1. The Development of Modern Atomic Theory

2. In the beginning…
Democritus- In the 4th century BC, he proposed that the universe was made of tiny indivisible units.
He called these atoms, from the Greek word atomos, meaning unable to be divided or cut.
He did not have any evidence to support his theory, but nonetheless, some people believed.

3. Dalton’s Atomic Theory
In 1808, he proposed a revised atomic theory.
According to Dalton, all atoms of a given element were exactly alike, and atoms of different elements could join to form compounds.
Theory was based on experimental evidence.
However, some parts of his work turned out to be incorrect.

4. Thomson’s Model of the Atom
1897- Experiments with electricity led to the discovery that atoms were NOT indivisible!
He conducted a famous cathode ray tube experiment which suggested that these rays were made of negatively charged particles that came from inside of atoms.

5. Results from the Cathode Ray Tube Experiment
Thomson knew that a magnet deflected charges, and he reasoned that because the tube was a vacuum the beam must have come from the cathode (positively charged plate) or anode (negatively charged plate).
The direction of deflection indicated a negatively charged beam, and electrons were discovered!!!!

6. Thomson’s Plum Pudding Model
Based on the outcome of his experiment, he proposed a new model of the atom.
He reasoned that the electrons were spread throughout the atom, just like in plum pudding, a dessert in his time.
We might use the chocolate chip cookie model to better visualize it today.

7. Rutherford’s Model of the Atom
Ernest Rutherford proposed his model shortly after Thomson.
He found that most of the mass of an atom was concentrated at the atom’s center, or the nucleus.
To understand this conclusion, you must learn about his famous gold foil experiment.

8. Gold Foil Experiment
A beam of positively charged alpha particles were aimed at a thin sheet of gold foil.
Because of Thomson’s model, Rutherford predicted that there would not be a large enough mass of positive charge to cause many particles to bounce back. He thought they would travel in a straight path through the foil.

9. Gold Foil Experiment
But, the observations did not match his hypothesis. Most passed straight through, but a few were deflected by a large amount.
A few even bounced back!
He said, “It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15- inch shell at a piece of tissue paper and it came back and hit you.”

10. Gold Foil Experiment

11. Results of Rutherford’s experiment
An atom’s positive charge in concentrated in the center of the atom, called the nucleus.
He is also credited with the discovery of the proton.
Here is his atomic model: