2. History of the Atom

3. Democritus Democritus: It was 400 BC when he came up with the idea that matter could not be divided indefinitely. This lead to the idea of an atom in a void. Aristotle Aristotle: In 350 BC he modified an early theory that matter was composed of 4 “elements” These were EARTH, FIRE, WATER, and AIR. fire air water earth He was incorrect but his theory persisted for 2000 years.

4. John Dalton: In 1800 he proposed the first atomic model based on experimentation and not just reason. His experiments lead to important facts concerning the atom: 1.All matter is made of atoms 2.Atoms of an element are identical 3.Each element has different atoms 4.Atoms of different elements combine in a constant ratio to form compounds 5.Atoms are rearranged in a reaction His ideas account for the Law of Conservation of mass. This law states that matter can be neither created nor destroyed but can only change form

5. Joseph Thomson: In 1897 Joseph Thomson discovered another aspect of the atom using a Cathode Ray Tube. The particles in the Cathode Ray were negatively charged and were deflected by both a magnetic and electric fields. This lead to the discovery of the Electron.

6. Ernest Rutherford: In 1910 he used the GOLD FOIL experiment to determine that atoms were mostly empty space and had a dense positive center. This lead to the discovery of the Proton.

7. If the atom were the size of a football stadium The nucleus would be the size of a marble sitting in the middle of the 50-yard line

8. Niels Bohr: In 1913 Bohr introduced a model of the atom where the electron orbits the nucleus in a shell If energy hits the electron it may jump to a higher shells and give of a line spectra.

9. Dalton’s model: The Billiard Ball Model Thomson Model: The Plum Pudding Model Rutherford Model: Atoms are mostly empty space with a positive nucleus Bohr Model: The electrons reside in energy shells at specific energies.