Download Democritus

ATOM S
(A short history of the knowledge of the atom)
Compiled by Jim Walker
Originated: Sept. 1988
Latest revision: Nov. 2004
atom n. A unit of matter, the smallest unit of
an element, consisting of a dense, central,
positively charged nucleus surrounded by a
system of electrons, equal in number to the
number of nuclear protons, the entire structure
having an approximate diameter of 10-8
centimeter and characteristically remaining
undivided in chemical reactions except for
limited removal, transfer, or exchange of
certain electrons.
The history of the study of the atomic nature
of matter illustrates the thinking process that
goes on in the philosophers and scientists¶
heads. The models they use do not provide an
absolute understanding of the atom but only a
way of abstracting so that they can make
useful predictions about them. The
epistemological methods that scientists use
provide us with the best known way of
arriving at useful science and factual
knowledge. No other method has yet proven
as successful.
I n the beginning
Actually, the thought about electricity came
before atoms. In about 600 B.C. Thales of
Miletus discovered that a piece of amber, after
rubbing it with fur, attracts bits of hair and
feathers and other light objects. He suggested
that this mysterious force came from the
amber. Thales, however, did not connect this
force with any atomic particle.
Democritus
Not until around 460 B.C., did a Greek
philosopher, Democritus, develop the idea of
atoms. He asked this question: If you break a
piece of matter in half, and then break it in
half again, how many breaks will you have to
make before you can break it no further?
Democritus thought that it ended at some
point, a smallest possible bit of matter. He
called these basic matter particles, atoms,
from the Greek atomos PHDQLQJ³LQGLYLVLEOH´
Unfortunately, the atomic ideas of Democritus
had no lasting effects on other Greek
philosophers, including Aristotle. In fact,
Aristotle dismissed the atomic idea as
worthless. People considered Aristotle's
opinions very important and if Aristotle
thought the atomic idea had no merit, then
most other people thought the same also.
For more than 2000 years nobody did
anything to continue the explorations that the
Greeks had started into the nature of matter.
Not until the early 1800's did people begin
again to question the structure of matter.
John Dalton
He got his ideas for electrons by studying the
beam in a cathode ray tube. The beam could
be bent by a magnet or an electric charge
which showed that the beam had a negative
charge.
In the 1800's an English chemist, John Dalton
performed experiments with various
chemicals that showed that matter, indeed,
seemed to consist of elementary lumpy
particles (atoms). Although he did not know
about their structure, he knew that the
evidence pointed to something fundamental.
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atom.
Dalton was able to figure out that compounds
could be made up of the same atoms in
different proportions, for example, H2O and
H2O2.
J. J. Thomson
His experiments changed the idea that the
atom was a hard sphere into the idea that it
had positive and negative parts. The negative
parts (the electrons) were the same in all
atoms, but the positive part was different from
atom to atom.
In 1897, the English physicist J.J. Thomson
discovered the electron and proposed a model
for the structure of the atom. Thomson knew
that electrons had a negative charge and
thought that matter must have a positive
charge. His model looked like raisins stuck on
the surface of a lump of pudding.
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atom.
Ernest Rutherford
they must get scattered by tiny bits of
positively charged matter. Most of the space
around these positive centers had nothing in
them. He thought that the electrons must exist
somewhere within this empty space.
Rutherford thought that the negative electrons
orbited a positive center like the planets orbit
the Sun in the solar system.
Rutherford's atom
Other particles were discovered around this
time called alpha rays. These particles had a
positive charge and physicists thought that
they consisted of the positive parts of the
Thompson atom (now known as the nucleus
of atoms).
In 1911 Ernest Rutherford thought it would
prove interesting to bombard atoms with these
alpha rays, figuring that this experiment could
investigate the inside of the atom (sort of like
a probe). He used Radium as the source of the
alpha particles and pointed them at the atoms
in gold foil. Behind the foil sat a fluorescent
screen on which he could observe the alpha
particles impact.
The results of the experiments came
unexpectedly. Most of the alpha particles
went smoothly through the foil. Surprisingly,
an occasional alpha bent sharply from its
original path, sometimes bouncing straight
back from the foil! Rutherford reasoned that
Rutherford knew that atoms consist of a
compact positively charged nucleus, around
which circulate negative electrons at a
relatively large distance. The nucleus occupies
less than one thousand million millionth of the
atomic volume, but contains almost all of the
atom's mass. If an atom had the size of the
earth, the nucleus would have the size of a
football stadium.
Not until 1919 did Rutherford finally identify
the particles of the nucleus as discrete positive
charges of matter. Using alpha particles as
bullets, Rutherford knocked hydrogen nuclei
out of atoms of six elements: boron, fluorine,
sodium, aluminum, phosphorus, an nitrogen.
He named them protons, from the Greek for
'first', for they consisted of the first identified
building blocks of the nuclei of all elements.
He found the protons mass at 1,836 times as
great as the mass of the electron.
RULE 1: Electrons can orbit only at certain
allowed distances from the nucleus.
But there appeared something terribly wrong
with Rutherford's model of the atom. The
theory of electricity and magnetism predicted
that opposite charges attract each other and
the electrons should gradually lose energy and
spiral inward. Moreover, physicists reasoned
that the atoms should give off a rainbow of
colors as they do so. But no experiment could
verify this rainbow.
Niels Bohr
RULE 2: Atoms radiate energy when an
electron jumps from a higher-energy orbit to a
lower-energy orbit. Also, an atom absorbs
energy when an electron gets boosted from a
low-energy orbit to a high-energy orbit.
Bohr's atom for Hydrogen
The electron can exist in only one of the orbits
at any one time. (The diagram shows only five
orbits, but any number of orbits can
theoretically exist.)
Source: http://www.nobeliefs.com/atom.htm
In 1912 a Danish physicist, Niels Bohr came
up with a theory that said the electrons do not
spiral into the nucleus and came up with some
rules for what does happen. (This began a new
approach to science because for the first time
rules had to fit the observation regardless of
how they conflicted with the theories of the
time.)
Bohr essentially said, "Here are some rules
that seem impossible, but they describe the
way atoms operate, so let's pretend they're
correct and use them." Bohr came up with two
rules that agreed with experiment: