1. Dates are included for a frame of reference.

2.  Around 400 B.C.  Atomos => indivisible  He suggested that all substance in the universe was made up of particles so tiny that nothing smaller was possible.  He regarded those particles as unchangeable and indestructible, and as the only content of the universe.

4.  1803  Atoms = small, dense, spheres  Billiard Ball Model  Atomic Theory

6.  Tiny particles called atoms make up all elements  Atoms of the same element are identical  Compounds are made with whole number ratios of atoms  Chemical reactions are when compounds are broken and reformed

7.  1897  Discovered the electron, and they were negative  Cathode Ray Tube experiment  Plum pudding model  Immobile electrons

9.  1911  Gold Foil Experiment  Discovered atom was mostly empty space  Discovered the nucleus  Dense, positive, small, in center  Atomic Model

11.  1913  Electrons were located in shells (fixed orbits)  Electrons can move to other shells  Planetary Model

13.  1916  Oil-drop experiment  Mass of an electron  Exact charge of an electron

15.  1926  Electron cloud of probability  Electron Cloud Model

17.  1932  Discovered the neutron