402 



PROTOPLASM 



one would expect to get if the electron could be photographed. 

 From Fig. 172, it is seen that there are certain preferred regions 

 for the electron, i.e., regions where the probabiUty of finding the 

 electron is relatively large, and unpreferred regions. 



The old and the new pictures of the atom are quite different 

 in their formulations and in their energy relations, except for 

 hydrogen. Here the old Bohr orbital picture and the new quan- 

 tum mechanical one give exactly the same energy relations for the 

 atom. 



With the foregoing as a basis of discussion, we may now con- 

 sider the forms of radiant energy given off by atoms. There is 

 first the energy emitted when an electron changes its orbit, from 

 an "excited" to a normal state. If we take the simple case of 

 hydrogen, we shall remember that its one electron can have a 



Fig. 172. — Photographs of the electron cloud for three states of the hydrogen 

 atom, obtained from a working model. The cloud shows where the electron of 

 hydrogen may be at any given moment. {From H, E, White.) 



number of possible orbits, in each of which it possesses a definite 

 energy value calculable in terms of the radius of the orbit, 

 according to the same principles for the calculation of the energy 

 associated with the rotation of a planet around the sun. The 

 greater the size of the orbit the greater the energy. Only by 

 acquiring energy can an electron be made to leave its orbit and 

 go into a larger one. In returning from a larger orbit to a smaller 

 one, it parts with this energy, which is radiated in the form of 

 light or X rays. 



Other types of energy radiated by an atom are alpha and beta 

 particles and gamma rays. Alpha particles are the nuclei of 

 helium atoms and therefore consist of two neutrons and two 

 protons {i.e., four neutrons, two of which have positive charges). 

 Alpha particles are, therefore, helium atoms which have lost 



