1. Development of Atomic Theory In 440 BC, Democritus, a Greek Philosopher theorized that if you cut an object in half, then cut that one in half, you would eventually end up with a particle that could not be cut. He called this an atom. From the Greek word Atomos, meaning “not able to be divided.”

2. Development of Atomic Theory In 1803 British chemist John Dalton published his atomic theory. In 1803 British chemist John Dalton published his atomic theory. All substances made of atoms, which are small, cannot be created or divided or destroyed. All substances made of atoms, which are small, cannot be created or divided or destroyed. Atoms of same element are alike; different elements made of different atoms. Atoms of same element are alike; different elements made of different atoms. Atoms join with different atoms to form new substances. Atoms join with different atoms to form new substances.

3. Development of Atomic Theory In 1897, the British scientist J.J. Thomson discovered small particles inside of atom. In 1897, the British scientist J.J. Thomson discovered small particles inside of atom. Negatively charged particles he called corpuscles (electrons). Negatively charged particles he called corpuscles (electrons). Cathode ray tube experiment. Cathode ray tube experiment. Plum-pudding model: Positive particles mixed with negative particles. Plum-pudding model: Positive particles mixed with negative particles.

4. Development of Atomic Theory J J Thomson: Cathode ray tube experiment

5. In 1909, Earnest Rutherford, a former student of Thomson performed “gold foil experiment.” http://www.mhhe.com/physsci/chemistry/esse ntialchemistry/flash/ruther14.swf In 1909, Earnest Rutherford, a former student of Thomson performed “gold foil experiment.” http://www.mhhe.com/physsci/chemistry/esse ntialchemistry/flash/ruther14.swf http://www.mhhe.com/physsci/chemistry/esse ntialchemistry/flash/ruther14.swf http://www.mhhe.com/physsci/chemistry/esse ntialchemistry/flash/ruther14.swf 1911 published revised atomic theory. 1911 published revised atomic theory. Nucleus: tiny, dense, positively charged center of atom where most of the mass is located. Nucleus: tiny, dense, positively charged center of atom where most of the mass is located. Development of Atomic Theory

6. 1913 Neils Bohr, Danish scientist, who worked with Rutherford. 1913 Neils Bohr, Danish scientist, who worked with Rutherford. Electrons traveled in definite paths at specific distances from the nucleus. Electrons traveled in definite paths at specific distances from the nucleus.

7. Development of Atomic Theory Austrian physicist Erwin Schr ö dinger & German physicist Werner Heisenberg Austrian physicist Erwin Schr ö dinger & German physicist Werner Heisenberg Electrons do not move in definite paths. Electrons do not move in definite paths. Electron paths cannot be predicted. Electron paths cannot be predicted. “ Electron cloud ” related to the paths described in Bohr ’ s model. “ Electron cloud ” related to the paths described in Bohr ’ s model.

8. Development of Atomic Theory Parts of an atom Parts of an atom Nucleus Proton Neutron Electron