190 CHEMICAL DISCOVERY AND INVENTION 



The name electron is now used to signify one atomic charge, 

 and from the account already given of these minute masses :* 

 must be inferred that the addition of one electron to an atomic 

 mass converts it into a univalent negative ion. The removal of 

 an electron from an atom converts it into a positive ion. When 

 electrolysis of hydrogen chloride, for example, occurs each atom 

 of chlorine set free receives one electron, while each atom of 

 "ionised hydrogen loses an electron, the other elements gaining or 

 losing two, three, or four electrons according to their valency or 

 combining capacity. According to this idea the movable 

 electron by its presence or absence determines the possibility of 

 chemical combination between atoms. It has been considered 

 as an element by Sir William Ramsay, who assigns to it the 

 symbol E, and has discussed the use of such formulae as NaECl 

 for ionisable compounds. 



In the story of the action of the electric discharge on gases the 

 electronic constitution of atoms has been described. If the 

 hypothesis just explained is accepted as to the process of elec- 

 trolysis, it must be admitted that while the electrons when free 

 are all of the same kind, no difference of mass among them having 

 been observed, their relation to atoms must be of two kinds. 

 It would appear that the majority enter into the permanent 

 constitution of the atom, while others, usually from one to four 

 and never more than eight, are capable of being detached in 

 chemical exchanges. These correspond to the valency of the atom. 



In considering such hypotheses we may observe that among 

 the elements known there are at least three kinds of molecules. 



(1) There are the elements of the argon group whose mole- 

 cules consist of one atom only which is electrically neutral and 

 incapable of taking up either a positive or negative charge, and 

 hence are chemically inactive and are unknown in the form of 

 any chemical compound either among themselves or with other 

 elements. 



(2) The elements of the zinc, cadmium, mercury group have 

 also monatomic molecules, but they are fairly active as chemical 

 agents. Zinc, for instance, dissolves readily in all acids, and 

 mercury, though not so easily attacked by acids, combines 

 with the halogens and even with many metals. An atom of zinc 

 therefore must be constituted internally quite differently from, 

 say, an atom of argon. 



(3) The great majority of the elements have polyatomic mole- 



