1. History of Atomic Models
The Atom

2. Early Greeks Believed matter could be broken into four categories:
Fire (Plasma)
Earth (Solid)
Water (Liquid)
Air (Gas)

3. Democritis More than 2400 years ago. Philosopher- did no experiments.
“can matter be broken down forever or is there a limit”
Called the smallest pieces ATOMOS (atom) meaning “not to be cut”.

4. Democritis Conclusions: Atoms are small, hard particles
Atoms are made of the same material that differs in size and shape.
Atoms are infinite in number, always moving, and able to join.

5. John Dalton Early 1800’s
Found that gases combine as if individual particles
Chemist who did experiments, had an interest meteorology and studied air and properties of gases.

6. John Dalton Early 1800’s Conclusions:
All elements are composed of atoms which are indivisible particles.
Elements contain atoms that are all the same for that element.
Atoms differ for different elements.
Compounds are formed by joining the atoms of two or more elements.

7. J J Thomson 1897
First hint that the atom is made of smaller particles.
He sent an electric current through a gas. The gas gave off rays that Thomson showed were negatively charged.
To this point in time, the uncharged atom was the only particle known.
Thomson concluded that a particle smaller than the atom had to exist.

8. J J Thomson 1897
Thomson called the negatively charge particles “corpuscles” now called electrons.
Thomson model had a problem. The atom is known to have a neutral charge and now a negative particle is known to exist. The solution was that a positive particle has to exist to balance the negative charge. Thomson was unable to prove that the positive particle existed.

9. JJ Thomson 1897

10. J J Thomson 1897
‘Plum Pudding‘ model. Plum pudding is a rich, boiled or steamed pudding made with flour, suet, raisins, currants, citron and spices.

11. Earnest Rutherford 1908
Rutherford was a British physicist who tested Thomson’s Model.
Gold Foil experiment.
Proposed that the atom has a small, dense, positively charged center called the nucleus.

12. Rutherford 1908

13. Rutherford 1908
The nucleus is tiny as compared to the whole atom. If a marble sits in the center of Heinz Field, the marble represents the nucleus while the stadium represents the whole atom.
The atom is mostly empty space.

14. Neils Bohr 1913 Bohr improved the Rutherford model.
Definite orbits like planets around the sun.
Orbits are energy levels, of which there are 7.

15. Werner Heisenberg 1926
Heisenberg was a German Physicist who proved that the position of the electron can never be exactly known.
Regions are known as orbitals where electrons might be found.
Electrons can jump from orbital to orbital.
Electron Cloud.