1. Atomic Model History

2. Early Ideas
Greeks thought that there were 4 elements that made up ALL matter:
air, earth, fire, and water
Democritus – first to suggest that all matter was made up of some tiny, fundamental particle.
Called this particle “ATOMOS” – meaning “indivisible”

3. John Dalton
Very first atomic theory (what atoms were made of and how they behaved)
Viewed atoms as tiny, solid spheres
“Billiard Ball” Model

4. Dalton’s Atomic Theory
Atoms are tiny, indivisible particles
All atoms of one element are the same.
Atoms of different elements are different.
Atoms of different elements combine to form compounds.

5. J.J. Thomson “Chocolate Chip Cookie” Model
(or “Plum Pudding”)
Cathode-ray tube experiments
Discovered electrons (particles with a negative charge)
Was the first to show that there are smaller pieces that make up an atom.

6. Ernest Rutherford
Gold Foil Experiment – positively charged alpha particles were passed through a thin sheet of gold foil.
Most particles continued through on a straight path
Discovery: the atom is made of mostly empty space

7. Gold Foil Experiment (cont.)
Ernest Rutherford
Gold Foil Experiment (cont.)
Some particles were deflected at large angles
Discovery: atoms have a very dense positive (+) center, called the nucleus
“Peach” Model
(Positive nucleus and mostly empty space)

8. Niels Bohr
Electrons move around the nucleus in specific layers, or shells.
Chemical properties of each element are determined mainly by the number of electrons in the outermost shell

9. James Chadwick Discovered neutrons
Particles with no charge found in the nucleus

10. Schrödinger & Heisenberg
Modern Atomic Model
Schrödinger & Heisenberg
Electrons move around the nucleus in a cloud
It is impossible to know the exact location of an electron. (There are 3-D areas that they have a probability of being found in – more on this later… )
“Quantum Mechanical”
Model