The Development of Atomic Theory

1. The Development of Atomic Theory • Ancient times to the present

2. Democritus • Atoms are indivisible, indestructible, fundamental units of matter. • “Atomos”

3. Robert Boyle • The Skeptical Chemist • Quantitative experimentation • Identification of elements

4. Antoine Lavoisier • Role of oxygen in combustion • Law of conservation of mass • First modern textbook

5. John Dalton • Atomic Theory -Unchallenged for 100 years • Law of multiple proportions

6. Atomic Theory of Matter The theory that atoms are the fundamental building blocks of matter reemerged in the early 19th century, championed by John Dalton.

7. Dalton’s Postulates Each element is composed of extremely small particles called atoms.

8. Dalton’s Postulates All atoms of a given element are identical to one another in mass and other properties, but the atoms of one element are different from the atoms of all other elements.

9. Dalton’s Postulates Atoms of an element are not changed into atoms of a different element by chemical reactions.

10. Dalton’s Postulates Atoms are neither created nor destroyed in chemical reactions.

11. Dalton’s Postulates Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kind of atoms.

12. Early experiments to characterize the atom

13. J. J. Thomson • Discovery of the electron • Cathode ray tube • http://www.chem.uiuc.edu/clcwebsite/video/Cath.mov

14. The Electron • Streams of negatively charged particles were found to emanate from cathode tubes.

15. The Electron Thomson measured the charge/mass ratio of the electron to be 1.76  108 coulombs/g.

16. The Atom, circa 1900: • “Plum pudding” model, put forward by Thomson. • Positive sphere of matter with negative electrons imbedded in it.

17. Henri Becquerel • Discovery of radioactivity • Nobel Prize

18. Radioactivity • Three types of radiation were discovered by Ernest Rutherford: •  particles •  particles •  rays

19. Robert Millikan • Oil Drop Experiment • Determined the charge and mass of the electron • http://search.eb.com/nobel/cap/omillik001a4.html

20. Millikan Oil Drop Experiment Once the charge/mass ratio of the electron was known, determination of either the charge or the mass of an electron would yield the other.

21. Millikan Oil Drop Experiment Robert Millikan (University of Chicago) determined the charge on the electron in 1909.

22. Ernest Rutherford • Existence of the nucleus and its relative size • Gold foil experiment

23. Discovery of the Nucleus Ernest Rutherford shot  particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

24. The Nuclear Atom Since some particles were deflected at large angles, Thomson’s model could not be correct.

25. Niels Bohr • Proposed improvements to Rutherford’s model. • Definite orbits in which an electron can travel around a nucleus without radiating energy. • Only works for hydrogen!

26. Other Subatomic Particles • Protons were discovered by Rutherford in 1919. • Neutrons were discovered by James Chadwick in 1932.

27. Erwin Schrödinger

28. What could be next?