November 2, 1917] 



SCIENCE 



425 



converts Uranitim 2 into Radium B, which 

 is a variety (isotope) of the element lead. 

 "We will assume that the nucleus of a 

 uranium 2 atom, so far as its composition, 

 but not its constitution, is concerned, Is 

 made up of the nucleus of a Radium B 

 atom (which nucleus we will designate by 

 (RaB)„, where the subscript n denotes that 

 it is the nucleus only), and 5 a** particles, 

 where the two plus signs serve to remind us 

 that the alpha particle carries a double 

 positive charge. Then the changes which 

 occur, beginning with Uranium 2, and end- 

 ing with Radium B, are such that in each 

 successive change one of these a* particles 

 is emitted by the nucleus. 



the fact that there is evidence in the chem- 

 ical properties that the number of valence 

 electrons decreases by two. According to 

 this idea, when the nucleus shoots out an 

 a* particle, the atom, as a whole, loses an 

 entire helium atom by the time it becomes 

 electrically neutral. That the loss of the 

 negative electrons in alpha disintegrations 

 has not been detected is probably due to the 

 low velocities with which such external 

 electrons leave the outer part of the atom. 



THE ELEMENTS OF EVEN ATOMIC NUMBER, OR 

 HELIUM SERIES ELEMENTS 



Wliile the alpha disintegrations of atoms 

 are known only among the heaviest atoms, 



The Changes iyi the Composition of the Nuclei of Atoms when they eject Alpha Particles (Nuclei of 



Helium Atoms) of Weight 4, and carrying Two Positive Charges, with Corresponding 



Changes in the Non-nuclear Electrons 



According to this table it would seem 

 that when the nucleus of an atom loses an 

 a** particle, and thus decreases its positive 

 charge by two, the outer atom must lose 

 two negative electrons in order to keep the 

 atom electrically neutral. That this is 

 actually the case seems to be indicated by 



3 The most doubtful feature of this table is the 

 assumption that the nuclear charge is equal to the 

 atomic number, but the insertion of 92 -F ^ for 92, 

 of 90 -|- /I for 90, etc., where ^ is a whole number, 

 sind probably either zero or else Tery small, re- 

 moves this doubtful feature. 



and extend downward from element ninety- 

 two (uranium) to element eighty-two 

 (lead), it occurred to me several years ago 

 that this system undoubtedly should ex- 

 tend downward still further, and quite pos- 

 sibly even to the lightest elements. The 

 indication that the system still holds 

 sliould be found in the atomic weights, for 

 these should increase in steps of four be- 

 tween the atoms of even number. Thus the 

 atomic weights of the lighter elements, if 

 exactly this same system holds, should be 

 as follows : 



