1. Development of Atomic Theory Ancient Times | Dalton's Postulates | Thomson's Discovery of Electron Properties | Rutherford's Nuclear Atom | Bohr's Nuclear Atom | Modern Atomic Theory

2. Learning Objectives Understand the experimental design and conclusions used in the development of modern atomic theory Development of Atomic Theory

3. Democritus proposed atomos – Indivisible, indestructible particles that made up matter – Different types possessed different properties – Changes in matter produced by changes in the composition of groups containing different types – Moved through empty space Aristotle’s beliefs – Matter composed of earth, air, fire, water – No such thing as empty space Idea of atom dismissed for thousands of years – Disagreement – Inability to prove ideas through experimentation Ancient Times

4. Dalton’s Atomic Theory – Based on experiments Elements made of small indivisible particles called atoms Atoms of an element are identical to each other and different from atoms of other elements Chemical reactions occur when atoms are combined, separated, or rearranged in simplified whole-number ratios Atoms not created or destroyed in chemical reactions Dalton’s Postulates Today’s revisions – Atom – the smallest unit of an element that demonstrates the properties of that element – Atoms of the same element can have different masses – Atoms can be divided into subatomic particles

5. Cathode ray – a stream of electrons that emanate from a negative conductor (http://www.youtube.com/watch_popup?v=XU8nMKkzbT8)http://www.youtube.com/watch_popup?v=XU8nMKkzbT8 Electron – a negatively charged subatomic particle – J.J. Thomson proved the existence of electrons Mass of electrons is much smaller than the least massive atom – Proved subatomic particles exist Thomson’s Discovery of Electron Properties

6. Plum pudding model postulated by Thomson – Atoms made of a positively charged mass – Electrons scattered throughout the mass – Failed to correctly describe the structure of the atom Thomson’s Discovery of Electron Properties

7. http://www.teachertube.com/video/rutherfords- experiment-8329 http://www.teachertube.com/video/rutherfords- experiment-8329 Rutherford’s Gold Foil Experiment

8. Nucleus – the small, dense, positively charged area in the center of an atom where protons and neutrons are found – Discovered during Ernest Rutherford’s gold-foil experiment Nuclei of atoms repelled some positively charged (alpha) particles, while other alpha particles traveled through empty space Rutherford’s Gold Foil Experiment

9. Nucleus – Small but accounts for most of an atom’s mass – Contains protons Proton – a positively charged subatomic particle that has a mass of approximately one atomic mass unit – Contains neutrons Neutron – a neutrally charged subatomic particle that has a mass of approximately one atomic mass unit Rutherford’s model showed the nucleus surrounded by orbiting electrons Rutherford’s Gold Foil Experiment

10. Discovery that the emission spectrum of hydrogen contains only certain frequencies of light – Prompted Niels Bohr to propose another atomic theory Attempted to describe the relationship between energy and movement of electrons Bohr’s Nuclear Atom

11. Electrons are arranged in circular orbits around the nucleus Electrons – In each orbit have a fixed amount of energy Nearest nucleus are in their lowest energy state – Gain and lose fixed amounts of energy Release energy as photons that correspond to specific frequencies of light Bohr’s Nuclear Atom

12. Bohr’s shortcomings – Only explained emission spectrum of hydrogen – Did not explain chemical properties – Electrons do not revolve around nucleus in circular orbits Quantum mechanical model – Electrons exhibit wave and particle properties – Impossible to know both the speed and location of electrons at the same time Region around the nucleus where finding an electron is most probable is called an orbital Modern Atomic Theory

13. Learning Objectives Understand the experimental design and conclusions used in the development of modern atomic theory Development of Atomic Theory