1. History of Atomic Theory
Review of In-Class Research
Tuesday, September 15th, 2015

2. Ancient Philosophy § Who: Aristotle, Democritus, Leucippus
§ When: More than 2000 years agoears 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 “atomos”.
§ Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

3. Like to the Greeks (Democritus)?
What did an Atom Look
Like to the Greeks (Democritus)?
This was Democritus’ atomic model. It was simply a round sphere with no electrons, protons, or neutrons. Democritus created the first atomic model. His contribution helped people to understand the idea of an atom, then other scientists looked further into the science of the atom and its general makeup.

4. Dalton’s Particle Theory of Matter
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.
What did an Atom look like to Dalton?

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

6. Thomson’s Cathode Ray Tube
Understanding Thomson’s Experiment

7. What did an Atom look like to Thomson?
Bunch of negative charged particles (raisins) in a sphere of positive charge (plum pudding)

8. Discovery of Nucleus Who: Ernest Rutherford When: 1911 Where: England
What: Conducted an experiment
that resulted in discovery of atom’s nucleus.
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.

9. Rutherford’s Gold Foil Experiment

10. What did an Atom Look Like to Rutherford?

11. Bohr’s Model of the Atom
Who: Niels Bohr
When: 1913
Where: England
What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model.
Why: Bohr was trying to show why each element emitted its own unique frequency of energy as electrons jumped orbits.

12. Bohr’s Model of the Atom

13. Our Current Model - The Electron Cloud
- Bohr said electrons orbit the nucleus (like the planets around the Sun)
- The electron cloud model says that we can't know exactly where an electron is at any given time, but the
electrons are more likely
to be in specific areas.

14. Our Current Model - The Electron Cloud
- Bohr assigned electrons to different shells
- Using quantum mechanics, the electron cloud model assigns electrons to different atomic orbitals. ** These are not all spheres

15. For the quiz/test
You should know the main scientists and their experiments/contributions.
You should be able to draw their models of the atom and explain differences.