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