1. AP ChemistryUnit 3 - Elements Lesson 2 – Noted Experiments Book Section: 2.2

2. The Electron • Streams of negatively charged particles were found to emanate from cathode ray tubes. • J.J. Thompson is credited with their discovery (1897).

3. The Electron • Thompson measured the charge/mass ratio of the electron to be 1.76 x 108 coulombs/g. • VIDEO: Cathode Ray Tubes

4. 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.

5. Millikan Oil Drop Experiment • Robert Millikan (University of Chicago) determined the charge on the electron in 1909. • -1.6 x 10-19 C • Also called the elementary charge • Video: Millikan Oil Drop

6. Radioactivity • Radioactivity is the spontaneous emission of radiation by an atom. • It was first observed by Henri Becquerel. • Marie and Pierre Curie also studied it.

7. Radioactivity • Three types of radiation were discovered by Ernest Rutherford: • α particles (helium nuclei) • βparticles (electrons) • γ rays (extremely high energy)

8. The Atom, circa 1900 • The prevailing theory was that of the “plum pudding” model, put forward by Thompson. • It featured a positive sphere of matter with negative electrons imbedded in it.

9. Discovery of the Nucleus • Ernest Rutherford shot αparticles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

10. The Nuclear Atom • Since some particles were deflected at large angles, Thompson’s model could not be correct. • Video: Rutherford Gold Foil

11. The Nuclear Atom • Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. • Most of the volume of the atom is empty space.

12. Practice AP Questions 1989 MC #18 – For the types of radiation given, which of the following is the correct order of increasing ability to penetrate a piece of lead? • Alpha particles < gamma rays < beta particles • Alpha particles < beta particles < gamma rays • Beta particles < alpha particles < gamma rays • Beta particles < gamma rays < alpha particles • Gamma rays < alpha particles < beta particles

13. Practice AP Questions 1989 MC #18 – For the types of radiation given, which of the following is the correct order of increasing ability to penetrate a piece of lead? • Alpha particles < gamma rays < beta particles • Alpha particles < beta particles < gamma rays – 57% correct, medium • Beta particles < alpha particles < gamma rays • Beta particles < gamma rays < alpha particles • Gamma rays < alpha particles < beta particles

14. Practice AP Questions 1989 MC #33 – Which of the following conclusions can be drawn from J.J. Thompson’s cathode ray experiments? • Atoms contain electrons. • Practically all the mass of an atom is contained in its nucleus. • Atoms contain protons, neutrons, and electrons. • Atoms have a positively charged nucleus surrounded by an electron cloud. • No two electrons in one atom can have the same four quantum numbers.

15. Practice AP Questions 1989 MC #33 – Which of the following conclusions can be drawn from J.J. Thompson’s cathode ray experiments? • Atoms contain electrons. – 34% correct, hard • Practically all the mass of an atom is contained in its nucleus. • Atoms contain protons, neutrons, and electrons. • Atoms have a positively charged nucleus surrounded by an electron cloud. • No two electrons in one atom can have the same four quantum numbers.

16. HW: 2.14, 2.16 • Thursday: Quantitative Analysis of a Soluble Sulfate Lab, Chemical Reactions Lab Due • Friday: Subatomic Particles (2.3) • 10/7 – Quantitative Analysis of a Soluble Sulfate Due • 10/18 – Gravimetric Analysis of a Chloride Salt Due • 10/20 – Elements Exam • 10/21 – Problem Set 2 Due