1 66 OUR PHYSICAL WORLD 



distinction between elements and compounds is still maintained. 

 But now it appears that elements in turn are made of protons 

 and electrons, and the difference in their properties is due merely 

 to the difference in the number and arrangement of these com- 

 ponent units in their atoms. The nucleus of every atom is 

 made of one or more protons, each holding at some distance an 

 electron that moves about the nucleus. Possibly the nucleus 

 is made of both protons and electrons, but if so the protons are 

 in excess, and it is the excess protons that hold the electrons that 

 move about the nucleus. It will simplify matters here to con- 

 sider only the excess protons and their attendant circling electrons. 



The atom of hydrogen, as we have seen, 

 consists of one excess proton and one elec- 

 tron. The helium atom has two protons in 

 its nucleus and two electrons that lie on 

 opposite sides of this nucleus. These two 

 electrons have pathways which are included 



in a sphere that is relatively distant from 

 FIG. 66.-The helium ^ central nucleus j ust as was the case 

 atom. In this and the . 



succeeding diagrams no m tne hydrogen atom. The electrons are 

 attempt is made to repre- symmetrically arranged with reference to 



sent relative sizes and ^ nuc l euS (Fig. 66). In all such Cases 

 distances accurately. A , , , , ,. , . 



the element seems to be relatively inactive 



chemically, and helium is a very inert gas. It is a very light gas, 

 not as light as hydrogen, but it is used in place of the latter in 

 filling balloons, for it is safer. It is obtained from natural gas. 

 Hydrogen is very active chemically, and forms with oxygen an 

 explosive mixture. 



The lithium atom, the next in the series, has three protons 

 in its nucleus and three electrons about the nucleus, two in a 

 sphere similar to that of helium, the third in a sphere 

 twice as far from the nucleus as the first sphere. Its mole- 

 cule is, therefore, larger than that of helium. Then come 

 beryllium with four electrons, boron with five, carbon with six, 

 nitrogen with seven, oxygen with eight, fluorine with nine, and 



o 



