APPENDIX 359compounds their atoms are always electrically neutral and the
number of planetary electrons is the same as the nuclear charge.
The atomic numbers give the sequence 2, 18, 36, 54, 86, and it is
obvious that these numbers of electrons must fall into layers of
great stability. The numbers in the succeeding shells are probably
as follows:
He 2
Ne 10
Al8
Kr 3 e
Xe 64
Nt 862 2 2 2 2 2
8
8
8
8
88
18
18
188
18
328
18 8To take for example the atom of xenon, the first three shells
are filled to the maximum numbers of 2, 8, and 18. There remain
26 electrons, but when the number 18 in the fourth shell is reached,
it is more difficult for the remaining electrons to enter this shell
than to begin building a new shell. The 8 electrons left are
just enough to give the stable configuration of 8 in the fifth
shell.
The neutral atom of cesium has one more electron than xenon,
and this extra electron stays in the sixth shell rather than forcing
itself into either the fourth or fifth shells and disturbing the stable
arrangements of 18 and 8 in these.
Positive Polar Valence. This single electron in the outer layer
has very little stability in its isolated position and is easily removed
altogether from the atom, leaving the atom with a net positive
charge of one unit. Thus the ease with which cesium changes
to cesium ion Cs+ is accounted for. Similarly, the formation of
barium ion Ba++ and lanthanum ion La+++ is accounted for.
Negative Polar Valence. There still exists the usual electrostatic
attraction between the electron and the kernel of the cesium atom.
The kernel consists of the nucleus and all the electron layers except
the outer layer, or valence layer. In fact, the electron would
ordinarily be held in the outer layer unless some other atom were
ready to take it up. Neutral atoms with nearly complete outer
shells show a strong tendency to take on enough electrons to com-
plete the shell. This tendency is strong enough to overcome the
electrostatic repulsion of the other electrons and impart to the
atom a net negative charge. Thus the chlorine atom Cln 2-8-7