Structure of the Atom. 495 



an a particle, and it has often been pointed out that the atomic 

 weights of many of the permanent atoms differ by about four 

 units. 



It will be seen later that the resultant positive charge on 

 the nucleus determines the main physical and chemical 

 properties of the atom. The mass of the atom is, however, 

 dependent on the number and arrangement of the positive 

 and negative electrons constituting the atom. Since the 

 experimental evidence indicates that the nucleus has very 

 small dimensions, the constituent positive and negative 

 electrons must be very closely packed together. As Lorentz 

 has pointed out, the electrical mass of a system of charged 

 particles, if close together, will depend not only on the 

 number of these particles, but on the way their fields interact. 

 For the dimensions of the positive and negative electrons 

 •considered, the packing must be very close in order to 

 produce an appreciable alteration in the mass due to this 

 •cause. This may, for example, be the explanation of the 

 fact that the helium atom has not quite four times the mass 

 of the hydrogen atom. Until, however, the nucleus theory 

 (has been more definitely tested, it would appear premature 

 to discuss the possible structure of the nucleus itself. The 

 general theory would indicate that the nucleus of a heavy 

 atom is an exceedingly complicated system, although its 

 dimensions are very minute. 



An important question arises whether the atomic nuclei, 

 which all carry a positive charge, contain negative electrons. 

 "This question has been discussed by Bohr *, who concluded 

 from the radioactive evidence that the high speed /3 particles 

 have their origin in the nucleus. The general radioactive 

 •evidence certainly supports such a conclusion. It is well 

 known that the radioactive transformations which are accom- 

 panied by the expulsion of high speed /5 particles are, like 

 the a ray changes, unaffected by wide ranges of temperature 

 •or by physical and chemical conditions. On the nucleus 

 theory, there can be no doubt that the a particle has its 

 origin in the nucleus and gains a great part, if not all, of its 

 •energy of motion in escaping from the atom. It seems 

 reasonable, therefore, to suppose that a /3 ray transformation 

 also originates from the expulsion of a negative electron 

 from the nucleus. It is well known that the energy expelled 

 in the form of ft and 7 rays during the transformation of 

 radium C f is about one-quarter of the energy of the expelled 

 a, particle. It does not seem easy to explain this large 



* Bohr, Phil. Mag. xxvi. p. 476 (1913). 



t See liutlierford and Robiiison, Phil. Mag. xxv. p. 301 (191o). 

 2 L2 



