Why should atoms contain outermost electron shells that have room
for a maximum of eight electrons? G. N. Lewis first proposed this
"rule of eight" at the turn of the century, and initially pictured
the electrons as occupying the corners of a cube around the atom.
Some of his sketches from a notebook page of 1902 are shown opposite.
Lewis soon realized that the eight electrons were arranged in four
electron pairs, like the electron pairs involved in chemical bonding.
No good explanation for these four electron pairs per shell was
forthcoming until physicists began applying quantum mechanics to
chemistry in the 1920's. We will use their results in Chapters 7
and 8, but will avoid the mathematics that led to the results. For
the moment we will say only that in the second electron shell there
are four different "orbitals" or ways of locating electrons in space
around the nucleus, with each of these orbitals capable of holding
two electrons. In Chapter 9 we shall see that the calculated arrangement
of these orbitals in space leads to a correct prediction of the
observed shapes of molecules, a powerful indication that
the quantum- mechanical treatment is correct.