168 B R I C K S O F T H E U N-i V E R S E
first shell and eight in the second. The next element, sodium, has
eleven electrons; the eleventh begins a third shell of orbits. When
there are eight orbits filled in the third shell we have the inert gas argon.
Argon is a sort of half-way house toward completion of the third
shell. Additional electrons go either to a fourth shell, or else are added
to the number on the third shell. Finally we reach the inert gas
krypton with the innermost three shells completely filled, two orbits on
the first, eight on the second, and eighteen on the third; beyond these
krypton has eight orbits on a fourth shell. The more elaborate elements
have still more shells of orbits.
INERT GAS STRUCTURE
In each of the inert gases the inner shells of orbits are completely
filled and there are eight orbits in the outermost shell (except, of course,
In the case of helium, which has only one shell with both orbits filled).
The outermost shell of eight orbits is often called the " outer octet."
When there are eight electrons in the outermost shell the atom appears
to be sated (or saturated). Such atoms are chemically inert; they do not
form molecules by joining with other atoms; they exist as self-satisfied
entities. The structure of the inert gas atoms is referred to as the Inert
gas structure.
.COMPLETING THE OCTET
There seems to be an urge amongst atoms toward saturation, an
urge toward completing the outer octet of electrons (or in the case of
hydrogen toward the structure of helium with its pair of outer electrons).
Atoms often link up in such a way as to achieve the Inert gas
structure; the two kinds of link are ways of obtaining it. When this
structure is achieved the compound becomes itself inert.
One of the links is called the ionized or electrovalent link. The
metal sodium has a single electron on its outermost shell. If it could
give up this electron it would become a positive ion with the inert gas
structure. Chlorine has seven electrons on the outermost shell. If it
could gain an extra electron it would be a negative ion with the inert
gas structure.
When an atom of sodium comes in contact with an atom of chlorine,
the exchange is made. The sodium atom gives up an electron to the
chlorine atom. Instead of two neutral atoms we now have two ions with
opposite charges. The ions attract each other, and are held together by
the attraction as a molecule of common salt,
THE CHEMICAL LINK
The other kind of link between atoms is called the chemical link.
Chlorine has seven electrons on its outermost shell. When two chlorine
atoms meet, each contributes an electron toward a link. These
electrons become a part of both atoms, so chat each atom has eight
demons on its outermost shell, and therefore has the inert gas structure.