POPULAR SCIENCE 629 



path when it approaches the centre of the atom itself. The 

 angular deflection of the a particle has been determined, and 

 from such measurements Rutherford has inferred the exis- 

 tence of a central nucleus in every atom, the nucleus having 

 the dimensions and mass already mentioned. It must not 

 be concluded, however, that the nucleus is all positive elec- 

 tricity, although its effective charge is positive. The nucleus 

 contains a number of electrons which are held extremely 

 tenaciously, much more firmly indeed than the outer " atmo- 

 sphere " of electrons. It is these outer satellite electrons 

 which give rise to the chemical reactivity of the atom, its 

 valency, etc., and it is upon these outer electrons that ultra- 

 violet light acts when it detaches an electron from the atom. 

 The outer " atmosphere " consists of a number of concentric 

 rings of electrons — at any rate, that is the simplest view of its 

 structure — the numbers in each ring going through certain 

 changes according to the nature of the chemical element. 

 The outermost ring of all appears to contain those electrons 

 which confer valency on the atom, enabling it to interfere with 

 the corresponding outer ring of another atom, in other words 

 to unite with another atom. The question of the structure of 

 the nucleus is practically unsolved at the present time. It is 

 unlikely that much information will be gained regarding it 

 until we are much more familiar with the relatively simpler 

 problem involved in the outer " atmosphere " of electrons. 



As regards the actual number of these outer electrons, we 

 are already in a position to say that the number is very nearly 

 one half of the ordinary atomic weight of the atom concerned. 

 In the case of nitrogen, for example, the number of electrons is 

 seven, in oxygen eight, in sodium eleven, and so on. In the 

 case of hydrogen it is now fairly certain that the outer atmo- 

 sphere consists of a single electron. That is, the neutral 

 hydrogen atom consists of a positive nucleus carrying unit 

 positive charge, round which a single electron rotates. It 

 follows, therefore, that the unhydrated hydrogen ion is itself 

 the hydrogen nucleus. 



The atomic model of Rutherford is the one attracting most 

 attention at the present time, and mainly for this reason : the 

 system is essentially an instable one, that is, unstable from 

 the standpoint of classical mechanics. Within recent years, 

 however, Planck has put forward a new concept known as 



