SOME FUNDAMENTAL PHYSICS AND CHEMISTRY 



33 



is greater than 10, the additional ones are in a shell outside of a first 

 shell of 2 and a second shell of 8. 



It is only the electrons of the outermost shell which enter into ordinary 

 chemical reactions. Different elements having the same number of 

 electrons, similarly placed, in this outermost shell tend to possess similar 

 properties and enter into similar reactions. A number of families of 

 elements are thus recognized whose properties are much alike, such as 

 the halogen family which includes fluorine (F), chlorine (CI), bromine 

 (Br), and iodine (I), in which there are 7 electrons in the outer shell — but 

 a different shell in each of these elements. 







e 











Fig. 23. — Two atoms of oxygen combined to complete their outer shells of electrons. 



Chemical Reactions. — Some elements react more easily than others. 

 The difference between them in this respect lies in their outermost shells 

 of electrons. -In each shell of an atom there is a maximum possible 

 number of electrons. An element which has this maximum number 

 of electrons in its outer shell does not react I'eadily; the inert gases, such 

 as helium used in balloons and neon in electric signs, are in this state. 

 Most elements, however, have less than the maximum number of electrons 

 in the outer shell, and it is easy for such elements either to complete that 

 shell or to lose the electrons which are already in it. Because of this ease 

 of reaction, two atoms of the same element sometimes join to complete 

 their outer shells. Oxygen, as already stated, has six out of a possible 

 eight in its second (outer) shell. If one atom shares two of its electrons 

 with the other atom, and in turn accepts two electrons from the latter, 

 each has a complete shell of eight electrons (four of them in common) 

 and the two atoms are combined (Fig. 23). A molecule of oxygen is thus 



