1. History of the Atomic Theory An atom is the smallest particle of an element that retains its identity in a chemical reaction. Our current theory of the structure of the atom has developed over ~2000 years.

2. Democritus 440 BC All matter is made of atomos that travel through empty space Atoms cannot be created, destroyed, or further divided NOT based on scientific experiment

3. Aristotle 340 BC One of the most influential philosophers Criticized Democritus’ theory Based on Aristotle’s influence, Democritus’ theory was rejected

4. Robert Boyle 1650 Studied properties of gases Concluded that gasses are made up of tiny particles that group together to make different substances

5. Antoine Lavoisier 1765 First to state Law of Conservation of Mass First recorded quantitative experiments Devised a chemical naming system Allowed future chemists to share data  Tax collector, guillotined during the French Revolution

6. John Dalton 1803 All matter is made from tiny, indivisible atoms First atomic theory based on scientific experiments. Atom cannot be broken down into smaller particles. Hydrogen atom is the smallest atom.

7. J.J. Thomson 1897 Studied cathode ray tubes Discovered the electron http://www.youtube.com/watch?v=O9Go yscbazk

8. Cathode Ray Tube

9. Attracted to a positively charged plate and repelled by a negatively charged plate Thomson concluded that a cathode ray is a stream of negative particles called electrons

10. J.J. Thomson 1897 “Plum Pudding Model” Neutral atom: electrons (-) in sea of positive charge.

11. Protons Eugen Goldstein (1886) and Wilhelm Wien (1898) discovered a particle with a positive charge using the cathode ray tube. This would later become known as the proton.

12. R.A. Millikan 1909 Oil Drop Experiment Determined a quantitative charge and mass of electron www.youtube.com/watch?v=UFiPWv03f6g

13. Ernest Rutherford 1911 Gold Foil Experiment http://www.youtube.com/watch?v=5pZj0u_XMbc

14. Ernest Rutherford 1911 Gold Foil Experiment

15. Ernest Rutherford 1911 Gold Foil Experiment Concluded that atom is mostly empty space All positive charge and most of the mass in a small center: nucleus

16. Niels Bohr 1913 Electrons travel around the nucleus in definite paths  Worked on the Manhattan Project

17. James Chadwick 1932 Discovered the neutron (neutral charge) Explained the mass of an atom  Worked on the Manhattan Project

18. “Rutherford/Bohr Model”

19. Quantum Mechanical Model (Electron Cloud Model) ~1926 Mathematical model of the atom Electron orbits determined by probability

20. CHECK 1.What was Democritus’ theory? 2.What did Aristotle contribute? 3.What did Thomson’s model of the atom look like? 4.What experiment did Rutherford do? What did he conclude?