1. Bellwork 10/12/06
How long have people been interested in understanding matter and its structure?
Thousands of years
Hundreds of years
A few years
Never

2. History of Atomic Structure

3. Ancient Philosophy Who: Aristotle, Democritus
When: More than 2000 years ago
Where: Greece
What: Aristotle believed in 4 elements: Earth, Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.
Why: Aristotle and Democritus used observation and inferrence to explain the existence of everything.

4. Democritus
Aristotle

5. Alchemists Who: European Scientists When: 800 – 900 years ago
Where: Europe
What: Their work developed into what is now modern chemistry.
Why: Trying to change ordinary materials into gold.

6. Alchemic Symbols

7. Particle Theory Who: John Dalton When: 1808 Where: England
What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.
Why: Building on the ideas of Democritus in ancient Greece.

8. John Dalton

9. Discovery of Electrons
Who: J. J. Thompson
When: 1897
Where: England
What: Thompson discovered that electrons were smaller particles of an atom and were negatively charged.
Why: Thompson knew atoms were neutrally charged, but couldn’t find the positive particle.

10. J. J. Thompson

11. Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England
What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.
Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

12. Ernest Rutherford

13. Atomic Structure II Who: Niels Bohr When: 1913 Where: England
What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.
Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

14. Niels Bohr

15. Electron Cloud Model
Electrons travel around the nucleus in random orbits.
Scientists cannot predict where they will be at any given moment.
Electrons travel so fast, they appear to form a “cloud” around the nucleus.

16. Electron Cloud Model

17. Atomic Structure Timeline
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