RADIANT ENERGY 399 



trical rather than gravitational forces, as in the case of our earth 

 and sun), the electron tends to occupy the innermost orbit K. 

 The radius of this orbit is that of the hydrogen atom. In going 

 from one orbit to another, the electron takes on energy when 

 passing outward and gives it off when passing inward, to the 

 amount of one quantum, or unit of energy, when jumping from 

 one orbit to another. This capacity of the electron to give off 

 explosive bursts, or quanta, of energy in going from one orbit, 

 or energy level, to another is the source of Roentgen rays. 



The second element in the periodic table is helium. Its atomic 

 number is 2, and its weight is 4. Helium has, therefore, two 

 planetary electrons in the outer 

 field and four neutrons in the 

 nucleus, two of which possess posi- 

 tive charges (i.e., are protons) and 

 two of which are of zero charge 

 (Fig. 171). This constitution of 

 atomic nuclei is characteristic of 

 all elements except hydrogen, viz., 

 enough neutrons to account for 

 the atomic weight and enough posi- 

 tive charges to balance the outer 



field electrically. Fig. 171.— Diagram of the helium 



Each succeeding element of 

 increased weight in the periodic table has one additional 

 electron in the outer field balanced by an additional positive 

 charge in the nucleus, with enough neutrons to account 

 for the increase in weight. For example, a moderately 

 heavy element, such as calcium, with atomic number 20 

 and atomic weight 40, possesses 20 outer electrons and a corre- 

 sponding number of protons, with 20 additional neutrons in the 

 nucleus. 



Having at our disposal all the elementary units necessary to 

 construct matter, it will be instructive to list them in the table 

 shown on page 400. 



One additional fact is necessary to complete the picture of 

 atomic structure and to emphasize the importance of the energy 

 relationships resident there. It also indicates how closely atomic 

 structure duphcates, in plan, the arrangement of bodies in our 

 solar system. Goudsmit and Uhlenbeck are given credit for 



