The animation shows the essence of salt structure: positive and
negative ions, attracted toward one another, and packed in a regular,
geometric array into a three-dimensional solid with an alternation
of positive and negative charges.
By contrast, in a metal
all of the ions are positive ions, and these are held together by
a sea of mobile electrons. We shall see how the arrangement of ions
and electrons in these two kinds of solids makes each behave in
its own way.
The ideas of periodicity of chemical behavior and of a periodic
table with horizontal rows corresponding to electron shells were
introduced at the end of Chapter 3.
These second-shell (or second-row) elements, Li to Ne, are a microcosm
of the entire periodic table, for the trends and behavior that we
will see in this chapter will be encountered again with the heavier
We are learning not only about the second-shell elements, but about
the basic principles of chemical behavior that are applicable to
all atoms. These principles appear in a particularly simple way
for the second-shell atoms.