Atoms with the same outer-shell structure have similar chemical
properties. These outer electrons are the most significant features
that a neighboring atom sees.
Sodium behaves like lithium, silicon and carbon having many properties
in common, and chlorine and fluorine are very much alike. This is
the most important single fact in this chapter, and we will develop
this theme at length.
Having said this, we must qualify matters by saying that there
are important differences in the chemical behavior of second-shell
and third-shell elements, which arise because the third-shell atoms
are bigger and have a weaker hold on their outer electrons.
Each third-shell atom has a lower first ionization energy than
the corresponding atom in the row above it. This property makes
every third-shell element less electronegative than its second-shell
analogue, and more metallic.
The three rows in the opening diagram, which represent the first
three electron shells, are the beginning of a very important means
of classifying chemical properties of atoms, the periodic table.
The periodic table is organized such that the successive addition
of electrons to the same shell occurs across horizontal periods,
and similar outer electron-shell structures and chemical properties
are found in vertical groups.
The periodic table was devised a century ago as a memory aid, to
make chemistry easier. It eventually became the great statement
of chemical reality, against which any theory of atomic structure
and properties had to be tested.
The second-shell elements are the elements of living organisms,
and the third-shell elements make up the framework of our planet.
The crust of the Earth is 60 atomic percent oxygen.
The other 40% is divided among silicon (21%), hydrogen 3%), and
third- and fourth-shell metals (16% together): sodium, magnesium,
and aluminium in the third shell; and potassium, calcium, and iron
in the fourth.
All of the remaining elements taken together make up less than
half of one percent of the Earth's crust. The six metals just mentioned
occur in combinations with oxygen as silicate minerals, water, and
the various metal oxides, carbonates, and nitrates.