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ATOMS
(A short history of the knowledge of the atom)
Compiled by Jim Walker
Originated: Sept. 1988
Latest revision: 2016
atom n. A unit of matter, the smallest unit of
an element.
The history of the atomic theory 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
thinking about matter that can be used to
make useful predictions.
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
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 meaning “indivisible”.
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.
We call this the “billiard ball model” of the
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
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 in an
English “Plum Pudding”.
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.
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.
Ernest Rutherford
Radioactivity provided a tool to explore
matter, alpha particles. 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
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
surrounded the small, dense, positive nucleus.
The electron can exist in only one of the orbits
at any one time. We call his model the
planetary model because the electrons had
certain “orbits”.
Bohr’s model explained the line spectrum
observed when electricity is passed through a
sample of hydrogen gas.
The Hydrogen Spectrum
Rutherford's atom
The Nuclear Model
Erwin Schrodinger
Niels Bohr
In 1912 a Danish physicist, Niels Bohr came
up with a theory that said the electrons have
certain allowed energy levels. 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:
RULE 1: Electrons can orbit only at certain
allowed distances from the nucleus.
RULE 2: Atoms radiate energy when an
electron jumps from a higher-energy orbit to a
lower-energy orbit.
A powerful model of the atom was developed
by Erwin Schrodinger in 1926. Schrodinger
generated a mathematical model for the
distribution of electrons in an atom known as
wave functions or “orbitals”. The electron is
a wave. It is difficult to imagine a physical
model of electrons as waves, but we have
accepted it as the “electron cloud” model of
the atom. This is quantum mechanics.
The Schrodinger model describes the regions
in space where electrons are most likely to be
found. Instead of trying to tell us where the
electron is at any time, the electron cloud only
tells us where it might be.