1. The History of Atomic Theory

2. The discovery of the atom as we know it today was a progression.
Many scientists contributed to the development of modern day atomic theory.

3. Democritus (~460 BC ~370 BC) Greece
Believed all matter was composed indivisible particles
“atomos”
Ideas were rejected by society (but correct)

4. Aristotle (~322 BC) Greece
Believed everything in universe was composed of four elements: earth, fire, water, air
Ideas accepted by society (but incorrect)

5. John Dalton ( )
England
Chemist

6. Dalton’s Atomic Theory
All matter is composed of atoms. Atoms are indivisible (didn’t know about protons, neutron, and electrons).
All atoms of an element are identical (didn’t know about isotopes).
Compounds are formed by two or more types of atoms (still true).
A chemical reaction is rearrangement of atoms (still true).

7. Dalton’s Billiard Ball Model of Atom
Solid sphere

8. J. J. Thomson ( )
England
Physicist

9. In 1897, Thomson discovered the electron while studying cathode rays using cathode ray tubes

11. Thomson’s Plum Pudding Model of Atom

12. Robert Milliken ( )
USA
Physicist

13. Milliken’s Oil Drop Experiment
1909
Discovered the charge on an electron

15. Ernest Rutherford ( )
New Zealand
Physicist

16. Rutherford’s Gold Foil Experiment
1911
Fired alpha particles at gold foil and measured deflection
Surprised most particles went straight through while a few bounced back
Discovered nucleus
Results not consistent with plum pudding model

17. Rutherford’s Gold Foil Experiment

18. Rutherford’s Nuclear Model of Atom
electrons circling in fixed orbits

19. Niels Bohr ( )
Denmark
Physicist

20. Bohr’s Planetary Model of Atom
Electrons move in fixed orbits at different distances from nucleus

21. Erwin Schrodinger ( )
Austria
Physicist

22. Schrodinger’s Electron Cloud Model of Atom
What we use today
Protons and neutrons located in small dense nucleus
Electrons move about nucleus in cloud

23. James Chadwick ( )
England
Physicist

24. In 1932, discovered neutrons