248 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



it became evident that for some of the elements the order based on 

 atomic weights did not agree with the order predicted on the basis 

 of chemical properties. The reason for this discrepancy came after 

 Rutherford's suggestion of the nuclear structure of the atom and as 

 a result of a series of investigations by Moseley and others on the 

 X-ray spectra of the elements. 



Turning now to a consideration of the structure of the nucleus, 

 it was discovered, in 1934, that besides the electron, and the proton, 

 which is the nucleus of the hydrogen atom, there also exists in all 

 the nuclei another elementary particle, the neutron. This particle 

 has zero charge and a very slightly higher mass than that of the 

 proton. Our present view, therefore, based on this discovery and a 

 larger number of observations, is that the nucleus contains both neu- 

 trons and protons. The number of protons is the same as the atomic 

 number (that is, the number of extranuclear electrons) and the num- 

 ber of neutrons is equal to the difference between the atomic mass 

 and the number of protons. 



Since the chemical properties of an element are governed solely by 

 the atomic number (that is, the number of protons), it is possible to 

 have two or more kinds of atoms which are chemically inseparable, 

 but which have different atomic masses. This is because of differences 

 in the number of neutrons in the nucleus. Such atoms are known 

 as isotopes. Thus, the nucleus of ordinary hydrogen is designated 

 the proton ; but there is also an isotope of hydrogen of mass 2, the 

 nucleus of which consists of a proton and a neutron. This nucleus 

 is designated the deuteron; and while it has the same charge as the 

 proton, it has about twice the mass of the proton. A large number 

 of the elements have two or more isotopes, and in the case of the 

 heavier nuclei, these are radioactive ; that is, they decay spontaneously 

 with emission of high-speed electrons, or gamma rays, or a particles. 

 The latter are the nuclei of helium atoms and consists of two protons 

 and two neutrons. Now the very important discovery has been made 

 that it is possible to transmute nuclei of a radioactive element number 

 X into nuclei of an element number X+1 or X+2 by bombardment 

 with protons, deuterons, or helium ions. Since protons and deuterons 

 have a nuclear charge of one positive unit and alpha particles have 

 a nuclear charge of two positive units, these particles may be acceler- 

 ated to very high velocities by means of high voltage in much the 

 same manner as electrons are speeded up by high voltage in an X-ray 

 tube. The velocities thus acquired by the positively charged particles 

 are of the order of one-tenth to one-half of the velocity of light. 

 Under these conditions, the particles gain sufficient energy to enable 

 them to penetrate and combine with the nucleus of a bombarded atom 

 of element number X. The result is an atom of element number X+1 

 or element number X+2, depending upon whether singly-charged 



