CHEMISTRY: I. LANGMUIR 
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radii of the shells form an arithmetric series 1, 2, 3, 4, and the effective areas 
are in the ratios 1 : 2^: 3^: 4^. 
3. Each shell is divided into cellular spaces or cells occupying equal areas 
in their respective shells and distributed over the surface of the shells accord- 
ing to the symmetry required by postulate 1. The first shell thus contains 2 
cells, the second 8, the third 18, and the fourth 32. 
4. Each of the cells in the first shell can contain only one electron, but each 
other cell can contain either one or two. All the inner shells must have their 
full quotas of electrons before the outside shell can contain any. No cell in 
the outside layer can contain two electrons until all the other cells in this 
layer contain at least one. 
5. When the number of electrons in the outside layer is small, these elec- 
trons arrange themselves over the underlying ones, being acted on by mag- 
netic attractive forces. But as the charge on the kernel or the number of 
electrons in the outside layer increases, the electrostatic repulsion of the un- 
derlying electrons becomes predominant and the outer electrons then tend to 
rearrange themselves so as to be as far as possible from the underlying ones. 
6. The most stable arrangement of electrons is that of the pair in the helium 
atom. A stable pair may also be held by: {a) a single nucleus; {b) two hydro- 
gen nuclei; {c) a hydrogen nucleus and the kernel of another atom; {d) two 
atomic kernels (very rare). 
7. The next most stable arrangement of electrons is the octet; that is, a 
group of eight electrons like that in the second shell of the neon atom. Any 
atom with atomic number less than twenty, and which has more than three 
electrons in its outside layer tends to take up enough electrons to complete its 
octet. 
8. Two octets may hold one, two, or sometimes three pairs of electrons in 
common. One octet may share one, two, three or four pairs of its electrons 
with one, two, three or four other octets. One or more pairs of electrons 
in an octet may be shared by the corresponding number of hydrogen nuclei. 
No electron can be shared by more than two octets. 
The inert gases are those having atoms in which all the cells in the outside 
shell have equal numbers of electrons. Thus according to the first four postu- 
lates the atomic numbers of the inert gases should be 2, 10, 18, 36, 54, and 86 
in agreement with fact. 
All atoms with an atomic number greater than that of helium, have as their 
first shell a pair of electrons close to the nucleus. The line connecting the two 
electrons establishes the polar axis for the atom. Neon has in its second shell 
eight electrons, four in each hemisphere (i.e., above and below the equatorial 
plane), arranged symmetrically about the polar axis. The eight electrons are 
thus nearly at the corners of a cube. In argon there are eight more electrons 
in the second shell. 
The eight electrons in the third shell of the atom of iron are arranged over 
the underlying electrons in the second shell. The two extra electrons in the 
