrange- 

 ments of pairs or octets. The hydrogen atom which contains only one electron tends 

 to share its electron with another hydrogen atom containing a " lone " electron — 

 thus a stable pair of electrons is shared between two hydrogen atoms and a relatively 

 stable hydrogen molecule is formed. Another possibility is for the hydrogen atom 

 to share its lone electron with a chlorine atom which has, in its outer orbit, an unstable 

 arrangement of seven electrons, thus making with the hydrogen atom a stable arrange- 

 ment of eight electrons. Hydrogen or chlorine atoms cannot, of course, exist by them- 

 selves but hydrochloric acid with its stable octet of electrons is relatively stable. 

 In aqueous solution, however, hydrochloric acid does not exist in molecular form — 

 it is ionised ; the hydrogen part of the molecule separates from the chlorine portion, 

 the hydrogen losing its electron temporarily and the chlorine taking it up. The 

 hydrogen ion, as it is now called, has no electrons, while the chlorine ion has eight, so 

 that both are relatively stable — neither has an unstable electronic arrangement. 

 But now the hydrogen ion has a positive charge, namely, the unit positive charge 

 on the nucleus which was previously balanced out by the single negative charge on 

 the electron. The chlorine ion has a single negative charge, namely, the charge on 

 the extra electron it has acquired from the hydrogen atom. The ionisation of 

 hydrochloric acid may be represented as follows, ® signifying the possession of a single 

 positive charge and © a unit negative charge : — 



HCl -> H© + CI© 



In the same way ferrous chloride is ionised in solution giving a ferrous ion, with 

 a double positive charge, and two chlorine ions : — 



FeCl2^Fe®©+2C10 



and ferric chloride gives a ferric ion, with a treble positive charge, and three chlorine 

 ions : — 



FeClg -^ Fe®©® + 3C1© 



The oxidation of ferrous chloride to ferric chloride : — 



FeCla + CI ^ FeClg 



is really a reaction between ions when it takes place in aqueous solution, so that it 

 should be wxitten : — 



Fe©® -f 2C1© -f CI -> Fe®©© -f 3C1© 

 If the essential reactants only are considered this equation becomes : — 



Fe©® + CI ^ Fe©®© -f CI© 



That is to say the reaction consists essentially of an exchange of an electron 

 between iron and chlorine. The ferrous ion loses an electron, which is taken up by 

 €hlorine. If we consider the iron system alone the reaction may be wTitten : — 



Oxidation 



Fe©© ^ Fe®®© + e (e represents an electron) 

 reduction 



On oxidation the ferrous ion, with its two positive charges, loses an electron and 

 becomes a ferric ion with three positive charges. Conversely on reduction the ferric 

 ion takes up an electron and becomes a ferrous ion. This gives us our general defini- 

 tions. Oxidation is a process involving the loss of electrons, and reduction one 





