STATIC MODELS 
IX. 2] 
123 
to the tendency to conform as nearly as possible to the structure 
of the rare gases. 
The most stable element of all is helium with two electrons. 
Each electron may be imagined as situated in one half of a 
complete shell (like a walnut shell) surrounding the nucleus. It 
is not necessary to regard the electrons as stationary, but for 
simplicity each may be thought of as occupying its average 
position. The grouping in pairs about a positive charge may be 
regarded as the first electronic ideal. 
The next most stable element is neon, atomic number 10. 
It is assumed that as in helium, two electrons are near the nucleus. 
The remaining 8 arrange themselves in a second shell, four in 
either hemisphere. This arrangement of 8 electrons may be 
called an octet. Next to the balanced pair of electrons the octet 
is the most stable grouping. 
The arrangement of the electrons in the other inert gases, 
argon, krypton, xenon and niton (radon), is indicated in the 
table on p. 74. 
The stability and inactivity of these elements is attributed 
to the electrons being placed in the most ideal position for the 
production of a balanced system, so that there is a minimum 
field of external force. 
The key to all chemical combination is found in the striving 
of all elements to become as nearly as possible like the inert gases. 
According to Lewis chemical compounds may be divided 
broadly into two types—polar and non-polar. As an example 
of the former type we take lithium fluoride. The single electron 
in the outer shell of the lithium atom may be supposed to pass 
to the outer shell of the fluorine atom so as to complete the 
octet. We then have a positively charged lithium ion and a 
negatively charged fluorine ion, and the electrostatic attraction 
between these ions results in the formation of a molecule of 
lithium fluoride (Li F). 
In chemical compounds of the second type no ionization need 
be involved, and the chemical bonds or valencies are determined 
by pairs of electrons. In the Lewis-Langmuir theory one 
electron held in common never holds two atoms together; two 
atoms held together by a single valency bond hold two electrons 
in common. This is illustrated by the case of the fluorine mole 
cule (Fig. 16). 
The double valency bond implies that four electrons are held 
conjointly by two atoms (Fig. 17). 
If the pair of electrons be regarded as the most stable group 
ing of all, it may be, as Lewis and Langmuir suggest, that the 
pairs of electrons held in common by two atoms are drawn 
closer together by the (magnetic) attraction between them. The