THE ATOMIC NUCLEUS 145 



As regards the deepest region of atomic structure, 

 wherein radioactive phenomena originate, the nuclear 

 atom is the only one proposed that has any direct 

 experimental foundation. It is based on the deflec- 

 tions suffered by the a-particle in its passage through 

 the atoms of matter, on the one hand, as Bragg 

 showed many years ago, on the exceedingly slight 

 deviation of the overwhelming majority of the 

 a-particles, and, on the other, on the subsequently 

 discovered large deviations suffered by a minute 

 proportion. The nuclear atom is a miniature solar 

 system, like most model atoms, the negative electrons 

 occupying the atomic volume by their orbits around 

 a relatively excessively minute central sun or nucleus, 

 wherein the atomic mass is concentrated, and con- 

 sisting of an integral number of atomic positive 

 charges equal to the atomic number of the element, 

 and the number of electrons in the outer shell. An 

 a-particle is the nucleus of the helium atom, and, 

 unless it passes very near the nucleus of the atom 

 through which it penetrates, its path is practically 

 undeflected. The few that chance to pass close to 

 the exceedingly small but massive central nucleus 

 are swung out of their path like a comet at perihelion, 

 save that the forces at work are regarded as repulsive 

 rather than attractive. 



It appears from radioactive change that atomic 

 disintegration occurs always in the central nucleus, 

 both a- and /3-particles originating therein. The 

 atomic number of the element is its nett nuclear 

 charge, the difference between the positive and 

 negative charges entering into its constitution. Of 

 all properties, mass and radioactivity alone depend 

 on the nucleus ; the physical and chemical character 

 and the spectrum of an element originate in the 

 outer shell. The character of the outer shell is fixed 

 by the nett charge, not at all by the mass or internal 



