When I started this page I didn't know it would grow to such size.  Don't panic, you don't need all
this for the class, but it is interesting.
Before the Beginning:
The real start to the atomic theory had little to do with anything like atoms, it was an argument over
change.
The ancient philosopher, Heraclitus, maintained that everything is in a state of flux. Nothing escapes
change of some sort (it is impossible to step into the same river twice). On the other hand,
Parmenides argued that everything is what it is, so that it cannot become what is not (change is
impossible because a substance would have to transition through nothing to become something
else, which is a logical contradiction). Thus, change is incompatible with being so that only the
permanent aspects of the Universe could be considered real. (this gives you a small idea of what
philosophy was like in ancient Greece.
Democritus

An ingenious answer was proposed in the fifth century B.C. by Democritus. He hypothesized that all matter
(plus space and time) is composed of tiny indestructible units, called atoms. This idea came from the
question of how small one can go on cutting up matter. While Democritus performed no experiments and
had only the flimsiest evidence for the existence of atoms, his theory was kept alive by the Roman poet
Lucretius who's work survived the Dark Ages to be discovered in 1417.
The atoms in Democritus theory themselves always remain unchanged, but move about in space to combine in various ways
to form all objects. Early atomic theory stated that the properties of an object are determined by the shape of its atoms. So,
for example, sweet things are made of smooth atoms, bitter things are made of sharp atoms.
In this manner permanence and flux are reconciled and the field of atomics was born. Although Democritus' ideas were to
solve a philosophical dilemma, the fact that there is some underlying, elemental substance to the Universe is a primary driver
in modern physics, the search for the ultimate smallest particle.
Aristotle

Using math as a guide, Aristotle believed that all things could be
infinitely divided. All substances were combinations of elements
and elemental qualities. The elements are: fire, water, earth,
and air. (Aristotle added later another "element" - Ether which
was a perfect substance an what the heavenly bodies are
composed of) The qualities are: hot, cold, wet, dry. The qualities
define the character of "elements". Fire was seen as ideal
mixture of hotness & dryness.
John Dalton

In the early 1800's Dalton determined that each chemical element is composed of a unique
type of atom, and that the atoms differed by their masses. He devised a system of chemical
symbols and, having ascertained the relative weights of atoms, arranged them into a table. In
addition, he formed the theory that a combination of different elements occurs in whole
number ratios by weight.

He then determined that compounds are made of molecules, and that molecules are
composed of atoms in definite proportions. Thus, atoms determine the composition of matter,
and compounds can be broken down into their individual elements.
J.J. Thomson

In 1897 the physicist Joseph John (J.J.) Thomson (1856–1940) discovered the electron in a series
of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray
tube—an area being investigated by numerous scientists at the time (and still used in our
televisions). Thomson interpreted the deflection of the rays by electrically charged plates and
magnets as evidence of "bodies much smaller than atoms" that he calculated as having a very
large electrical value with a very small mass. Later he estimated the value of the charge itself. In
1904 he suggested a model of the atom as a sphere of positive matter in which electrons are
positioned by electrostatic forces later called the 'Plum Pudding Model.
Because Thompson could easily get negatively charged particles off of the atom
(electrons) but could not easily get positive charges off of it, his model has the
atom still as a 'solid' piece.  The 'pudding' is made of a positively charged
substance with the smaller negatively charged particles embedded into it. Plum
pudding was a common English dessert somewhat similar to our raisin bread.
Ernest Rutherford

Even though the plum pudding model of the atom was the current model, Rutherford believed that the Bohr model would
be more accurate.(Bohr was his top assistant at the time) The Gold foil experiment was an experiment done by Ernest
Rutherford to determine the layout of the atom.
He bombarding gold foil with alpha particles, and observed the scattering of these particles, a procedure requiring many
hours in a darkened room watching for tiny flashes of light as the scattered particles struck a screen that would give out
a flash when hit by alpha particles.
Max Plank

In 1900, Max Plank proposed a most intriguing theory to explain how energy is absorbed and
released by matter. The thinking of the day was that energy was exchanged from matter and its
environment on a flowing continual basis. It does appear that energy flows continually from one
place to another. Our experiences in the macroscopic (large enough to see) world tell us that
energy is exchanged continually. However, intuitively, Plank proposed that on the
sub-microscopic level of the atom energy is absorbed or released in packets or chunks of
energy. Plank called these packages of energy "quanta". In order for an atom to absorb a packet
of energy, it must absorb the whole packet or none at all.
Neils Bohr

Ernest Rutherford's model of the atom pictured negatively
charged electrons moving in circular orbits about a positively
charged nucleus. The electrons did not have anything to do with
energy in this model. Niels Bohr provided the explanation by
incorporating Max Planck's quantum theory into Rutherford's
atomic model. He envisioned specific energy levels (i.e., shells)
for the electrons within which they could move yet not emit
energy. Only if the electrons dropped to a lower energy level, or
were raised to a higher level, would they emit or absorb energy.
The energy of the emitted or absorbed radiation must equal the difference between the
original and final energy levels of the electrons explained why atoms only absorb certain
'packets' or amounts of wavelength's of light.  

To put things in more understandable terms, this is why something that is 'red' absorbs some
colors of light but the red wavelength bounces off.  So, when the red light hits our eyes we
see it as red.
From this point on the work of many scientists, in many areas of physics, developed ideas that combined to make our current
model of the atom.
Erwin Shroedinger (1926)
describes the  form of the
describes the  form of the
probability waves that
govern the motion of small
particles and how these
waves are altered by
external influences.external
influences.
Chadwick
(1931)
England - discovered the
neutrally-charged neutron.
Geiger (1925)
alpha particles
and other
radiations.
Glen T. Seaborg
(1951)
Added elements number
94 - 102, and 106.94 -
102, and 106.
Otto Hahn & Lise Meitner  
(1938), Germany  -
discovered nuclear fission, in
which the nucleus of an atom
breaks up into two separate
nuclei, while experimenting
with uranium.
Lise, being Jewish received
no credit for her discovery
until many years later.
In this diagram of the electron
cloud you can see the 'clouded'
areas in the shell of the atom.
Notice that the clouds in the first
orbit effect the placement of the
clouds in the second, that will effect
the cloud in the third, and so on.  
Keep in mind though that these
cloud areas are moving.
The work of these scientists
lead to the development of the
electron cloud model of the
atom.  In this model the 'shell'
that the electron move in is
constantly 'vibrating' and so we
cannot really know where the
electron is
.
Our final 'picture' of the
atom looks like this.  The
electrons are found in
areas around the
nucleus.  Their exact
position can not be
know.  This is part of what
we call the
uncertainty
principle
which is more
than can be covered in
this class.
Rutherford observed that most of the
particles passed through the foil
without any deflection; under the plum
pudding model, charge would hit the
much larger atoms within the foil, and
very few particles would avoid
deflection. This result implied that, on
the contrary, most of the apparently
solid foil was, in fact, empty space.
Even more surprising, a tiny number
of the particles underwent larger
deflections in their path: some were
even bounced back from the foil.
Under the plum-pudding model, thus
would be like a sheet of paper to
bouncing back bullets.
History of the Atom
Beyond the Book