THE STRUCTURE OF ATOMS 29 



have their electrons arranged exactly like those of the neon 

 atoms and the ions of sodium and fluorine. Therefore, 

 the crystalline form of magnesium oxide and sodium flu- 

 oride should be identical, and this prediction of the theory 

 has been confirmed experimentally by Dr. A. W. Hull by 

 the X-ray method. Because of the much greater forces 

 acting between the ions as a result of the double charges, 

 the stability of the magnesium oxide is much higher than 

 that of the sodium fluoride. This is manifested by the 

 high melting point, low conductivity, low solubility, and 

 hardness of magnesium oxide. 



Phosphorus (N == 15) and sulfur (N = 16) have, re- 

 spectively, 5 and 6 electrons more than neon, and are thus 

 capable of giving up these numbers of electrons. If these 

 elements are brought into contact with an excess of fluor- 

 ine (which because of its proximity to neon has a par- 

 ticularly strong tendency to take electrons) all the extra 

 electrons pass to fluorine atoms. Thus a sulfur atom will 

 supply electrons to 6 fluorine atoms and will form the 

 compound SF 6 . The force acting between the fluorine 

 ions and the central sulfur ion is still electrostatic in na- 

 ture but it must be nearly 6 times greater than the force 

 between sodium and fluorine ions. Furthermore, the 6 

 fluorine ions would surround the sulfur ion so that there 

 would be little stray field of force. Therefore, we should 

 not expect sulfur fluoride to be salt-like in character but 

 to consist of very stable molecules having weak external 

 fields of force and, therefore, readily existing in the form 

 of a gas. As a matter of fact this extraordinary sub- 

 stance has these properties developed to such a degree that 

 it is an odorless and tasteless gas with a boiling point of 

 62. Phosphorus pentafluoride, as would be expected 

 from its less symmetrical structure, is a gas having greater 

 chemical activity. 



The fluosilicate ion SiF 6 has a structure exactly like 



that of the sulfur fluoride molecule since the number and 

 arrangement of the electrons are the same. This is clear 



