1. Atom Model History
- Democritus a fifth century B.C. Greek philosopher proposed that all matter was composed of indivisible particles called atoms (Greek for uncuttable).
Billiard Ball Model (1803)
- John Dalton viewed the atom as a small solid sphere.
-Dalton developed the first atomic theory model of the atom

2. Atom Model History
- Each element was composed of the same kind of atoms.
- Each compound was composed of different kinds of atoms.
- Chemical reactions are rearrangements of atoms.

3. Atom Model History Plum Pudding Model (1904)
J.J. Thomson thought that an atom consisted of a “pudding” of evenly distributed positive and negative particles.

4. Atom Model History Solar System Model (Rutherford, 1911 and
Bohr, 1913)
Ernest Rutherford discovered that the atom is mostly empty space with a dense positively charged nucleus surrounded by negative electrons by using alpha particles (2 protons and 2 neutrons).
Electrons orbit in “shells” 

5. Atom Model History Discovery of the Neutron (Chadwick, 1932)
Chadwick did an experiment which detected neutral particles. 
Electron Cloud Model
(early 1900’s)
-An atom consists of a
dense nucleus composed of protons and neutrons surrounded by a “cloud” of electrons
-This model best predicts where electrons can be found  

6. Atom Model History Discovery of the Neutron (Chadwick, 1932)
Chadwick did an experiment which detected neutral particles based on mass and charge

7. Atom Model History Electron Cloud Model (early 1900’s)
-An atom consists of a
dense nucleus composed of protons and neutrons surrounded by a “cloud” of electrons
-This model best predicts where electrons can be found
-Based on Heisenberg Uncertainty Principle which states that it is impossible to know both the momentum and location of an electron at the same time

8. Atom Model History Wave Model
(De Broglie and Shrödinger, early 1900’s)
-Electrons exist as both waves and particles
-Shrödinger Wave Equation allows scientists to determine the probable location of an electron based on its energy

9. Atom Model History Subatomic Particles p+ e- n0 proton electron
neutron
Symbol p+ e- n0
Charge +1 -1
Mass 1
1/1837

10. Particle Check
Draw pictures of proton, electron, and neutron that will help you to remember their characteristics.
Include an explanation as to what your picture means. It should describe the charge and the mass of the particle.