52 SCIENCE PROGRESS 



Under the influence of the attraction of the nucleus, and of 

 their mutual repulsions, the electrons tend to arrange them- 

 selves in definite groups. In the first shell the stable formation 

 is the pair, in subsequent layers the octet appears to be the 

 most favourable group. In this case the actual arrangement 

 of the electrons in space is at the eight corners of a cube. If 

 the electrons in a single atom are unable to dispose themselves 

 in such a permanent configuration, they tend to associate 

 themselves with the electrons in other atoms in such a way 

 as to do so. This is the explanation of chemical com- 

 bination. 



The realisation of a stable formation in a single atom results 

 in the production of the inert gases of the first group in the 

 periodic table. With Helium (N = 2) * the positive charge 

 on its nucleus is 2, and it has 2 electrons. These dispose 

 themselves symmetrically on either side of the nucleus, each 

 in the centre of its shell. The symmetrical distribution results 

 in almost complete external compensation of the electric forces 

 with little stray field. Helium is therefore the most stable 

 element known. Because the electrons are as suitably grouped 

 as possible, with little external field of force, there is no tendency 

 to associate with atoms either to form molecules or chemical 

 compounds. Consequently helium is an inert gas with a 

 boiling-point only 4^ degrees above absolute zero. 



The next element in this group is Neon (N = 10), with a 

 positive charge of 10, and 10 electrons. The first 2 electrons 

 complete the first shell, which corresponds to that of the helium 

 atom, and the remaining 8 form an octet in the layer lla. 

 Thus a second stable configuration has been attained, and 

 Neon is the second most stable element. The following element 

 in this group, Argon (N = 18), has a nucleus with 18 positive 

 charges and 18 electrons disposed around. Here we have 2 

 electrons in the inner shell, 8 in layer lla, corresponding to 

 Neon, and another 8 in layer 11^. Argon is, therefore, Neon 

 with an extra octet. Since the electrons are again completely 

 satisfied, we have another inert gas with low boiling-point and 

 no tendency to form molecules. 



In the third shell there can be 18 electrons, 9 in each hemi- 

 sphere. One of these electrons will go to the pole, forming a 

 pair with the corresponding electron in the other hemisphere, 

 and the remaining 16 are distributed symmetrically about them, 

 8 in each hemisphere, forming 2 octets. This arrangement 

 corresponds to Krypton. Xenon has a further 18 electrons 

 in the lllb layer, and Niton follows at the end with 32 electrons 

 in the fourth shell, 16 in each hemisphere. These inert elements 

 are therefore built up of consecutive layers of electrons 



^ N = Atomic number. 



