1. History of Atomic Theories
(The Changing Views of Atomic Structure)

2. (Greek Philosopher, 450 B.C.)
Empedocles
Empedocles
(Greek Philosopher, 450 B.C.)

3. Everything that existed was thought to be composed of four elements.
Empedocles
(i) Water
(ii) Fire
(iii) Air
(iv) Earth

4. (Greek Philosopher, 400 B.C.)
Democritus
Democritus
(Greek Philosopher, 400 B.C.)

5. Democritus
All matter is made of tiny hard indestructible particles which are indivisible (i.e. can not be broken down). The Greek word for “indivisible” is atomos, therefore all matter is made up of atoms.

6. (Greek Philosopher, 350 B.C.)
Aristotle
Aristotle
(Greek Philosopher, 350 B.C.)

7. Believed in Empedocles’ “four element” model
Believed in Empedocles’ “four element” model. -Despite the more recent “atomic model” proposed by Democritus, Aristotle was so influential that the “four element” model was accepted for almost 2000 years.
Aristotle

8. Philosophers
Note: The Greek philosophers did not test their ideas with experiments. They were thought of as great “thinkers”, but not scientists.

9. (Philosophers, Mystics, Magicians, Chemists, 500-1600 A.D.)
Alchemists
Alchemists
(Philosophers, Mystics, Magicians, Chemists, A.D.)

10. Alchemists
Performed experiments and devised chemical symbols for substances that we now call elements and compounds. They also invented many lab tools that are still used today…

11. (English Scientist, 1800’s)
Dalton
John Dalton
(English Scientist, 1800’s)

12. Using experiments, he discovered that
Atoms are tiny indestructible particles that cannot be broken down
2. Atoms combine with other atoms to form molecules (e.g. carbon + oxygen  carbon dioxide)
Atoms
Molecule
3. Atoms of an element are identical (e.g. H2 gas are all H atoms)
Dalton
4. Atoms do not lose their identity during chemical reactions (i.e. cannot be created and/or destroyed)
5. Molecules of a compound are identical (e.g. water = H2O)

13. Dalton developed a 6 part theory based on these experiments which explains
The Law of Conservation of Mass
The total mass of the reactants = the total mass of the products
Dalton
2nd law: any pure substance has a fixed composition in terms of the chemical elements, regardless of where located (earth, moon) (ex. Iron oxide located in Australia is SAME as iron oxide found in Zanzibar)
The Law of Constant Composition
A compound always contains the same elements in the same proportions by mass.

14. Dalton Model: Billiard Balls Theory Model Analogy (Featureless Sphere)
Indivisible atoms
(Featureless Sphere)
(Billiard Balls)
Dalton

15. Thompson
J.J. Thompson
(Physicist, 1904)

16. Thompson
Thompson discovered electrical particles can be lost or gained from elements. He named these particles electrons. He also believed that atoms were made of positively charged matter with negatively charged electrons scattered throughout.

17. Model: The Raisin Bun Model
Theory
Model
Analogy
Electrons embedded within a positive sphere
Net charge of zero
(Uniform Charge Distribution)
(Raisin Bun)
Thompson

18. Nagaoka
Hantaro Nagaoka
Hantaro Nagaoka
(Japanese Scientist, 1904)

19. He modeled the atom as a large positive sphere surrounded by a ring of negative electrons.
Nagaoka

20. Model: The Saturn Model
Theory
Model
Analogy
Positive sphere with ring of electrons
(Ring of Flat Electrons)
(Saturn)
Nagaoka

21. Rutherford
Ernest Rutherford
(Nuclear Physicist, 1911)

22. Rutherford
At McGill University (in Montreal), Rutherford designed the “Gold Foil Experiment” to test Thompson’s and Nagaoka’s models

23. The Gold Foil Experiment
Positive alpha particles were fired at a thin sheet of gold foil
Rutherford predicted that the alpha particles would pass directly through the metal foil, untouched.
Rutherford

24. The Gold Foil Experiment
Rutherford

25. Rutherford The Gold Foil Experiment The Results…
Most alpha particles passed through in a direct path, however, some deflected and bounced back.
Rutherford

26. “Its like shooting a bullet at a piece of tissue paper and having the bullet bounce off”
Rutherford

27. An atom is mostly empty space !!!
The Gold Foil Experiment
An atom is mostly empty space !!!
The Conclusion…
Atoms are mostly empty space with a tiny central nucleus which contains almost all of the total mass of the atom and is positively charged. He called these positively charged particles protons. Protons have a mass nearly 200x greater than electrons.
Rutherford
The nucleus is surrounded mostly by empty space, containing rapidly moving negative charges called electrons.

28. Model: The Nuclear Model
Theory
Model
Analogy
Small positive nucleus surrounded by negative electrons
(Nuclear Model)
(Beehive)
Rutherford

29. Chadwick
James Chadwick
(Nuclear Physicist, 1932)

30. Chadwick
Chadwick discovered particles in the nucleus of an atom possessing no electrical charge (neutral). He called them neutrons. The neutrons make up the remaining mass of the nucleus.

31. Bohr
Neils Bohr
(Danish Physicist, 1920)

32. Suggested electrons moved around the nucleus in a definite “orbit” arranged in “shells”.
Bohr

33. Model: The Planetary Model
Theory
Model
Analogy
Explains periodic law
Electrons are quantized in energy
levels
(Planetary Model)
(Planets Orbiting Around the Sun)
Bohr

34. Summary Neutrons + Protons are in the nucleus
Electrons in orbits around the nucleus
Summary
Protons = Positive Charge
Electrons = Negative Charge
Neutrons = Zero Charge

35. HOMEWORK Read pg. 82 – 85 Read pg. 90 – 92 Answer # 1 – 6, 9 on pg. 85

36. The End
The End