138 THE PATH OF SCIENCE 



The structure of the chemical elements, therefore, the 

 charge on the nucleus, which is the same as that of the atomic 

 number, and the nuinber of electrons were all worked out. 

 One difficulty still remained, ho^vever. The atomic weights 

 of the elements are not the same as their atomic numbers. 

 The atomic weight, for instance, of helium is 4; its atomic 

 number is only 2; and it has only 2 electrons. If the hydro- 

 gen nucleus, which is generally called a proton^ has a weight 

 of 1, helium might be expected to have 2 protons in its nu- 

 cleus, ^vhich would give it t^vo positive charges. Having 2 

 electrons, it would be neutral, and its atomic weight should 

 be 2. The problem was solved when James Chadwick— like 

 Moseley and Aston, one of Rutherford's collaborators— found 

 that, under some circumstances, from atoms exposed to radia- 

 tion, particles could be obtained having a mass equal to 

 that of the proton but no electric charge. They are called 

 neutrons^ and they represent the missing units in the struc- 

 ture of the nucleus of the atom. The helium nucleus, for 

 instance, contains 2 protons and also 2 neutrons, these sup- 

 plying the necessary units of weight to account for the atomic 

 weight of the element as a whole. 



The discovery of the neutron made possible an explanation 

 of the nature of the isotopes, discovered by Aston. The chem- 

 ical properties of an element depend upon the number of its 

 electrons, and the nucleus must have a number of protons 

 equal to the electrons to maintain electric balance in the atom 

 as a whole. The number of neutrons in an atoin, however, 

 do not affect the chemical properties, so that it is possible to 

 have two atoms with the same number of electrons, the same 

 atomic number, and the same chemical properties, but a 

 different total mass, because of a difference in the number 

 of neutrons present in the nucleus. Thus, in the case of the 

 two isotopes of neon that Aston discovered in the mass spec- 

 trograph, the particles in the rays had different masses. The 

 neon with an atomic weight of 20 has in its nucleus 10 protons 

 and 10 neutrons; its atomic number is 10, and it has 10 elec- 

 trons; but the neon with an atomic weight of 22 has the same 



