THE ELECTRON 99 



creased, certain arrangements recur. When the num- 

 ber is increased to fifteen there is an outer ring of nine, 

 an inside ring of five, and in the center of this a single 

 magnet. As the number of magnets increases certain 

 configurations appear more or less periodically, that is, 

 at definite intervals. 



It is reasonable, therefore, to expect, as prominent 

 scientists 1 have done, that whatever the positive 

 nucleus of the atom may be, atoms differing in their 

 compositions by definite numbers of electrons may in 

 part have similar configurations for their electrons. 

 In so far as the electron configuration as well as the 

 number determines the characteristics, such atoms 

 should have certain similarities in chemical properties. 

 If a list is made of atoms in the order of the number of 

 electrons they contain, those of similar properties might 

 be expected to occur periodically in the list. 



To some extent this is reasoning after the fact. That 

 there is such a periodicity to a list of the elements, 

 arranged in order of their atomic weights, was noticed 

 years before the electron was discovered. Also, some 

 chemical elements are similar in their properties, e.g. 

 lithium and sodium are both soft white metals. They 

 form chlorides, LiCl and NaCl, respectively, which 

 have quite similar properties. Potassium is also very 

 similar to these two elements. We find such a similar- 

 ity between the compounds of sodium and potassium ; 

 thus we are all familiar with the similarity of caustic 

 soda and caustic potash, NaOH and KOH, respectively. 

 The atomic weights of lithium, sodium, and potassium 



1 Cf . J. J. Thomson, "The Corpuscular Theory of Matter," 

 Chap. VI, Charles Scribner's Sons, 1907. 



