THE GROWTH OF CHEMICAL IDEAS 137 



beta rays or electrons, and almost ahvays some of the gamma 

 or x-rays. Uranium has an atomic weight of 238. It passes 

 through five transformations in becoming radium, which has 

 an atomic weight of 225; and the radium passes through 

 nine transformations before becoming lead with an atomic 

 weight of 206, the last element before lead being polonium. 

 Thorium, in the same way, goes through a series of trans- 

 formations before the atom stabilizes as an atom of lead, 

 with an atomic weight, however, not of 206 but of approxi- 

 mately 208. 



H. G. J. Moseley, a young student working with Ruther- 

 ford at Liverpool in 1913, measured the wave lengths of the 

 x-rays emitted by various elements when they were used as 

 the anti-cathode in an x-ray tube; that is, when the stream 

 of electrons falling upon them in the tube produced x-ray 

 emission. Using Rutherford's picture of the atom, Moseley 

 was able to show that the frequency of the x-radiation is pro- 

 portional to the square of the number of the element, the 

 number being the position of the element in the list of all 

 known elements; that is, the number of hydrogen, the light- 

 est element, is 1; that of helium, 2; of lithium, 3; and so on. 

 This discovery enabled Moseley to assign the numbers to all 

 the elements and thus to show what elements were missing 

 from the list, the numbers of the kno^vn elements being re- 

 lated to their chemical properties by the periodic classifica- 

 tion. When it was realized, after the ^vork of Rutherford 

 and Bohr, that an atom consisted of a positively charged nu- 

 cleus surrounded by electrons traveling in orbits, the total 

 charge of ^vhich was equal to that of the nucleus (Chapter V, 

 p. Ill), it became clear that the chemical properties of the 

 atom depend upon the electrons in the outermost orbit. 

 From the periodic classification, it "was realized that the in- 

 nermost orbit can contain at most two electrons, that the next 

 two orbits may contain eight each, and then the orbits con- 

 tain eighteen electrons, and so on. The number of electrons 

 in the atoms of each element can be stated definitely and 

 corresponds to Moseley's atomic number. 



