FREE ESSAY ON ATOMIC THEORY

ATOMIC THEORY

In ancient Greek the word atom meant the smallest indivisible particle that could be
conceived. The atom was thought of as indestructible; in fact, the Greek word for atom
means not divisible. Knowledge about the size and make up of the atom grew very slowly as
scientific theory progressed. What we know/theorize about the atom now began with a core
theory devised by Democrotus, a Greek philosopher who proposed that matter consisted of
various types of tiny discrete particles and that the properties of matter were
determined by the properties of these particles. This core theory was then modified and
altered over years by Dalton, Thompson, Rutherford, Bhor, and Chadwick. The atoms
original structure was simple, but as more and more research was done the atom became
more complex and puzzling. 
Our first benefactor of atomic theory was John Dalton, a man later nick-named the Father
of atomic theory for his contribution of many theories and laws to modern atomic theory.
His theories answered many questions of skeptical scientists: elements combine with one
another to form chemical compounds and the atom doesn't change, atoms link together in
definite proportions, all atoms of any element are all the same, the law of multiple
proportions which states that a given mass of one element can combine with various masses
of another element (or elements) but always in small whole number ratios, and the law of
conservation of matter, matter can neither be created or destroyed, but it can change
form. 
One of the next most recognized people involved in the progression of atomic theory was
J.J. Thompson. Thompson had researched the work of William Crookes whose research
concluded that cathode rays were deflected by magnetic fields. Thompson elaborated on
this conclusion and found that cathode rays were also deflected by an electric field.
With much experimentation Thompson theorized that although the atom was made up of small
particles it was not the same indestructible model proposed by Dalton. A man named
Milikin determined the mass of an electron to be 0 amu and the relative charge to be
negative 1. With the discovery of these electrons by Thompson and the discovery of
protons several years later Thompson was able to devise a new model of the atom. Thompson
stated that protons and electrons were evenly distributed throughout the atom. This model
was labeled the plum-pudding model. Thompson described the atom as a gooey mass of
positively charged particles (protons) with raisins (electrons) embedded all around it.
Ernest Rutherford was a major contributor to the atomic model because he proved that
although Dalton and Thompson were partially correct there was still a flaw in both of
their models of the atom. Rutherford devised an experiment to either prove or disprove
Dalton's model and or Thompson's model. Rutherford and his assistants put a piece of
radioactive material in a lead box with a small hole in one side to direct the alpha
particles towards the gold foil, which was surrounded by zinc sulfide screens. The zinc
screens showed flashes of light where the alpha particles were going to go, straight
through (Thompson's model) or straight back (Dalton's model). To his astonishment
particles were through, came back, and some also were deflected to the sides of the gold
foil. He proved that Dalton and Thompson were incorrect and the atom consisted mostly of
space. At the center of this space is a very small core, called the nucleus, which can
justify the deflected particles. Rutherford established that the mass of the atom is
concentrated in its nucleus. He found that an electron is 1/1836 the mass of a proton and
he also proposed that electrons travel in orbits around the nucleus.
With all of these alterations to the theory of an atom a few, five to be exact, problems
arose. One of the major problems was the size of an atom. If each electron had its own
orbital and the atom had 23 electrons then the atom would be enormous. Another problem
with the orbital of an electron was that no energy could be observed by the electron
orbit decay. Next, if the center of an atom was composed of protons (+) and the electrons
(-) orbited this positive core why didn't the electrons crash into the protons, causing
an ultra violet catastrophe. Also, if the core was composed of just positive protons and
opposite charges repel then how did the protons stay together. And the final problem, the
atom didn't weigh enough. When scientists added the weight of the electrons and the
weight of the protons and subtracted that from the overall weight of the atom there was a
remainder. Something had to be missing from the model of an atom to make up for the
weight difference. The answers to these questions came from the next big contributors to
this theory, Niles Bohr and Chadwick.
Niles Bohr developed a theory known as the Bohr theory of the atom. He assumed that
electrons are arranged in definite energy levels, or quantum levels, at a specific
distance from the nucleus. The arrangement of these electrons is called the electron
configuration. There are seven levels, which were derived from the seven colors he saw,
each of which has a specific number of electrons that it has capacity for. The first
level can only accommodate two electrons, the second can hold up to eight electrons, the
third can hold up to eight-teen, and so on. If an atom had four electrons you wouldn't
find two in the first, one in the second, and one in the sixth. Electrons always occupy
the lowest energy levels first. Electrons in a ground state are in their regular energy
level and give off no energy; however, if an electron is in an excited state it sends
energy in quantum packets (photons) and light is observes. When excited electrons jump up
a level they give off light energy: however, they can never go down a level, energy can
never be lost only gained. 
Chadwick discovered the last piece of information about the atom. In 1932, through mass
spectronomy, he discovered a heavy neutral particle with the mass of a proton. He called
it a neutron. This answered the few remaining question about the atom. There was no
longer a missing part because the neutron made up for the original weight loss. And it
answered the question about how the protons stayed together. The neutrons had a soothing
effect on the protons. Between every two protons lies a neutron, which attracted the
protons, which made the nucleus very condensed.
With the last discovery, of the neutron, scientists were able to complete the model of
the atom. The completed model that they came up with is the model that students now learn
about in school. These scientists did exactly what scientists are supposed to do: test,
experiment, and answer questions. Because of the years of study they did we now have a
strong idea of what an atom is and what its components are. A theory never becomes fact
until all of the bugs are wiped out, if this is true then this atomic theory is well on
its way to becoming the facts about atoms.
Bibliography
Sources: 
Textbook: Chemistry: The Study Of Matter, Simon and Schuster, copyright 1992
Internet Source: http://encarta.msn.com/find/Concise.asp?ti=03E68000