1. 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!

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. 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 cloud-like 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.