Discovering the Electron

1. Discovering the Electron Chapter 4, Section 2

2. Crooke’s Tube Crooke’s tubes were developed in the 1870’s - kind of like early neon lights. Sealed glass tube with a small amount of gas inside and metal electrodes (+, -) at either end. Pass electricity through the tube.

3. Victorian Party Novelty

4. Crookes – cathode ray • Working with a tube that had a coating at the end. The coating produced a flash of light when it was hit by radiation. • There were rays (radiation) traveling inside the tube from the cathode (-) to the anode (+). • Called a cathode ray.

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6. source source Movie of cathode ray tube

7. Cathode Rays – by late 1800’s • Actually a stream of charged particles. • Particles carry a negative charge. • Didn’t matter what gas (low P) was inside the tube or what metal the electrodes were made of. So the negative particles were in all forms of matter.

8. Divisible!!! • Negative particle in all forms of matter. • Called electrons. • The atom is DIVISIBLE!

9. An electric field or a magnetic field will deflect a beam of charged particles.

10. J.J. Thomson “discovered” the electron in 1897 source

11. Thomson’s Cathode Ray Tube source Thomson proposed that cathode rays were streams of particles much smaller than atoms. He found the charge-to-mass ratio of the electron. (He called it a corpuscle.)

12. Atom is Divisible! • Thomson’s discovery meant that the atom was divisible! • He knew there had to be an equal amount of positive charge because matter is neutral.

13. Thomson’s Plum-Pudding Model The positive charge is evenly smeared out. The negative charge is in bits – like chips. source

14. Robert Millikan - 1909 • Oil drop experiment – determined the charge of the electron: 1.60 X 10-19 coulomb. • Thomson had determined the charge-to-mass ratio as 1.76 X 108 coulomb per gram. • So the mass of the electron is 9.09 X 10-28 grams. Animation of Oil-Drop Experiment

15. Proton – Discovered by 1920 animation • Thomson & Goldstein – 1907 • Discovered a heavy particle with a positive charge in some cathode ray tube experiments. • Rutherford – 1918 • Shot alpha particles at nitrogen gas and got hydrogen. Figured out that the hydrogen had to come from the nitrogen. Suggested that the hydrogen nucleus was an elementary particle. Named it proton.

16. Rutherford • Famous for a lot of experiments. • Discovered the proton. • Figured out  and  radiation. • Changed our idea of the atom! NUCLEAR model. • One of the most elegant experiments in the history of science!

17. Rutherford’s Experiment - 1911 source

18. source The steel marble will move in a straight line until it hits something. If it hits something heavy, like the rim of the table, it will rebound back.

19. Rutherford’s exp’t: animation Compared to an electron, an alpha particle is massive & fast. If Thomson’s model was correct, the alpha particle wouldn’t be much affected. No big deflections. electrons are tiny positive charge uniformly spread

21. Results of Rutherford’s Expt • Most of the alpha particles went straight through – they didn’t bump into anything so most of the atom was empty space. • Some of the alpha particles were deflected back – they must have hit something really heavy that Rutherford called the nucleus. • Results do NOT match Thomson’s model.

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23. What did Rutherford conclude from the particles that went straight through? What did Rutherford conclude from the particles that bounced back? source

24. So how big is the nucleus compared to the entire atom? • If the atom was as big as a football stadium, the nucleus would be smaller than a flea on the 50-yard line! • If the atom was as big as a period at the end of a sentence in a standard textbook, it would have the mass of 70 cars!

25. Rutherford proposed the nuclear atom. source Rutherford did not speculate on how the electrons were arranged around the nucleus. source

26. Neils Bohr - 1913 Planetary model source

27. Electrons travel only in specific orbits. • Each orbit has a definite energy. The orbit closest to nucleus has the lowest energy. • Atoms emit radiation when an electron jumps from an outer orbit to an inner orbit. • Outer orbits hold more electrons than inner orbits. • Outer orbits determine atom’s chemical properties. source

28. Schrodinger – 1926 Mathematically - treated electrons as waves rather than particles! Quantum mechanical model or Modern model.

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30. Modern Model • Electron’s energy has only certain values – it is quantized. (Bohr model had quantization too!) • Electrons are located in “probability regions” or atomic orbitals. These are not circular orbits! • Electrons move around the nucleus at near the speed of light.

31. Schrodinger’s Model • We talk about the probability of locating an electron at a certain place. • Also called: Quantum Mechanical Model, Wave Mechanical Model, or Modern Model • Orbitals – standing wave patterns with definite energy.

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33. The development of atomic theory represents the work of many scientists over many years. source

34. James Chadwick - 1932 • Discovered the neutron in cloud chamber experiments. • About the same mass as a proton. • Electrically neutral.