182 Prof. W. L. Bragg on the 



11. These empirical relations will now be considered with 

 reference to the theory of atomic structure proposed by 

 Lewis*, which has been greatly extended by Langmuir f. 

 Briefly stated, some of the principal features of the theory 

 in the form in which Langmuir presents it are the 

 following : — 



(a) The electrons surrounding the positively charged 

 nucleus of an atom are either stationary, or oscillate 

 about certain fixed positions. 



(6) The electrons are distributed in a series of approxi- 

 mately spherical shells surrounding the nucleus. 



(c) Certain arrangements of electrons around a nucleus, 



those of the atoms of the inert gases, are very 

 stable. These arrangements are: — Helium, where 

 a nucleus with two unit positive charges is sur- 

 rounded by a shell containing two electrons. Neon, 

 nuclear charge .10, surrounded by an inner shell 

 containing two electrons and an outer shell con- 

 taining eight electrons. Argon, nuclear charge 18, 

 surrounded by three shells containing two, eight, 

 and eight electrons. Krypton, nuclear charge 36, 

 shells containing two, eight, eight, and eighteen 

 electrons. Xenon, nuclear charge 54, shells con- 

 taining two, eight, eight, eighteen, and eighteen 

 electrons. Niton, nuclear charge 86, shells con- 

 taining two, eight, eight, eighteen, eighteen, and 

 thirty-two electrons. 



(d) The chemical properties of the elements depend pri- 



marily on the tendency of the atom to surround 

 itself with a more stable arrangement of electrons. 

 The most simple chemical properties are exhibited 

 by those atoms which revert most easily to the inert 

 gas form, i. e., the atoms nearest the inert gases in 

 the periodic table. An electropositive element is 

 one which tends to give up electrons in doing this, 

 an electronegative element one which tends to take 

 up electrons. 



12. Broadly speaking, tlie theory distinguishes between 

 two different types of combination in chemical compounds. 

 The first type is represented by a compound such as KC1. 

 The potassium atom has a nuclear charge of 19 units, and is 

 surrounded by 18 electrons arranged in the same way as those 

 of the argon atom, with in addition an electron which finds 



* G. N. Lewis, loc. cit. 

 f I. Langmuir, loc. cit. 



