History of the Atom
The idea of the atom has been around since 400 B.C. when
Democritus, a Greek philosopher and mathematician, first
suggested the idea of atoms. Democritus said that atoms were indivisible,
indestructible, fundamental units of matter. The word atom is derived from the
Greek word atomos, which means indivisible. Democritus thought and talked
about atoms, but scientific experiments were unknown in his world. Therefore,
Democritus did no experiments to test his theories. The connection between
chemical changes and individual atoms was not to be established for 2200
years.
An English school teacher, John Dalton, studied chemistry very
differently than Democritus, who only philosophized about
atoms. Dalton performed experiments to arrive at his atomic
theory. He was the first person to explain chemical reactions in
terms of atoms. Dalton devised his theory around 1800 and
named it Dalton's Atomic Theory. While some exceptions to
Dalton's atomic theory were eventually discovered, the theory itself has never
been discarded, only modified and expanded as the world of the atom was
explored.
Dalton's Atomic Theory
1. All matter is composed of indivisible and indestructible atoms.
2. Atoms of a given element are identical, but different than atoms of other
elements.
3. Atoms combine in simple, whole-number ratios to form compounds.
4. Chemical reactions occur when atoms combine, are separated, or
rearranged.
With Dalton's Atomic Theory in place, the first model of the atom
was developed. It was not very sophisticated, but it was a start on
the road to identifying the structure of the atom. Dalton's Model of
the atom was a solid, indivisible, indestructible sphere. It would be almost 100
years before scientists were able to expand on Dalton's Model.
Experiments by several scientists in the middle of the
19th century led to a major alteration to Dalton’s
Theory and his model of the atom. The atom was
found not to be indivisible after all. The first
evidence that the atom consists of smaller particles
was obtained by early researchers in the field of
electricity. They studied the flow of electric current in a device called a
Crookes Tube. This was a glass tube containing a small amount of gas and had
metal plates at each end called electrodes. When a voltage source was
connected to the electrodes, a glowing beam of particles formed, which were
called cathode rays.
In 1897 Joseph J. Thomson, an English physicist, discovered
that electrically charged plates and magnets deflected the
straight paths of the cathode rays. The direction of deflection
showed that the particles in the cathode rays must be negatively
charged. Thomson named the small, negatively charged
particles in the cathode ray electrons. This was the first step in
learning about the structure of the atom. Later experiments
determined the mass of the electron and found it to be over
2000 times less than the mass of the lightest atom. The lightness of electrons
led to the belief that there must be other particles in the atom to account for the
rest of the mass of the atom. Scientists also knew that atoms were electrically
neutral, so there must be a positive charged particle to offset the negative
charge of the electron. That positively charged particle turned out to be the
proton.
With Thomson's discovery of the electron,
the model of the atom had to be revised.
Thomson had the idea that the atom was a
ball of positive charge with negatively
charged electrons embedded inside. His
model was named the "plum pudding
model" becasue it resembled plum pudding,
a British dessert that consists of a ball of
sweet bread with pieces of fruit embedded
in it.
A physicist by the name of Ernest Rutherford
performed an experiment that would give the next clue
as to how the atom was structured. Rutherford
directed a stream of positively charged particles (alpha
particles) at a very thin sheet of gold foil. Rutherford
believed that by observing the alpha particles as they
passed through the gold foil he would get a better idea
of the atom's structure. Rutherford thought that the alpha particles would pass
straight through the gold foil, but the results were very different than what he
expected.
Most of the alpha particles passed straight through the gold foil
like Rutherford predicted, but some were deflected and a small
number actually bounced back. From these observations
Rutherford was able to conclude that atoms consisted of very
small, dense regions of positive charge, which he called the
nucleus. This would explain why a small number of the alpha
particles bounced back. Alpha particles were also deflected when they came
close to a nucleus since like charges repel each other. This would mean that the
model of the atom had to be modified.
Rutherford’s Model



Discovered dense positive piece at the center of the atom
Nucleus
Electrons moved around

Mostly empty space
Although Rutherford's model was the best representation of the atom at the
time, it had some shortcomings. His model had the electrons traveling around
the nucleus, but since opposite charges attract, what was keeping them from
collapsing in on the nucleus? Also, electric charges moving in curved paths
should give off light. If electrons would give off light they would lose energy
and collapse into the nucleus.
In 1913, the Danish physicist Niels Bohr proposed improvements
to Rutherford's model. The key idea in Bohr's model is there are
certain definite orbits in which an electron can travel around a
nucleus without radiating energy. Each one of these orbits is a
fixed distance from the nucleus and have a definite amount of
energy. The farther away from the nucleus, the greater the energy of the
electron in that orbit.
Bohr stated that electrons in different orbits had different
amounts of energy and he called the different orbits,
energy levels. When an electron loses energy it must
fall from one energy level to another. When electrons do
not give off energy they are stable and stay in their orbit
or energy level. When electrons are stable, they are said
to be in their ground state. When they absorb energy and
move up to a different energy level they are said to be in the excited state.
During the 1930's and 1940's experiments
provided more information about the stucture
of the atom and Bohr's model had to be
updated. These changes produced the
Quantum Mechanical Model. This new model does not show the paths of
electrons, only the most probable locations of finding them. This model shows
the electrons as a diffuse cloud of negative charge. Wherever the cloud is the
most dense, there is a greater chance of finding an electron.
The picture to the right
shows what the Quantum
Mechanical Model would
look like. Sometimes it
is also called the Charge
Cloud Model, since the
electrons make up a
cloud of negative charge. Bohr's idea of energy levels is still true in the charge
cloud model, but when electrons move from one energy level to another, the
shape of the cloud changes.
You have just made the journey from the first idea of atoms up to the modern
model of the atom. The journey has taken over 2000 years and is not at its end
yet. Scientists are still learning more about atoms all the time. The modern
model of the atom is likely to change as we continue to learn more. Now that
you have learned a little of the history behind the atom, take a look at the
pictures of atoms in the STM Gallery. This web site will explain how they are
able to get colorized pictures of atoms and then has several thumb nails that
you may enlarge.