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Presentation transcript:

Ancient Greece Democritus: . “All matter is made of tiny, unbreakable particles; atomos” Aristotle: “All matter is made of 4 elements; earth, air, fire and water” ~400 B. C. Aristotelian viewpoint triumphs for 2000 years…

Dalton’s Atomic Theory 2000 years later… John Dalton, a British Teacher in the late 1700’s, combined ideas of elements with that of atoms and developed Dalton’s Atomic Theory

Dalton’s Atomic Theory: All matter is made of tiny indivisible particles called atoms. Atoms of the same element are identical. Atoms combine to form compounds in simple whole number ratios. (example: electrolysis) Chemical reactions involve the rearrangement of atoms; no matter is created or destroyed.

100 years later, another classy brit enters the scene: J.J. Thompson: English Physicist (1856 – 1940) Nobel Prize in Physics 1906 Famous for: Cathode Ray Experiments “Discovery” of proton, neutron and electron ‘Plum Pudding’ Model of the Atom

- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end

Thomson’s Experiment Voltage source + - By adding an electric field he found that the moving pieces were negative (electrons!)

This, and Thompson’s other experiments showed that atoms had… positive, (protons) negative, (electrons) and neutral (neutrons) …pieces inside them. (but where in the atom were they?)

Thompson suggested a “Plum Pudding” Model of the atom: electrons positively charged “pudding” - + It explained conduction of electricity: - + conduction of electricity

Thompson’s Model of the Atom: A bunch of positive stuff, with the electrons able to be removed “Plum Pudding” Model; a bunch of positive ‘pudding’, with removable negative ‘plums’ (electrons)

Plum Pudding gets spilled by: Ernest Rutherford: New Zealand Physicist (1871-1937) Nobel Prize in Physics 1908 Famous for: Gold Foil Experiment “Discovery” of the nucleus Nuclear Model of the Atom

Rutherford’s experiment Took positive charges (alpha particles; radioactive particles that are emitted by Uranium) and fired them at Gold Atoms His surprise: Some of them bounced back.

How it worked: When the alpha particles hit a florescent screen, the screen glows. So, he could keep track of where the positive charges ended up after he fired them at some gold foil.

What he expected What he saw:

He Expected The alpha particles to pass through without changing direction very much Because The positive charges were spread out evenly. Alone they were not enough to stop the alpha particles…

Because, he thought the mass was evenly distributed in the atom

How he explained what he saw: + Atom is mostly empty Small dense, positive piece at center Alpha particles are deflected by it if they get close enough

Result of Rutherford’s Gold Foil Experiment: Result of Rutherford’s Gold Foil Experiment: “Discovery” of the Nucleus Rutherford’s Scattering Experiment metal foil alpha particles Ernest Rutherford 1871-1937 film loop what he expected: ricocheting alpha particles! what he got:

Goodbye, Plum Pudding; Hello, Nuclear Model Thomson’s Atom: diffuse mass and charge + alpha particles would have passed through - + ++ Hello, Nuclear Model ++ 10-8 cm 10-13 cm Rutherford’s Atom: concentrated mass and positive charge at the nucleus electrons roam empty space around the nucleus some + alpha particles repelled Prediction based on Rutherford’s nuclear atom: number of scattered alpha particles is proportional to: thickness of foil square of the charge on the nucleus 1/sin4(theta/2) 1/velocity4

Niels Bohr: Danish Physicist (1885 – 1962) Nobel Prize in Physics 1906 Famous for: Planetary Model of the Atom “Father of” Quantum Theory / Mechanics “Electrons have specific energy levels, …like planets have specific orbits.”

energy levels Bohr said: n = 1 n = 2 n = 3 Electrons can only be at certain, specific energy levels (distances from the nucleus) n = 1 n = 2 n = 3 n = the primary quantum number (distance from nucleus

Electrons are found in ‘shells’ = ‘energy levels’ can hold 18 electrons can hold 8 electrons can hold 2 electrons

Valence Shell: Valence Electron: The outermost shell that contains electrons. Valence Electron: An electron in an atom’s valence shell

Bohr Model Diagrams …show energy levels (shells) and the electrons in them. nucleus is not shown. Inner shells fill first; 1st shell (n = 1) gets 2 e- 2nd shell (n = 2) gets 8 e- 3rd shell (n = 3) gets 8 e- …before 4th shell starts to fill