1. History of the Atomic Model
Seeing the Invisible

2. Can matter be divided into smaller and smaller pieces forever?
A Big Debate
Can matter be divided into smaller and smaller pieces forever?
What do you think?
YES!
NO!
Democritus
Aristotle

3. Atomic Models: Greek
Democritus ( B.C.)  Matter can not be divided forever
Smallest piece = “atom”
(Gk “atomos” = “not to be cut”)
He had no way of knowing what atoms looked like!

4. The word “atom” comes from a Greek word that means “unable to be cut”
… and you kept on cutting the leftover piece in half…
Imagine you had a piece of gold that you then cut in half…
…and then you cut one of these smaller pieces in half…
Gold Go ld

5. The word “atom” comes from a Greek word that means “unable to be cut”
Eventually you would have 1 piece of gold left. If you cut it in half, you wouldn’t have gold any more – you’d have something else. This tiny, tiny single piece of gold is called an atom of gold. An atom is the smallest particle of an element that acts like the element.
…and kept going…
…and kept going…
…and kept going…
An atom of gold

6. He hypothesized that atoms were:
Democritus
He hypothesized that atoms were:
Small & Hard
Diff in shape & size
Infinite
Always moving
Capable of joining
But no one believed him!

7. Time Goes By…
1600s-1700s: Key experiments occur which support Democritus’s ideas.
Robert Boyle ( )
Antoine Lavoisier ( )

8. Dalton (1766-1844) English chemist and school teacher
Did many experiments, studying gasses and proposed atomic theory

9. Dalton’s Atomic Theory
1. All matter is made up of atoms
Atoms cannot be divided into smaller particles
All atoms of the same element are exactly alike
4. Different elements are made of different kinds of atoms

10. Dalton’s Atom Model
Solid Sphere 10

11. Atomic Models: J.J. Thompson
Passed electricity through an uncharged gas
The gas gave off rays to show it was NEGATIVELY charged
How?
Negative charges must come from inside the atom!
=> Electrons!
Click here
for more
on the
Discovery of
the electron

12. But wait…
How can an atom be NEUTRAL if it is full of negatively charged particles (electrons?)

13. Atomic Models: J.J. Thompson
The atoms are neutral… How?
+ charges must be present to balance - charges
+ & - lumped in a cluster he said looked like “plum pudding”

14. Thompson’s Model

15. Atomic Models: Rutherford
Passed + Charged Particles through gold foil
Most passed right through
Atom is mostly empty space
Some bounced off at odd angles
Nucleus must be +
Calculate size of nucleus

16. Rutherford’s Gold Foil Experiment
start
experiment
DO NOT OPEN
RADIOACTIVE
MATERIAL
INSIDE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
LOOK INSIDE
THE GOLD

17. Rutherford’s Work What's Different This Time??? start experiment
DO NOT OPEN
RADIOACTIVE
MATERIAL
INSIDE + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
LOOK INSIDE
THE GOLD

18. Rutherford’s Model

19. So the atom is made up of positive and negative particles.
Okay…
So the atom is made up of positive and negative particles.
Where are the electrons found in the atom?

20. Atomic Models: Bohr Electrons are found in specific energy levels
Like planets around the sun

21. Bohr’s Model

22. Atomic Models: Wave
Electrons move so fast that it is impossible to determine their location
Move in all directions around the nucleus

23. Electron Cloud Model
Current Atomic Model- Electrons travel in a region surrounding the nucleus called the electron cloud