Atomic Theory Notes Day 1

1. 08.07.13 Boon Chemistry Atomic Theory Notes Day 1

2. Facts, Hypotheses, Theories, and Laws in Science • Fact: An observation that has been repeatedly confirmed and accepted as “true.” Truth in science, however, is never final. • Hypothesis: A tentative statement about the natural world. It must be testable. Experiments either support or disprove a hypothesis. • Law: A descriptive generalization about how a part of the natural world behaves under stated circumstances. Laws are the basis for theories and laws may or may not be facts. • Theory: Awell-supported explanation of some aspect of the natural world that can incorporate facts, laws, inferences, and tested hypotheses.

3. …but first, a short history lesson and the story of Mr. Rutherford, who had a little too much time… Atoms and Their Structure

4. History Democritus (460 BC) • The idea of an atomic theory is more than 2000 years old. • Ancient Greece: • Democritus: Atoms are the Smallest indivisible particles • Plato & Aristotle: there is not an ultimately indivisible particle Plato (428 BC) Aristotle (384 BC)

5. 16oo’s - Isaac Newton & Gassendi Theorized that all matter was made of very small indivisible particles.

6. 1808 - John Dalton – Beginning of modern atomic theory : Billiard Ball Dalton’s atomic theory had three main ideas: An element is composed of extremely small particles called “atoms.” All atoms of a given element show the same chemical properties. Atoms of different elements have different properties. In the course of an ordinary chemical reaction, no atom of one element disappears or is changed into an atom of another element.

7. Billiard Ball Model: John Dalton 1808 Description: The atom is the smallest particle of an element. The atom is a solid, indestructible unit. Atoms of different elements have different masses. Simulation: Write down observations in the space on your notes worksheet.

8. JJ Thomson’s Cathode Ray Experiments 1890’s • Description: A cathode ray is a beam of particles smaller than an atom that appears when a vacuum sealed glass tube is hooked up to an electric charge. • By passing the cathode ray through magnetic plates and observing that the ray moved away from the negative plate, Thomson discovered the negatively charged subatomic particle – the electron

9. Plum Pudding Model: JJ Thomson 1903 Description: The ‘pudding’ is the positive material of an atom. The embedded ‘raisins’ are negative electrons. Note: Thomson discovered the electron in the cathode ray experiment. Simulation: Write down observations in the space on your notes worksheet.

10. Rutherford’s Gold Foil Experiment Description: Rutherford shot a beam of alpha particles at a thin piece of gold foil. 98% of the particles went straight through the foil. Some particles bounced straight back.

11. Flourescent Screen Lead block Uranium Gold Foil

12. What he expected

14. What he observed

15. + Assumptions: • Atom is mostly empty. • Small dense, positive piece at center. • Rays deflected by center.

16. +

17. Solar System Model: Ernest Rutherford 1911 Description: The mass and positive charge of an atom is concentrated in a small core called the nucleus. Negative electrons orbit the nucleus. The atom is mostly made up of empty space. Note: This model was the result of the gold foil experiment. Simulation: Write down observations in the space on your notes worksheet.

18. Bohr Model: Neils Bohr 1923 Description: Electrons are arranged around the nucleus in discrete energy levels or shells. The idea of specific energy levels explained why different elements produced different light emission spectrums. Simulation: Write down observations in the space on your notes worksheet.

19. Quantum Model: Heisenburg, deBroglie, Schrodener, Pauli 1935 Description: Electrons exist in orbitals. i.e. an area surrounding the nucleus that has a 90% probability of  containing an electron. Simulation: Write down observations in the space on your notes worksheet.

20. Evolution of the Atom Model