The electronic structures of the first eighteen elements are shown
on the opening page, arranged in three rows of the periodic table
as it was introduced at the end of Chapter 3.
These are not pictures of the atoms, but are schematic diagrams
of electron-shell structure. Electrons are represented by black
dots. Each shell is shown in white while it is being filled, and
with a colored background when full, so that attention will be focused
on the next shell being filled.
The second and third shells are divided into four boxes as a reminder
that the eight electrons in a full shell occur as four electron
pairs. Below each shell diagram is a Lewis electron-dot diagram
of the atom, which conveys the same information about outer-shell
Each of the four compass points around the atom symbol in a Lewis
diagram represents one of the four possible positions for electron