1. The Changing Models of the Atom

2. Democritus 460 – 370 BCE

3. Democritus 460 – 370 BCE
Democritus believed matter was made up of “atoms” (Greek for indivisible)
Democritus thought:
Each element had its own kind of atom.
Atoms of all elements had different shapes and sizes.
All atoms moved around in space.
granite
wood
air
copper
water

4. Aristotle 400 BCE claimed that there was no smallest part of matter
different substances were made up of different proportions of fire, air, earth, and water.
The four element theory

5. Four element example Wood: Copper:
contains “earth” because it has weight
contains “fire” because it can burn
contains “air” which is released as smoke
contains water before it dries out
Copper:
More earth because it is heavier
Less fire, but can still be burned or melted
Very little air
Very little water

6. Aristotle’s Fault
Even though he was wrong, his theory persisted for some 2000 years!

7. John Dalton

8. John Dalton 1850
Dalton stated that all matter is made of indivisible and indestructible atoms, which differ from element to element
He also said atoms of different elements have different properties
Different elements combine in specific ways to form new substances.
The billiard ball theory

9. J. J. Thomson
Cathode ray tube

10. J.J. Thomson 1897
Experimenting with vacuum tubes (cathode rays), scientists discovered that an electric current could pass through a tube as a bright light
What do you think that bright
light might have been?

11. J.J. Thomson 1897
Thomson found that the glowing stream would bend toward a positive charge, and bend away from a negative charge.
What could this mean?
Thompson theorized that negatively charge particles were responsible and called them electrons.

12. Thomson’s Plum Pudding Theory Or Raisin Bun Theory

13. Ernest Rutherford

14. Ernest Rutherford
Rutherford fired tiny alpha particles at a very thin piece of gold foil.

15. Ernest Rutherford
He found that most of the alpha particles passed right through the gold foil as he expected
However, a small number of alpha particles were deflected or bounced right back.
What can we conclude?

16. Rutherford’s Planetary model
In 1911, Rutherford proposed a revolutionary view of the atom.
Proposed that the atom consisted of a small, dense core of (+) particles in the nucleus of the atom.
Electrons (-) floated around the nucleus like planets around the sun.

17. Rutherford’s planetary model

18. James Chadwick 1932 The Chadwick problem:
Positive charges repel positive charges
How then, can a nucleus not be pushed apart as two magnets are?
Finally, if protons weigh 1amu, where is all this extra mass coming from?
What did he discover???

19. The Nuclei of the atoms have too many particles to stick together.
Point of Interest
This is why all of those giant elements at the bottom of the periodic table are unstable or radioactive.
The Nuclei of the atoms have too many particles to stick together.
Too much charge!!

20. James Chadwick 1932 Chadwick discovered the neutron.
Neutrons reduce the repulsion between protons and stabilize the atom's nucleus.
Neutrons are in the Nucleus and weigh 1amu
Neutrons do not have any electrical charge, they are neutral

21. James Chadwick
The three isotopes of hydrogen

22. Neils Bohr

23. Bohr’s Model 1913
Bohr was studying electrons in atoms and proposed the idea that electrons travel in specific shells around the atom's nucleus.
Each of these shells can hold a certain amount of electrons which must be filled in a certain order
We now know his theory is not entirely correct but it is still useful today.

24. Bohr’s model

25. The Quantum Mechanical Model
This is the correct model used today.
The electron cloud model uses the basic idea of Bohr’s model except that the electrons are not found in distinct orbits but their position can be thought of as in a 3D cloud that has a particular energy

26. http://videos. howstuffworks. com/science/atoms-videos-playlist

27. Q’s
Pg 177 # 1, 3, 4, 6, 9, 11