1. Chapter 4, Section 2 Sub-Atomic Particles and Nuclear Atoms

2. Copyright © by Holt, Rinehart and Winston. All rights reserved. Accidental Discoveries? Does anything get discovered by accident? Yes –Vulcanized rubber –Aspartame (Nutrasweet) –Electrons

3. Copyright © by Holt, Rinehart and Winston. All rights reserved. Self Taught Class Who it is? When? What did He do? How was it important to understanding the Atom?

4. Copyright © by Holt, Rinehart and Winston. All rights reserved. Discovering the Electron Sir William Crookes, early 1800’s –What is the relationship between electricity and matter? Static from combs Static from carpets Recent inventions: –Vacuum pump –Cathode Ray Tube (CRT) Cathode (+) at one end of vacuum tube Anode (-) at other end

5. Copyright © by Holt, Rinehart and Winston. All rights reserved. Discovering the Electron Crookes was in a darkened room. –Noticed flashes of light within his tube (coated inside with light producing chemicals) –Further work: “rays” going from cathode end to anode end (hence cathode ray tube) –Cathode Ray Tube is basis for TV and computer monitors

6. Copyright © by Holt, Rinehart and Winston. All rights reserved. Crookes CRT

7. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Development of the Atomic Theory Thompson’s Discovery of Electrons Thompson experimented with a cathode-ray tube. He discovered negatively charged particles known as electrons. Chapter 4

8. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Development of the Atomic Theory Chapter 4 Thompson’s Cathode-Ray Tube Experiment

9. Copyright © by Holt, Rinehart and Winston. All rights reserved. Thompson’s Model Thompson proposed a new model of the atom. –electrons are mixed throughout an atom, like plums in a pudding (or raisins in raisin bread). –Called Plum Pudding model

10. Copyright © by Holt, Rinehart and Winston. All rights reserved.

11. Discovering the Electron By late 1800’s Further work led to conclusion that: –Cathode Rays were actually stream of charged particles –Particles carried a negative charge –These particles were found in all matter –Particles were called ‘electrons’ CRISIS: Dalton was wrong, Atoms did have smaller particles

12. Copyright © by Holt, Rinehart and Winston. All rights reserved. Discovering the Electron 1909 – Robert Millikin (US) –Determined charge of an electron –Determined mass of an electron 9.11 X 10 -28 g = 1/1840 mass of a hydrogen atom

13. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Development of the Atomic Theory Rutherford’s Atomic “Shooting Gallery” In 1909, Ernest Rutherford aimed a beam of small, positively charged particles at a thin sheet of gold foil. The next slide shows his experiment. Surprising Results Rutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected. Chapter 4

14. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Development of the Atomic Theory Chapter 4 Rutherford’s Gold-Foil Experiment

15. Copyright © by Holt, Rinehart and Winston. All rights reserved. The Nuclear Atom Rutherford concluded Thompson was wrong: –There must be a tiny, very dense region of the atom, called the ‘nucleus’ Must be very dense (like all the mass of an atom) Must have a positive charge to keep the electrons attracted –Between atoms and nucleus must be a lot of empty space How Much? –Nucleus the size of a quarter has electrons over 1 mile away

16. Copyright © by Holt, Rinehart and Winston. All rights reserved. The Nuclear Atom Rutherford Model Explains: –Why alpha particles (electrons) bend on their way through nucleus –Why some alpha particles are deflected at very sharp angles Did not explain all of the Atom’s Mass

17. Copyright © by Holt, Rinehart and Winston. All rights reserved. Section 1 Development of the Atomic Theory Where Are the Electrons? Far from the Nucleus Rutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus. Chapter 4

18. Copyright © by Holt, Rinehart and Winston. All rights reserved.

19. Discovering Protons and Neutrons 1919 Rutherford Later Experiments –Concluded nucleus must contain positive particles called ‘protons’ –With co-worker James Chadwick showed nucleus also contained a neutral particle called ‘neutron’ Mass of neutron almost same as proton No electrical charge

20. Copyright © by Holt, Rinehart and Winston. All rights reserved. Summary to Date Atoms are composed of: –Protons (+ charge, 1 mass unit) –Neutrons (no charge, 1 mass unit) –Electrons (- charge, very little mass) Most of an atom’s size is electrons moving through empty space –Electrons are held to nucleus by +/- electrical attraction

21. Copyright © by Holt, Rinehart and Winston. All rights reserved. Summary of Models