History of the Atom
How the work of one scientist built
upon the work of others to bring us to
greater and greater understanding of
the atom!
The Greeks
2400 years ago:
Democritus named the smallest
piece of matter the ATOM.
Democritus stated that matter was
made of atoms. The difference
between atoms was size, shape and
weight. Differences in these physical
properties explained the differences
in matter.
Unfortunately for Democritus, and
mankind in general, his ideas were
largely ignored for the next 2000
years.
Dalton’s
Theory
1803-John Dalton
Different atoms mean
different elements.
Summary of Dalton’s Theory:
1. Matter is composed of small
particles called atoms.
2. All atoms of an element are
identical, but are different from
those of any other element.
3. During chemical reactions, atoms
are neither created nor destroyed,
but are simply rearranged.
4. Atoms always combine in whole
number multiples of each other. For
example, 1:1, 1:2, 2:3 or 1:3.
Radioactivity
1896-early 1900s
Radioactive elements
show that atoms are made
of even smaller particles
In 1896, Henri Becquerel discovered
that uranium exposed a photographic
plate.
Pierre and Marie Curie went on to
discover lots of other elements that
disintegrated in the same way.
Marie Curie called the disintegrating
elements radioactive.
Atoms must be made of even smaller
parts if pieces were coming off of
them.
Positive and
Negative Charges
1898 Thomson’s Model
Atoms have positively and
negatively charged
particles.
Using cathode ray tubes,
Thomson finds that atoms all
have a negative part. Since
atoms are neutral, he
theorizes that all atoms must
also have a positive part.
The Discovery of
the Nucleus
1911 Rutherford’s Model
Atoms have a nucleus
By passing particles through a
thin piece of gold foil,
Rutherford finds that most
particles pass right through,
but some bounce back
because they have hit the
nucleus of a gold atom.
The Bohr Model
1913 Niels Bohr
Electrons orbit the nucleus
Bohr added the orbit of the
electrons around the nucleus
to our understanding of the
atom. Although it later
proved to be too simplistic, it
is far easier to draw than the
more realistic version of
electrons.
The Electron Cloud
Model
Today-Schrodinger
Electrons orbits are more
like clouds than perfect
rings.
Erwin Schrödinger took Bohr’s work a
little farther. We can identify a cloudlike area around the nucleus where
electrons are likely to be found. We
can’t say for sure where the electron
actually is at any point in time, but we
can describe where it ought to be.
This is the electron cloud model.