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

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

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

5. Where did the idea originate?
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
water
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. H O

10. Dalton’s Model
Dalton’s Model
An indivisible sphere

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

12. Results
Law of Definite Composition: Compounds have a fixed ratio of component elements.
(CO2 in the U.S. is the same as CO2 in Russia is the same CO2 you breathe out is the same CO2 plants absorb)
Law of Multiple Proportions: Elements can combine in different ratios to make different compounds.
(C3H8O is rubbing alcohol and C12H22O11 is sugar)

13. Now it’s your turn… Read pp. 70-74 about J. J
Now it’s your turn… Read pp 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.