WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 567 



are most simply explained by considering the atom to be pos- 

 sessed of a very small nucleus carrying a positive charge equal 

 to its atomic number. That the electron is a constituent of all 

 matter can no longer be doubted. Then, since the atom as a 

 whole is electrically neutral, enough electrons to neutralize the 

 nuclear charge must be arranged about the central nucleus.^ 

 Thus the hydrogen atom is a small positive nucleus bearing a 

 single positive charge and accompanied by a single electron; 

 the helium atom is a doubly charged nucleus accompanied by 

 two electrons; and so forth. 



Loosely-bound electrons. — Some of the electrons in an atom 

 may be expected to be more tightly bound than others. Close 

 to the positively charged nucleus the electrical forces (and pre- 

 sumably also the magnetic forces) should be much stronger than 

 farther away from the nucleus. Consequently those electrons 

 which are close to the center of the atom will be held with greater 

 force than the more distant electrons. 



There are numerous indications'- that the atoms do contain 

 a small number of more or less weakly bound electrons. The 

 application of the electron theory to the dispersion of radiation 

 indicates the existence of such electrons. By using the relations 

 which have been developed for the theory of dispersion it is pos- 

 sible to calculate roughly the number of dispersing systems in 

 each molecule. If such a calculation is carried out for quartz, 

 for instance, it is found that each molecule possesses three to 

 four, probably four, "dispersion electrons." These calculations 

 must be inexact, however, because the proper application of the 

 expressions used requires a knowledge of all the vibrating sys- 

 tems in the molecule. Experimental difficulties, especially in 

 the extreme ultra-violet, make this impossible. 



Studies upon the absorption of light and the optical proper- 

 ties of metals furnish similar results. Metallic conduction is 

 assumed to be due to electrons which possess a certain amount 

 of freedom of movement. Then there are those electrons which, 

 as is well known, can be Hberated by hght and heat; these also 



^2 Campbell. Modern electrical theory. 



