8 Dt. B tatty on 



atom of the gas helium, which contains only two electrons, can be 

 deprived of these two, but in a short time it will attract two 

 electrons again from some body which has them to spare. 



The atom under normal conditions has the positive charge 

 on its nucleus neutralized by the negative charge on the surrounding 

 electrons so that it exerts no electrical effects un other atoms 

 distant more than three or four atomic diameters from it. 



The electrons are grouped round the central nucleus in 

 concentric shells or rings like the layers of a golf ball. The inner 

 shells experience greater forces from the positive centre and so are 

 more rigid and more difificult to move by external agencies. The 

 outer groupings of electrons are more easily detached and oscillate 

 more slowly when slightly disturbed. This, then, is the specification 

 of the atom which the physicist presents to us to-day. Let us see 

 what is to be expected from it. 



Firstly, as we pass from one element to another in the 

 direction of increasing atomic weight, we have a series of nuclei 

 whose positive charge increases by one unit in each case. Accord- 

 ingly each atom will have one more electron than the one just 

 lighter than it. Thus the number of electrons in the outer ring 

 will increase till it becomes so unstable that the addition of another 

 electron would upset the whole ring. At such a stage we should 

 have an atom which can easily part with an electron from its outer 

 ring but which shews no inclination to admit an extra electron 

 into its system. This corresponds to the behaviour of an electro- 

 negative element. As the atomic weight increases the stability of 

 the outer ring will wax and wane periodically, giving the alternation 

 of electro-positive and electro-negative character, which is such a 

 feature of MendeleePs periodic classification. 



The outer rings of electrons will be the first to get in touch 

 with each other when two atoms approach, and their reactions on 

 each other will determine whether chemical combination will occur 

 between these two atoms or not. On this view the chemical 

 behaviour of bodies depends only on the outer electrons, the 

 nuclei and inner rings playing an inactive part. Undoubtedly the 

 addition to or removal from the outer ring of a single electron 



