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