1. Evolution of the Atomic Model
The history of the atom

2. The evolution of the atomic model

3. Early thoughts about atoms:
In the 5th century B.C., Democritus held that matter was not infinitely divisible, and that if divided, it would become smaller and smaller etc.
He called the smallest particle the atom.
Later Aristotle held that matter was infinitely divisible and that there was no need for atoms. He introduced the idea of elements.
Aristotle recognized four elements: fire, air, earth, and water; he said that you can go from one material to another by varying pro portions of four elements.
Following this, the idea of the atom was dropped for 2000 years.

4. Dalton: the first modern atom
John Dalton tried to explain observations by a new atomic theory. He said:
All matter is composed of indestructible and indivisibile particles called atoms
(Law of Multiple Proportions)
Atoms are neither created nor destroyed in any chemical reaction
(Law of Conservation of Mass)
All atoms of a given element are identical, but they differ from those of any other element
A given compound always has the same relative numbers and types of atoms
(Law of Definite Composition)

5. Law of Definite Composition
Atoms combine in unique ways to form unique compounds
Atoms combine in fixed ratios by mass
%H in water = 11.1%
%O in water = 88.9%
Percent composition

6. Law of Multiple Proportions
When two elements combine to form more than one compound
If the mass of one of the elements is held constant, then the ratio, by mass, of the other element in each compound is a whole number

7. Law of Multiple Proportions
Carbon and oxygen combine to form more than one compound
1 g C x = 1.33 g O  1
1 g C x = 2.66 g O  2

8. Law of Conservation of Mass
2H2O  2H O2
intramolecular bond
Chemical change

9. Law of Conservation of Mass
Intermolecular forces between water molecules
Physical Change
Intermolecular force

10. Water Cycle

11. Nitrogen Cycle

12. Carbon Cycle

13. Crooke’s tube phenonmenon
The beam in the discharge tube bends toward the positive end of a magnet.

14. Thomson and the atom
Thomson demonstrates that the atom contains negatively charged particles

15. Thomson’s Photoelectric effect.
When light is shown on any metal, the metal atom emits negatively charged particles.

16. Thomson called these negatively charged particles electrons
The beam is made of particles.
The particles are negative in charge.
The particles all have the same mass.
Since all metals exhibited the photoelectric effect, Thomson concluded that the electron is a fundamental particle of the atom.

17. Thomson proposes the first modern atomic model.
Thomson proposes a model with electrons embedded in a positive sphere. This was called the “grapes in jello” or “plum pudding” model.

18. The discovery of radioactivity
Becquerel, Rutherford and the Curies discover and categorize radioactivity.

19. Implications of the experiment.
One type of radiation is a positive particle. α
One type of radiation is a negative particle. β
One type of radiation is either neutral or not a particle. γ
The overall implication: atoms have parts

20. Questions remained.
What was the exact nature of the positive piece of the atom?
What was the exact location of the positive area?
What was the exact location of the electron?

21. The Rutherford Gold Foil Experiment

22. An atomic view of the Gold Foil Experiment
Observe what is occurring at the atomic level.
What is happening to most alpha particles?
7999/8000 alpha particles pass through the foil
What is happening to a few alpha particles?
1/8000 alpha particles are deflected by the foil

23. How did the results of the Rutherford Gold Foil Experiment impact the model of the atom?
Since most of the alpha particles passed through the gold foil, the atom must be mostly empty space
Since only a very small amount of alpha particles bounced back, there must be a small, dense and positive mass in the center of the atom which is called the nucleus.
The atom is a nuclear atom.

24. A picture of the Rutherford atom