The Electron Theory

    All matter is made up of ATOMS.  A simple atom consists of a NUCLEUS - made up of
PROTONS (which have a + charge), and NEUTRONS (which have no charge), surrounded
by orbiting ELECTRONS (which have a - charge).

Electron Shells
 
 

Electrons orbit the nucleus in shells.  The innermost shell will contain
only 2 electrons.  The second will contain up to 8, the third up to 18, the
fourth up to 32, the fifth up to 50 and the sixth up to 72.  The electrons
in the outermost shell are the most important in electricity, because these
are the ones that can be displaced easily.  This outer ring is known as the
Valence Shell.  If the Valence Shell contains 3 electrons or less, the electrons
can be displaced easily (such as in this copper atom).  If the Valence Shell
contains 5 to 8 electrons, the electrons are difficult to move from their orbit.
If the Valence Shell contains 4 electrons, it forms a special material that
we will discuss later known as a Semi-Conductor.

    The ELECTRONS in the inner orbits are known as BOUND electrons, and are difficult
to move from their orbits.  The ELECTRONS in the outer orbit are known as FREE electrons,
and are more easily moved from their orbits.
 

    A balanced atom will contain an equal number of protons and electronics. If the atom
contains more electrons - it is a negative ion.  If it contains fewer electrons, it is a positive ion.
If an additional ELECTRON is placed in the outer orbit by an outside SOURCE, the atom
becomes unbalanced because it has an excess of electrons.  This results in one of the
ELECTRONS attempting to move from its orbit to another atom which has a shortage
of electrons.

Conductors, Insulators and Semi-Conductors

In the Copper Atom above, only 1 electron is located in the Valence Shell.  As a
result, it can be easily displaced or can have an additional electron added to it.
This results in a material that can carry electrons easily - known as a
Conductor.  If the material has 5-8 electrons in the Valence Shell, it is
much more difficult to move the electrons in and out of the outer orbit, and
this results in the electrons being more firmly bound to the atom - producing
an Insulator.

Semi-Conductors

In this Silicon atom model, there are 4 electrons in the Valence Shell of
each electron.  When this occurs, neighbouring atoms can share the
electrons in their Valence Shell to form a special Crystal Lattice, which
produces an Insulator with special qualities.  Because the Valence
Shells are shared - this is known as a Covalent Bond.
 

If a Phosphourus atom (with 5 electrons in its Valence Shell) is added
to the Silicon lattice, a material is produced that forms a Semi-Conductor
with an extra electron that can be displaced under the proper conditions.
 

If a Boron atom (with 3 electrons in its Valence Shell) is added
to the Silicon Lattice, a material is produced that forms a Semi-Conductor
with a missing electron position (a + hole ) that can be filled with an
extra electron under the proper conditions.

Next - Electrical Circuits

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