Laboratory Methods of Inorganic Chemistry, 2nd English Ed. 1928

12 THE ELEMENTS.

number into the various levels. To take a concrete example:

Xenon has an atomic number of 54, that is, the nucleus has 54

unit charges and the number of planetary electrons in the neutral

atom is 54. If the attraction of the nucleus were unopposed the

first three levels would be filled to 2, 8 and 18 electrons respec-

tively and the fourth layer would hold 26. Actually the first

three layers are held by the strong nuclear charge, but a fourth

layer of 26 is unstable and eight are driven out into a fifth layer

so that the configuration becomes XeM 2-8-18-18-8.

Now there is something inherently stable in layers having the

numbers 2, 8, 18, 32 ... and to acquire these numbers very con-

siderable counter forces can be overcome in atom and molecule

building. The Xenon atom with layers all containing the stable

numbers and with the nuclear charge to just neutralize the elec-

trons in these layers, is an extremely stable structure. This

accounts for the chemical inertness of Xenon. Likewise the in-

ertness of the other rare gases is accounted for.

Elements are known with practically all the atomic numbers

from 1 to 92. With elements other than the inert gases the tend-

ency is to acquire layers in conformity to the nearest inert gas

atom, thus upsetting the electrical neutrality of the atom and

giving us an "ion." Thus cesium, Cs 6 6, loses an electron and

becomes a cesium ion Cs+ when some atom such as chlorine is at

hand to take on the electron.

Chlorine Cli 7 by taking on the electron acquires the structure

of argon A« 2-8-8 and thus becomes a negative ion Cl~.

The tendency for atoms to acquire an outer layer of eight where

this is in any way possible is very strong. This is often accom-

plished as just indicated through the formation of ions, but there
is another way which was first recognized by G. N. Lewis in 1916,

and which in fact smooths out most of the previously irreconcilable

differences between the conceptions of valence held by inorganic

and organic chemists. This is that the two layers of eight may be
filled by a small number of electrons simultaneously holding posi-

tions in the outer layers, or "octets," of two atoms. For example,

in the chlorine molecule CU we have the two atoms Cln 2-8-7 with

only 14 electrons out of which to make two octets. If two elec-
trons are held jointly in the outer layer or "sheath" of each atom,

the two octets are completed.