The History of Atomic Theory

1. The History of Atomic Theory

2. Democritus • Greek philosopher • 2400 years ago • The Atom • Could matter be divided into smaller and smaller pieces forever? • Or was there a limit to the number of times a piece of matter could be divided?

3. Atomos • Matter could NOT be divided into smaller and smaller pieces forever • This piece would be indivisible. • “atomos” =“not to be cut.”

4. Atomos • Atoms= small, hard particles that were all made of the same material • Different shapes and sizes. • Atoms were infinite in number, always moving and capable of joining together.

5. Aristotle • Same time period • Denounced the idea of atomos • Earth, Water, Air, and Fire

6. Alchemists • Middle Ages • Worked on turning ordinary metals into gold • Derived modern chem

7. John Dalton • English chemist • Early 1800’s • Revisited the particle theory • Laid the groundwork for Modern theory • Father of the Atom

8. Dalton’s Theory 1. All matter  small particles a.k.a atoms 2. Atoms of the same element are exactly alike. Atoms of different elements are different. 3. Atoms cannot be divided, created, nor destroyed

9. Dalton’s Theory 4. Atoms of different elements can combine to form compounds 5. In chemical reactions, atoms are combined, separated, or rearranged Known as the “Billiard Ball Model”

10. J. J. Thomson • English scientist • 1897 • The electron • The first to believe an atom is made of even smaller particles.

11. Thomson Model • Atoms were made from a positively chargedsubstance with negatively charged electrons scattered • A.k.a: the “Plum Pudding Model”

12. Thomson Model: Cathode Ray • Thomson studied the passage of an electric current through a gas. • As the current passed through the gas, it gave off rays of negatively charged particles.

13. Robert A. Millikan • American physicist • 1909 • Determined the mass of an electron relative to the atom

14. Millikan’s: Oil Drop Experiment • Mist of fine oil droplets are dropped into the chamber • Some of the droplets were negatively charged • No electricity electrons fell through: gravity is only acting force

15. Millikan’s Oil Drop Experiment • Voltage adjusted so down force = up force • Oil drop suspended between plates • The charge on the droplet was calculated • Mass of electron = charge of electron: charge-to-mass ratio of electron • Mass = 1/2000 of the Hydrogen atom

16. Ernest B. Rutherford • English physicist • 1908 • Gold Foil Experiment

17. Gold Foil Experiment: Results • Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. • Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid.

18. What the Gold Foil Experiment Proves

19. Niels Bohr • Danish scientist • 1913 • Solar System Model

20. Bohr Model • According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. • These orbits are located at certain distances from the nucleus based on relative energies.

21. James Chadwick 1932 Neutron: There was unaccounted mass so there had to be another subatomic particle Isotopes: Atoms of the same element (identical protons) but different numbers of neutrons and therefore different masses

22. Electron Cloud Model

23. The Electron Cloud Model • Heisenburg Uncertainty Principle: cannot know speed and location of electron at same time • Electrons in organized orbitals • Electrons travel so fast that they seem to form a “cloud” around the nucleus • Location of electron depends on its energy • Light spectrum

24. Electron Cloud: • Electrons: low NRG are closest to nucleus • Electrons: high NRG are further from nucleus

25. Historical Models