Transcript Document
History of the Development of Atomic Theory Your first task… • In each envelope you will find 7 yellow pictures illustrating the various models that have been used to explain the structure of the atom • Using clues from the pictures, put the pictures in chronological order (which model do you think came first, second, etc.) Your second task… • Using the clues from the 7 blue scientist cards, match up the models (yellow cards) with the appropriate scientist card • When you feel that you have made the appropriate matches, make sure that your cards are in chronological order (Hint: There are dates on the blue cards!) Democritus, 400 B.C. • Elements consist of tiny, solid particles that can not be subdivided. • He called these particles atomos, meaning “uncuttable.” John Dalton, 1805 • 1. All matter is made of tiny indivisible particles called atoms. • 2. Atoms of the same element are identical; those of different elements are different. • 3. Atoms of different elements combine in whole number ratios to form compounds • 4. Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed. Legos are Similar to Atoms H2 H H H O + H2 H H O2 O H 2O O H 2O H H O H Lego's can be taken apart and built into many different things. Atoms can be rearranged into different substances. Joseph John (J.J.) Thompson, 1904 • Discovered the electron. • Atoms contain small, negatively charged particles. • The electrons are evenly embedded throughout a positively charged sphere (like chocolate chips in a ball of cookie dough) • Known as the “Plum Pudding Model” A Cathode Ray Tube Source of Electrical Potential Stream of negative particles (electrons) Metal Plate Gas-filled glass tube Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58 Metal plate Thomson’s Experiment ON - OFF voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end Thomson’s Experiment ON - OFF voltage source + Thomson’s Experiment ON - OFF voltage source + + - By adding an electric field… he found that the moving pieces were negative. He concluded that these negative pieces must be suspended in a positive sphere Ernest Rutherford, 1911 • Almost all of the mass of an atom- and all its positive chargesare concentrated in a small, central atomic nucleus surrounded by electrons. • Used a “gold foil” experiment to come to his conclusions Results of foil experiment if plum-pudding had been correct. Electrons scattered throughout - + - positive charges + + + + - - + + + - Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57 - Rutherford’s Apparatus Rutherford received the 1908 Nobel Prize in Chemistry for his pioneering work in nuclear chemistry. beam of alpha particles radioactive substance circular ZnS - coated fluorescent screen gold foil Niels Bohr, 1913 • Electrons travel in fixed orbits around the atom’s nucleus (like the planets around the sun). James Chadwick, 1932 • A student of Rutherford, concluded that the nucleus contained positive protons and neutral neutrons • Discovered the neutron Electron Cloud Model, Current • Electrons do not follow fixed orbits but tend to occur more frequently in certain areas around the nucleus at any given time. • These areas are called electron clouds.