1 68 OUR PHYSICAL WORLD 



elements in this series are unknown, uranium with ninety-two 

 electrons in its atom being the heaviest known substance. 



Chemists believe that the elements differ in the construction 

 of their atoms, as indicated above, for several reasons, the chief 

 of which is that when the elements are arranged in such a scheme 

 they are in the order of their increasing atomic weights, and their 

 properties are a function of their position in the scheme. 



The first clear apprehension that the elements are so related, 

 that they form several series in which correspondingly placed 

 members in these series exhibit similar properties, was due to 

 Mendeleeff. The law has come to be known as the periodic 

 law or, since any element has properties closely approximating 

 the eighth one before or after it, if the elements are arranged 

 on the basis of the atomic weights, it is also known as the law 

 of octaves. The arrangement of the elements in the periodic 

 scheme is shown in the table on pages 1 70 and 171. The explana- 

 tion of their atomic structure in terms of protons and electrons 

 is very recent, and is a tentative theory that may have to be 

 much modified. 



When elements unite to form a chemical compound, a positive 

 element usually unites with a negative one. Thus positive 

 sodium unites with negative chlorine to make common salt or 

 sodium chloride. Positive elements do not unite with positive or 

 negative with negative. Moreover, elements always unite in 

 definite proportions by weight. That is one reason the atomic 

 theory was adopted. If one atom of sodium always unites with 

 one of chlorine to form a molecule of sodium chloride, then evi- 

 dently they must unite in amounts proportional to the relative 

 weights of the atoms. Sometimes, however, one atom of one 

 element unites with two of another. Thus Mg unites with Cl 

 to form MgCl 2 , which means that one atom of magnesium has 

 united with two atoms of chlorine to form one molecule of 

 magnesium chloride. The number of bonds an atom of one 

 element has, by which it attaches itself to the atom of another 

 element, is designated the valence of the element. It will be 



