1. Atomic Theory Democritus to Rutherford

3. Democritus - Ancient Greece w Philosophical question: Could matter be divided into smaller and smaller pieces infinitely? w Democritus thought that everything in the universe was made of tiny particles called “atomos” (atoms) w “atomos” means indivisible, not to be cut w Democritus was a philosopher, not a scientist. w He never tested his theory

4. John Dalton (1766-1844) We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen

7. w Matter is made up of atoms w Atoms are indestructible They cannot be created or destroyed w Atoms of an element are identical and have same mass* w Atoms of different elements have different mass w Compounds are formed by combining atoms Dalton’s Atomic Theory

8. JJ Thomson’s work w studied electricity w knew that: atoms were neutral negative charges from his electric stream w charged particles from individual atoms w termed the particle “electron” w model had positive material to balance the negative electrons

11. Ernest Rutherford w 1910 - devised a method to test Thomson’s atomic model w Passed alpha particles (positive charged particles) through a piece of gold foil. w He predicted the alpha particles would pass straight through. w 1 in every 20 000 bounced straight back. Some deflected at an angle.

15. Early Theory Summary w Democritus: named “atom” w Dalton - 1808: atomic theory w J J Thomson - 1897: “plum pudding” model, termed “electron” w Ernest Rutherford - 1910: atom with a positive nucleus surrounded by orbiting electrons