1. History of Atomic Structure

2. Democritus
Aristotle

3. Ancient Philosophy Who: Aristotle, Democritus
When: More than 2000 years ago 460 BC
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 inference to explain the existence of everything.

4. Any experimental support for ideas?

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. John Dalton

8. Particle Theory Who: John Dalton When: 1766-1844
Where: England, school teacher
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.

9. Does this part hold true today?
4 Part Atomic Theory
1. Elements are composed of tiny indivisible
particles (atoms).
Does this part hold true today?

10. Does this part hold true today?
2. Atoms of the same element are identical
and different from other atoms.
Does this part hold true today?

11. Does this part hold true today?
3. Atoms can physically mix and chemically combine in whole number ratios.
Does this part hold true today?

12. Does this part hold true today?
In chemical reactions, atoms separate, join and rearrange.
Atoms cannot be changed into other atoms.
Does this part hold true today?

13. Did Dalton have experimental support for his theory?

14. Dalton’s Solid Sphere Model of the Atom

15. Discovery of Electrons
Who: J. J. Thompson
When: 1897
Where: England, physicist
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.

16. J. J. Thompson

17. Thomson’s Cathode Ray Tube

18. Thomson’s Plum Pudding Model of the Atom

19. Who: Robert Milliken
When:
Where: United States, physicist
What: Conducted an experiment to determine the quantity of charge on an electron, 1-, in his famous oil drop experiment
Why: He knew that atoms had electrons, wanted to determine charge

20. Robert Milliken

21. Atomic Structure I Who: Eugene Goldstein When: 1850-1930
Where: England
What: Conducted an experiment with a cathode-ray tube and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles- protons.
Why: He concluded that they were composed of positive particles- protons

23. 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.

24. Ernest Rutherford

25. Who: James Chadwick
When: 1932
Where: England, physicist
What: confirmed existence of the neutron
Neutrons have no charge and have a
mass nearly equal to that of a proton

26. 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.

27. Niels Bohr

28. 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.

29. Electron Cloud Model