Development of the Atomic Theory

Definition of an Atom

• An atom is the smallest particle into which an element can be divided and still be the same substance.

Democritus Proposes the Atom

Can you keep cutting something in half forever?

Aristotle disagreesDemocritus’s ideas were largely ignored for centuries

John DaltonCreated an Atomic theory based on experiments.

Dalton’s Atomic Theory• Atoms are small particles that cannot be

created, divided, or destroyed• Atoms of the same element are exactly

alike.• Atoms join with other atoms to make new

substances.

J. J. Thomson

• Discovered that atoms can be divided into smaller particles

• Discovered one such particles called the electron.

Cathode-Ray tube Experiment

1. When particles were passed through a magnet they were attracted to the positive end, causing the beam of particles to bend downward.

2. Thompson inferred that there must two different sections of the atom, one positively charged and the other negative.

3. He called the negative portion of the atom the electron.

The Thompson Model1. Electrons are

embedded in a positively charged sphere, much like chocolate chips in cookie dough.

Ernest Rutherford• Discovered the Nucleus of the Atom• At the center of an atom is a small dense

positively charged nucleus.• The atom is mostly empty space through

which electrons travel

Gold Foil Experiment

The Gold Foil Experiment

1. Shot a beam of particles at a thin piece of gold foil (so thin it was only 1 atom thick).

2. Behind the foil was a piece of photographic film

3. Most of the particles passed right through the foil. But, occasionally one would be reflected right back at him, because it hit the hard dense nucleus.

Neils Bohr• Suggested that electrons travel around the

nucleus in definite paths• States that electrons can jump between

energy levels

Bohr Model of the Atom

Modern Theory• Electrons do not travel in definite paths• Instead they reside in electron clouds• Electron clouds are regions around the

atom in which the electrons likely to be found

Modern Theory

http://www.youtube.com/watch?v=t7YBmOk1nQw