172 OUR PHYSICAL WORLD 



In a similar way magnesium has twelve electrons, two in the 

 inner sphere, eight in the second sphere, and two in the third. 

 Six more would be needed to supply this outer sphere, and it is 

 difficult to get them. But oxygen has eight electrons, two in 

 its inner sphere, six in its second sphere, and needs two more to 

 satisfy this sphere. If an atom of magnesium and one of oxygen 

 unite by using the two electrons in the third sphere of the mag- 

 nesium atom to fill up the second sphere of the oxygen atom, 

 we will have the substance known as the oxide of magnesium. 

 Evidently it would take two atoms of fluorine to unite with one 

 of magnesium to make magnesium fluoride whose formula is 

 written, then, as MgFl 2 . 



But valences are not the only properties of the elements that 

 seem to be sequentially arranged on the basis of this periodic 

 law. The elements in any one column are very similar to each 

 other in their physical properties and chemical behavior. Thus 

 all the elements in the zero group or column are very inactive 

 chemically. They may be regarded as having no tendency to 

 combine with other substances they have a valence of zero. 

 The metals are more vigorously metallic in their characters as 

 you go down the columns, and the non-metals are less vigorous 

 in their non-metallic characters. Thus in column VII, fluorine 

 is the most vigorous non-metal known, chlorine slightly less so, 

 etc. Fluorine, chlorine, bromine, and iodine are so much alike 

 they have been grouped together as the "halogens" for a long 

 time. Color, density, and solubility of similar salts increase down 

 each column. Thus fluorine is pale yellow, chlorine greenish yel- 

 low, bromine red, iodine purplish black. The melting-point of 

 the elements decreases as you go down each column while the 

 boiling-point increases. 



The elements in the right column (VIII) do not fit well into this 

 scheme, and chemists suspect that this periodic law is but a par- 

 tial expression of the truth. In time we shall discover a better 

 statement of it which will take in these apparent exceptions. It 

 is, however, a working hypothesis and helps one to recall atomic 



