100 THE REALITIES OF MODERN SCIENCE 



are 7, 23, and 39, respectively ; hence the difference 

 between sodium and lithium is the same as that between 

 potassium and sodium. (Table I shows equal differences 

 in the number of electrons.) 



The classification of elements according to atomic 

 weights was first worked out by Newlands in 1864. It 

 was, however, much extended and elaborated by Men- 

 delejeff in 1869. Finding in one of the periods a place 

 where an element was apparently missing, Mendelejeff 

 predicted that an element of about a certain atomic 

 weight, with a certain valence, which should form 

 certain compounds typical of its group, might later be 

 discovered. He made two other predictions of this 

 character. The three elements, gallium, scandium, 

 and germanium, were discovered during the next twenty 

 years and found to have essentially all the properties 

 which had been predicted for them. 



The characteristics of the atoms of the elements 

 are believed to-day to be due to the number and con- 

 figuration of the electrons. The power of combination 

 is undoubtedly to be explained in terms of the forces ex- 

 isting between the electrical charges of atoms which are 

 brought close together. Although positive and nega- 

 tive electricities in the atoms are equal, their distribu- 

 tion might well be such that a net force would act be- 

 tween two dissimilar atoms if they were very close 

 together. The cause of attraction would then be some- 

 what similar to that discussed on page 95, the result- 

 ant force being due to the fact that the attracting 

 electricities of the two atoms were on the average closer 

 to each other than were the repelling electricities. 



