﻿662 Sir J. J. Thomson : Further Studies on 



anything but neutral systems ; in other words, the solid may 

 be regarded as built up of units, each of which contains as 

 much positive as negative electricity. It is noteworthy that 

 according to the Electron Theory of Chemical Combination, 

 two similar atoms if they have each more than four dis- 

 posable electrons, like the atoms of sulphur and phosphorus, 

 can combine and form a saturated molecule, which is 

 electrically neutral. 



Thus we are led to distinguish three types of solids : — 



a. A type where the atoms are arranged in lattices, 

 and the electrons in other lattices coordinated with 

 the atomic ones. In this type each electron has no 

 closer connexion with a particular atom than it has 

 with several others. Thus, for example, when the 

 electrons form a simple cubical lattice with the atoms 

 at the centres of the cubes, each electron has 8 atoms 

 as equally near neighbours ; so that an electron is not 

 bound to a particular atom. This type includes the 

 metals ; it also includes boron and carbon in the form 

 of diamond, which are insulators. 



b. A type represented by the salts ; here the atoms are 

 again arranged in lattices, but each electron has much 

 closer relation with one particular atom than it has 

 with any other. Thus to take the case of Na . (31, 

 where the Braggs have shown the atoms to be 

 arranged according to the following scheme : — 



Na CI Na CI 

 CI Na CI Na 



Na CI Na CI 



We suppose that each sodium atom has lost an 

 electron, while each chlorine atom has gained one ; 

 thus each chlorine atom has eight electrons around it, 

 and each electron is much more closely bound to one 

 particular chlorine atom than to any other. It is s<> 

 closely associated that it is not dissociated from it s 

 partner in either the solid or liquid state of the 

 substance. Thus the chlorine system always has a 

 negative charge, the sodium one a positive. These 

 atoms do not move when the substance is in a solid 

 state, though they may do so when it is liquefied. 



If the distance of the electrons from the chlorine 

 atoms were to increase until it was not far from half 

 the distance between the sodium and chlorine nuclei 

 this type would approximate to type a. 



