Chapter 4: Atomic Structure Table of Contents Chapter 4: Atomic Structure Atomic History Democratis Dalton Johnson Rutherford Bohr
Democritus First to theorize about the atom in 400 B. C. The string analogy
Development of the Modern Atomic Theory 1782 - Lavoisier - law of conservation of matter 1803 - Dalton - atomic theory of matter – explains the observations of Lavoisier & others Burning steel wool
1897 – J.J. Thomson – Cathode Ray Tube discovered the electron. Tesla coil
Cathode Ray Why does the beam deflect?
+ Cathode-Ray Tube electrodes vacuum - With charge, opposites attract, similar repels - Rays bent toward positively charged plate & away from negatively charged plate Charged electrodes → rays flow from one electrode to the other TV
Cathode-Ray Tube Conclusions from Thomson’s Cathode-Ray Tube: Discovered electrons: negatively charged particles These electrons had to come from atoms Atoms can’t be entirely negatively charged so… Plum Pudding Model
Rutherford’s Gold Foil Experiment 1912 - Ernest Rutherford – Gold Foil Experiment
Gold Foil Experiment What were the 3 paths of the particles? What was discovered?
The Gold Foil Experiment coated with zinc sulfide, which glows when struck by the positively charged particles of the beam
2 1 3 Discoveries from Rutherford’s Gold Foil Experiment Atoms are mostly space Atoms contain a dense center (nucleus) The nucleus is positively charged
Niels Bohr Proposed that electrons must have enough energy to keep them in constant motion around the nucleus. Electrons can move from one path to another by taking in or letting off energy 1913 – Bohr – Planetary Model
Evidence for Energy Levels Electrons can have only certain amounts of energy & orbit around nucleus Energy levels: The regions of space in which electrons can move around the nucleus of an atom
The emission spectrum of an element: The spectrum of light released from excited atoms
Other Particles 1920 - Rutherford discovered the positively charged proton in the center of the atom 1932 - James Chadwick discovered the neutron – another particle in the atom’s center with no charge but a similar mass to the proton.
The Electron Cloud Model 20th century – energy levels are not organized like planets Energy levels are spherical regions of space around the nucleus in which electrons are most likely to be found. e- are tiny and move quickly through space Electron cloud: The space around the nucleus of an atom where the atom’s electrons are found
The Electron Cloud Model
Review: identify each model Thomson’s Plum-Pudding Model Bohr’s Planetary Model Present Electron Cloud Model