Chapter 4, Section 2

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

2. Accidental Discoveries? • Does anything get discovered by accident? • Yes • Vulcanized rubber • Aspartame (Nutrasweet) • Electrons

3. Self Taught Class • Who it is? • When? • What did He do? • How was it important to understanding the Atom?

4. 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. 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. Crookes CRT

7. Section1 Development of the Atomic Theory Chapter 4 Thompson’s Discovery of Electrons • Thompson experimented with a cathode-ray tube. • He discovered negatively charged particles known as electrons.

8. Section1 Development of the Atomic Theory Chapter 4 Thompson’s Cathode-Ray Tube Experiment

9. 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

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. Discovering the Electron • 1909 – Robert Millikin (US) • Determined charge of an electron • Determined mass of an electron • 9.11 X 10-28g = 1/1840 mass of a hydrogen atom

13. Section1 Development of the Atomic Theory Chapter 4 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 ResultsRutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected.

14. Section1 Development of the Atomic Theory Chapter 4 Rutherford’s Gold-Foil Experiment

15. 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. 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. Section1 Development of the Atomic Theory Chapter 4 Where Are the Electrons? • Far from the NucleusRutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus.

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. 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. Summary of Models