PRESIDENTIAL ADDRESS SECTION B. 53 



and is therefore regarded as helium nucleus. The only explanation 

 of these contradictions that suggests itself to me is that of rearrange- 

 ment. Helium nucleus with atomic number 2 is unsymmetrical 

 (see Fig. 6) viz. (2H+20 2H+), but when two electrons are taken on 

 to give helium gas, the arrangement 8(211+26 2H+)0, changes into 



6 



H+ H + 



6 ti 



H+ H+ 



h 

 Although there is experimental evidence regarding the nucleus in 

 this case there is none in the case of any of the others. Carbon 

 nucleus as (12H+ + 60) is merely a deduction from helium nucleus 

 (4H+ + 26). If the atomic number of carbon is 8, not 6, the 

 nucleus is (12H+ + 46) which can be constructed as a tetrahedral 

 model, whereas (12H+ + 60) cannot. (See Fig. 7.) In addition 

 the four electrons which would then be required to make a kernel 

 out of a nucleus could also be arranged tetrahedrally, and finally 

 the remaining four valency electrons would form an outer tetra- 

 hedron. 



At this stage a brief resume of the experimental evidence for 

 the present-day view of the atom is necessary. The essential con- 

 ception, as I have mentioned in the introduction, is that the atom 

 is hollow, consisting of a sphere, or concentric spheres, of electrons 

 with an outer diameter of about 10 -8 cm, and an exceedingly small 

 nucleus (about joVo °f the diameter of the electron sphere), which, 

 however, contains nearly all the weight of the atom, and in the 

 case of a heavy element a very high positive electric charge. Just 

 outside this nucleus there is an immensely powerful electric field 

 due to the action of the positive nucleus and the sphere or spheres 

 of electrons. This conception, which is due to Sir E. Rutherford, 

 arose in order to explain the fact that the a -particles of radium, 

 which are about 7,000 times as massive as electrons and which are 

 positively charged and travel with a speed of about 50,000 miles a 

 second, when passed through thin metallic sheets, are deviated 

 through large angles just as the tiny electrons can be deviated by 

 moderate electric fields. The highly-deviated particles are those 

 which have not only penetrated the electron-sphere of the atom 

 but have passed uncommonly near to the nucleus and have thus 

 been thrown into a hyperbolic orbit despite their immense velocity. 

 It is conceived also that those which "ctually aim at the nucleus 

 of a larger atom are turned back in their path. 



This conception holds in its simplest form for the first 

 20 (or so) elements which have only one sphere of electrons. 

 For the higher elements it is necessary to assume two or 

 three concentric spheres (or octets, see page 51) of electrons 

 and a bigger and more highly charged nucleus, until at 

 the top of the series the nucleus becomes unstable owing to its 

 excessive charge and disintegrates automatically, giving rise to 

 radioactivity. Since atoms are electrically neutral, the total num- 

 ber of external electrons in the spheres must be equal to the posi- 



