The Advent of Modern Atomic Theory

Vocabulary:1. theory2. Democritus3. science4. John Dalton5. Dalton’s model6. Dalton’s atomic theory7. Law of Definite Composition8. Law of Multiple Proportions

“Black Circle” (1913) Kasimir Malevich

Russian

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!)

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.

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?

Where did the idea originate?

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?

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.

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

The first scientific atomic theoryJohn Dalton, 1807

Drew conclusions which became his atomic theory from repeatable observation and experimentation.

Dalton’s ModelBy 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 H O

water

Dalton’s Model

Dalton’s Atom

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.

ResultsLaw 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 C12H11O22 is sugar)

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.