1. Development of the Atomic Theory Matter Unit

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3. 460 B.C. – Democritus Democritus believed that all matter was composed of atoms that could be divided no further.

4. 350 B.C. - Aristotle Modified an earlier theory that matter was made of four “elements”: earth, fire, water, air. This theory was wrong. However, his theory persisted for 2000 years.

5. 1800 – John Dalton Dalton's Atomic Theory: Matter is composed of small particles called atoms. All atoms of an element are identical, but are different from those of any other element. During chemical reactions, atoms are neither created nor destroyed, but are simply rearranged. Atoms always combine in whole number multiples of each other. For example, 1:1, 1:2, 2:3 or 1:3.

6. 1897- J.J. Thomson Thomson’s 'Raisin in the Pudding' model of the atom He discovered the electron and proposed a model for the structure of the atom. Positive Sphere Negative Charges

7. Discovery of the Electron

8. 1910 – Ernest Rutherford

9. Rutherford’s Gold Foil Experiment

10. Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target?

11. The Answers Target #1Target #2

12. Rutherford’s Findings  The nucleus is small  The nucleus is dense  The nucleus is positively charged “Like howitzer shells bouncing off of tissue paper!” Conclusions:

13. 1912 – Neils Bohr Bohr came up with two rules which agreed with his experiment: RULE 1: Electrons can orbit only at certain allowed distances from the nucleus. RULE 2: Atoms radiate energy when an electron jumps from a higher-energy orbit to a lower-energy orbit. Also, an atom absorbs energy when an electron gets boosted from a low- energy orbit to a high-energy orbit.

14. Quantized energy levels Electron moves in a circular orbit Electron jumps between levels by absorbing or emitting photon of a particular wavelength Bohr’s Finding:

15. Practice Bohr Diagrams

16. James Chadwick 1932 Discovered the Neutron.

17. Quantum Mechanical Model Electrons are located in specific energy levels. There is no exact path around the nucleus. The model estimates the probability of finding an electron in a certain position.

18. The Electron Cloud  The electron cloud represents positions where there is probability of finding an electron.  The cloud is less dense where there is less probability of finding an electron.

19. Modern Atomic Theory Element have a characteristic average mass which is unique to that element. Atoms cannot be subdivided, created, or destroyed in ordinary chemical reactions. However, these changes CAN occur in nuclear reactions! All matter is composed of atoms Atoms of any one element differ in properties from atoms of another element