1. The Advent of Modern Atomic Theory Vocabulary: 1. theory 2. Democritus 3. science 4. John Dalton 5. Dalton’s model 6. Dalton’s atomic theory 7. Law of Definite Composition 8. Law of Multiple Proportions “Black Circle” (1913) Kasimir Malevich Russian

2. How do you know that atoms exist? n After all, atoms are “only a theory?” (By the way, “only a theory” is perhaps my least favorite phrase ever!)

3. Theory n A set of statements or principles devised to explain a group of facts or phenomena, that has been repeatedly tested and can be used to make predictions about natural phenomena.

4. n Think about this: ä Imagine that the line below is a piece of iron. ä Could you cut it in half? ä Could you cut the half in half? ä And again? Where did the idea originate?

5. n Is there a point that you could not cut the wire in half and still have the same substance, in this case iron?

6. Where did the idea originate? Around 400 BC, Greek philosopher Democritus reasoned that there must be a point where matter can not be cut in half and still be the same substance; therefore, all matter must be made of indivisible particles that he called “atomos” meaning indivisible.

7. Do you think that Democritus’ idea is scientific? Explain.

8. The first scientific atomic theory John Dalton, 1807 Drew conclusions which became his atomic theory from repeatable observation and experimentation.

9. Dalton’s Model By observing that compounds always decompose to form fixed ratios of the elements in the compounds no matter how much of the compound you started with, Dalton concluded that there must be individual particles (atoms) combining in fixed ratios to form the compounds. HH O water

10. Dalton’s Model Dalton’s Atom

11. Dalton’s Atomic Theory 1. All matter is composed of atoms (still supported.) 2. Atoms of the same element are identical (not supported any more.) 3. Atoms can not be created or destroyed. (This is now called the law of conservation of mass.) 4. Atoms of different elements combine in simple whole-number ratios to form compounds. 5. In chemical reactions, atoms are combined, separated, or rearranged.

12. Results Law of Definite Composition: Compounds have a fixed ratio of component elements. (CO 2 in the U.S. is the same as CO 2 in Russia is the same CO 2 you breathe out is the same CO 2 plants absorb) Law of Multiple Proportions: Elements can combine in different ratios to make different compounds. (C 3 H 8 O is rubbing alcohol and C 12 H 11 O 22 is sugar)

13. Now it’s your turn… Read pp. 70-74 about J.J. Thomson and Ernest Rutherford. Take notes about: Who they were. What they discovered. How they discovered it (and what equipment/experiment they used to do it.) What they thought the atom looked like.