1. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Democritus of Abdera, about 460-370 BCE Democritus was a Greek philosopher who theorized that all matter was made of invisible particles called atoms.

2. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory His views contrasted those of Aristotle, who believed in the four elements; earth, water, air, fire.

3. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Most of our knowledge of Democritus comes from negative remarks about him in others’ writings.

4. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory John Dalton 1766-1844 Dalton, a British chemist and teacher, did studies and experiments in weather, colorblindness, and gases.

5. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory He noticed that elements combine in specific proportions to form compounds, and theorized that their atoms combine at the same proportions

6. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Joseph John “J.J.” Thomson 1856-1940 Thomson’s experiments using a cathode-ray tube showed that smaller particles make up atoms

7. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Thomson received the Nobel Prize in physics in 1906 for his discovery of the electron.

8. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Ernest Rutherford 1871- 1937 Rutherford, a former student of Thomson’s from New Zealand, tested his teacher’s theories in his Gold Foil Experiment. He expected his alpha particles to go straight through the foil, and most of them did.

9. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Rutherford’s Gold Foil Experiment But some of the particles were deflected or bounced straight back! This showed that a nucleus with a positive charge makes up the center of an atom.

10. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Niels Bohr 1885-1962 Bohr, a Danish scientist who worked with Rutherford, described the motion of electrons around the nucleus.

11. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Bohr’s Atomic Model Bohr said that electrons orbit the nucleus at specific energy levels, and can move from one level to another.

12. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory To do this, Bohr said, the electrons must absorb or release energy, often in the form of light.

13. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Schrödinger & Heisenberg Erwin Schrödinger and Werner Heisenberg’s work with the uncertainty principle explained that electrons do not travel in orbits. In fact, the exact path of a moving electron cannot be predicted.

14. © Fall 2005, Pflugerville ISD, 8th GradeUnit A : Chapter 2 Development of the Atomic Theory Electron Cloud Model The current atomic theory states that there are regions inside an atom where electrons are likely to be found. These regions are called electron clouds.