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.)

(Hint: There are dates on the blue cards!) 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 H O H2 O2 H2O + 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”

particles (electrons) A Cathode Ray Tube Source of Electrical Potential Metal Plate Gas-filled glass tube Metal plate Stream of negative particles (electrons) J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out. Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

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

Thomson’s Experiment voltage source ON - OFF +

- Thomson’s Experiment + voltage source + - OFF + + - 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 charges- are 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 MODERN ALCHEMY “Ernest Rutherford (1871-1937) was the first person to bombard atoms artificially to produce transmutated elements. The physicist from New Zealand described atoms as having a central nucleus with electrons revolving around it. He showed that radium atoms emitted “rays” and were transformed into radon atoms. Nuclear reactions like this can be regarded as transmutations – one element changing into another, the process alchemists sought in vain to achieve by chemical means.” Eyewitness Science “Chemistry” , Dr. Ann Newmark, DK Publishing, Inc., 1993, pg 35 When Rutherford shot alpha particles at a thin piece of gold foil, he found that while most of them traveled straight through, some of them were deflected by huge angles. 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.