- Democritus first proposed the theory of materialism, that everything is composed of indivisible atoms. Antoine Lavoisier established the law of conservation of mass in 1789, stating that matter can be rearranged but not created or destroyed in chemical reactions. John Dalton created the first tables of relative atomic mass and established the law of definite proportions in 1805.

1. Democritus Abdera 460-370 B.C. He first attempted to create materialism. Everything is composed of atoms .

2. Antione Lavoisier 1772 Total mass of a product of a chemical reaction is always the same thing as the total mass Conservation of mass

3. Law of Conservation of Mass 1789 Matter can not disappear it can only be rearranged At high speeds mass changes to energy

4. Joseph Louis Proust 1798 Created the law of constant composition Constant composition

5. Dalton’s Atomic Theory 1799 Law of constant composition Oxygen and Carbon combined make 2 compounds

6. John Dalton 1803 Recorded the first tables of relative atomic mass Atomic number

7. Law of Definite Proportions 1805 Law say if formula AB is constant then the weight ratio will stay the same Weight ratio stays the same

8. Dmitri Mendeleev 1869 Created the periodic table List of elements

9. Eugene Goldstein 1886 Investigated X-rays Discovered cathode rays

10. Erwin Schrodinger 1887-1961 Created the Schrodinger Wave Equation Used to describe behavior of electrons in atoms

11. Cathode Ray Tube 1893 2 positive leads connected into a neon tube will make it light up Electric makes light

12. J.J. Thompson 1897 Discovered that cathode rays are small steams of atoms An electron has a negative charge

13. Plum Pudding Atomic Model 1897 He said that there has to be a positive and negative charge to balance out an atom Atoms are neutral

14. Ernest Rutherford 1898 Discovered and studied alpha particles and discovered the nucleus Nucleus

15. Marie Curie 1902 Discovered the radio active elements Radio active elements

16. Rutherford Model 1909 Model of a nucleus and electrons Nucleus is in the center and the electrons orbit it

17. Robert Millikan 1911 Made an oil drop experiment Made electron have an electric charge

18. Bohr Planetary Model 1913 He thought that electrons only orbit in one direction They are called stationary orbits

19. Niels Bohr 1913 Considered the term of stationary orbits for the electrons and that there is more than one level of orbit Electron cloud

20. Henry Mosdeley 1913 Numbered the elements Order of elements

21. Quantum Mechanical Model Mathematical equation developed by Schrodinger Calculated where electrons are at any given moment

22. Gold Foil Experiment 1920 It showed that the nucleus is positive and the electrons orbit it

23. James Chadwick 1932 Found the neutron His discovery led to nuclear fission and the atomic bomb

24. Lise Meitner 1966 First discovered nuclear fission