820 Sir E. Rutherford and Dr. J. Chadwick on the 



Since probably the forces holding the H satellites in equi- 

 librium increase with the nuclear charge of the element, it 

 is to be anticipated that the H satellites are closer to the 

 nucleus for aluminium than for nitrogen. The velocity of 

 escape of the H atom, however, does not seem to be very 

 simply connected with the nuclear charge of the disintegrated 

 elements. For example,, the range of the H atoms from 

 boron (charge 5) is greater than that for nitrogen (7) ; while 

 the range of the H atom from aluminium (13) is greater 

 than that expelled from phosphorus (14). 



An a particle in a direct collision with a free H nucleus 

 at rest communicates to it "4 of its momentum and *64 of its 

 energy. In order to communicate greater energy and 

 momentum, the nucleus must be in motion in the opposite 

 direction to that of the a particle. The momentum com- 

 municated to an H satellite will thus depend on two factors: 

 (1) the velocity of the H satellite in its orbit or, in more 

 general terms, the closeness of its binding to the central 

 nucleus ; (2) the nearness of the satellite to the central 

 nucleus.* 



In order that the colliding a particle may communicate 

 much of its momentum to the satellite, the latter must be 

 held by strong forces to the nucleus. If, however, the 

 II satellite is very close to the nucleus, the a particle may 

 have to communicate a considerable fraction of its momentum 

 to the central nucleus, and the velocity of escape of the 

 H satellite is correspondingly reduced. This, for example, 

 may be the explanation why the IT atoms from aluminium 

 are faster than those from phosphorus of higher nuclear 

 charge. In phosphorus, the H satellites may move so close 

 to the nucleus, that the a, particle is able to give a smaller 

 share of its momentum to the H satellite than in the case of 

 the more distant satellite of aluminium. 



The failure of the a particle to release H atoms from 

 elements of mass greater than phosphorus may be due to the 

 fact that the H atoms either move very close to the nucleus 

 or are incorporated in its structure. 



It should be borne in mind that the chance of ejecting an 

 H satellite at great speed from a nucleus is much smaller than 

 for setting a free H atom in correspondingly rapid motion. 

 The number of swift, H atoms released from the nucleus is, 

 as we have seen, very dependent on the velocity of the 

 u particle and also on. the structure of the nucleus, but on 

 an average is not more than 1/20 of the corresponding 

 number of swift II atoms which are set in motion by collision 

 of the a particles with free hydrogen nuclei. This shows 



