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Atomic Theory


     The Greek concept of atomos: the atom Around 440 BC leucippus of Miletus
originated the atom concept. He and his pupil, Democritus of abdera refined it
for future use. Their atomic idea has five major points. All original writings
of leucippus and Democritus are lost. The only sources we have for there
atomistic ideas are inquotations from other writers. Democritus was known as the
"laughing philosopher" because he enjoyed life so much. At this time

Greek philosophy was about 150 years old, emerging in the sixth century bc,
centered in the city of miletus on the ionian coast in Asia minor, which is now
turkey. The work of leucippus and Democritus was further developed by epicures
(341-270 BC) of Samos. He made ideas more generally known. Aristotle also quotes
both of them in arguing against their ideas. Most of what we know about
leucippus and Democritus was found in a poem entitled "de rerum natura"
(on the nature of things) written by Lucretius (95-55 BC). This poem was lost
for over a thousand years and was discovered in 1417. These are the basic points
of their theory. #1 - all matter is composed of atoms, which are bits of matter
to small to be seen. These cannot be split any smaller. " The atomists hold
that splitting stops when it reaches indivisible particles and goes on no
more" Which means there is a limit to division of matter that we cannot go.

Atoms are very hard so they cannot be divided. In Greek "a" means not
and "tomos" means cut. So our word comes from atomos, meaning
uncuttable. He reasoned that if matter could be infinitely divided, it could
also completely disintegrate and cannot be put back together, however matter can
regenerate. Even though matter can be destroyed by splitting, new things can be
made by joining other matter together. This process is reversible. The idea of
reversibility means there must be a limit to splitting. If it could be split
forever, there is nothing to stop it from destroying itself. Epicures insisted
on an upper limit also, that atoms are always invisible, it seems obvious; all
matter that can be seen is still divisible, so they can't be atoms. #2- there is
an empty space between atoms. " Unless there is a void with a separate
being of its own 'what is' cannot be moved-nor again can it be 'many' since
there is nothing to keep things apart." So there is an empty space between
atoms, or a vacuum. Given that all matter is composed of atoms, then all changes
must be a result of movement of atoms. So the movement within the atoms is
allowed by a space so atoms can move from place to place. #3- atoms are
completely solid. If there is a space outside there cannot be a space inside,
which would cause to disintegrate. But we knowthis is wrong, in 1919 Ernest

Rutherford discovered the nucleus, demonstrating that there is an empty space.
#4- atoms are homogeneous (no internal structure) The solidarity of atoms means
that atoms are the same all over, or has no internal structure. There was
speculation about sub-atomic structure in the 1800's introduced it on solid
scientific basis, not until 1897, J.J. Thomson's discovery of the electron that
it had internal structure. #5- atoms are different in... 1- there size. 2- their
shapes. " Democritus and leuccippus say thatthere are indivisible bodies,
infinite number and shape" Aristotle " They have all sorts of shapes
and appearances and sizes" Democritus Aristotle and others opposed almost
all of the ideas of the atom, so most of the information was lost. There is a
pattern of atomic thought but only a few scholars gave it real thought. It
wasn't until 1803 that john Dalton (1766-1844) a schoolteacher put the atom on a
solid scientific base. Dalton's gift for analyzing data allowed him to recognize
the connection between atomic weight and weight relations in chemical gases. He
was the first to put the idea of atoms and stoichiometry together. Dalton's
atomic laws are in the following points. #1- all matter consists of tiny
particles called atoms. The existence of atoms first came up 2000 years ago.

Though they remained pure speculation for most of this time. #2- atoms are
indestructible and unchangeable. Atoms of an element cannot be created,
destroyed, broken onto smaller parts or be changed into another element. Dalton
based this on the law of conservation of mass and experimental evidence. With
the discovery of subatomic particles after Dalton. Atoms could be broken into
smaller pieces. It was also discovered that atoms could be changed into
different elements. Most don't consider this a chemical process because the
nucleus is unaffected. #3- elements are characterized by the mass of there
atoms. With the discovery of isotopes, it was found that elements are
characterized by their atomic number, or the number of protons. #4- when
elements react, their atoms combine in simple whole number ratios. This suggests
a practical strategy for determining the atomic weights of atoms. Atomic weights
could then be used to explain the fixed mass percentages of elements in all
compounds. This explained the law of definite proportion and multiple
proportions. Some of daltons original atomic theories were wrong but the basic
concepts (like chemical reactions can be explained by the union and separation
of atoms and these characteristic properties.) these ideas are still the basics
of modern physical science. While these beliefs were the mainstay of atomic
theory. The belief that they were strutureless and indestructible was demolished
by J.J. Thomson's (1856-1940) discovery of the electron in 1897. It was soon
realized that the mass of an atom is from the positively charged electron.

Thomson used cathode rays and passed them through a glass bulb. He changed the
direction of the rays with an electric field and concluded they were of negative
charge. He knew he could move them with a magnetic field. Sending each beam in
opposite directions, he concluded that the particles (or corpuscles as he called
them) were a hundred times smaller. He found the first sub-atomic particles.

Through Thomson's discovery of the electron, Robert milikan (1868-1953)
discovered how to determine the charge of an electron. He sprayed tiny drops
onto the space above two metal plates. A drop of oil would fall through now and
then through a tiny hole in the plate. The rate of fall was determined by
observation through a telescope. Then the tope plate was connected to a
positively charged battery, and the bottom was connected to a negatively charged
battery. A beam of x-rays was passed between the plates. The rate of fall of the
oil would change in sudden jumps. He determined it was because of a gain or loss
of electrons. He created an equation for this fall. Electric force = E * e * n

Then in 1909 a man named Ernest rutherford (1871-1937) used alpha particles to
bombard a piece of thin gold foil. He noticed almost all went through the gold
but 1/8000 would bounce back in a wild direction. He quoted " as if you
fired a 15 inch naval shell at piece of tissue and the shell came right back and
hit you " from this he concluded that there was a highly concentrated, very
small positively charged nucleus, while electrons inhabit the furthest from the
atom. His experiment was similar to this. He also discovered that the number of
protons makes up the atomic number. And that the number of protons and neutrons
makes up the atomic mass. Some years later Neil's Bohr (1885-1962) came up with
his model of the atom. Bohr's model described how electrons performed within an
atom. In his model he stated that electrons that raced around the nucleus would
make standing wavelengths. He imagined the orbit as a pilot wave, continuous.
"It was like a vibrating string" He also came up with the following
terms. Energy level- the specific number of energy that an electron can have.

Principal quantum number- the integer used to identify each energy level. Ground
state- the lowest level of energy Excited state- when the electrical causes it
to move to a higher energy level. Though the information and discoveries about
atomic theory has changed over the years, the atom plays a very important role
in our existence.

Bibliography

On internet, Atomic Theory 1996. http://www.antoin.fsu.umd.edu Dalton's

Atomic Theory http://dbhs.wvusd.k12.ca.us The Greek Concept Of Atomos John L.

Park 1996 Parry/ Dietz/ Tellefsen/ Steiner Chemistry experimental foundations
pretence hall 1983 G. Raymer-Cantham, Chemistry addison-wesley publishers