1. History of Atomic Structure

2. 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 “atomos” (greek word meaning “unable to divide”). Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

3. Aristotle Democritus

4. 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. Atoms join to form compounds Why: Building on the ideas of Democritus in ancient Greece.

5. John Dalton

6. 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: Came up with plum pudding model which said the atom was a solid + charged sphere w/ negatively charged electrons embedded in it.

7. J. J. Thompson

8. J J Thomson (Plum pudding model)

9. Atomic Structure I Who: Ernest Rutherford When: 1911 Where: England What: Conducted the gold foil experiment and discovered the nucleus. Why: Decided that the atoms were mostly empty space, but had a dense + charged central core (nucleus) surrounded by negatively (-) charged electrons

10. Ernest Rutherford

11. Atomic Structure II Who: Niels Bohr When: 1913 Where: England What: Proposed that electrons traveled in fixed paths (solar system model) 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.

12. Niels Bohr

13. Atomic Structure III Who: Erwin Schrodinger When: 1926 What: Proposed the Electron Cloud Model which said electrons travel around the nucleus in random orbits. Why: Scientists cannot predict where they will be at any given moment. Electrons travel so fast, they appear to form a “cloud” around the nucleus.

14. Electron Cloud Model