History of Atomic Theory Resources & Practice

Below are two videos that I think are helpful to watch if you don't feel 100% comfortable with the history and scientists of atomic theory.

Ok, let's look at some of the most important scientists and their models.

John Dalton

Dalton's atomic theory was the first complete attempt to describe all matter in terms of atoms and their properties. Dalton based his theory on the law of conservation of mass and the laws of definite and multiple proportions.

His theory had four main parts, or postulates.

JJ Thomson

This video does an awesome job stepping through how JJ Thomson used the Cathode Ray Tube to discover the electron!

Big take aways: Thomson used the Cathode Ray Tube and magnets to discover that atoms have negative charges in them (electrons). He created the Plum Pudding Model -- in which negative electrons were floating around in a positive atom.

Rutherford

Rutherford did the Gold Foil experiment -- if you're not 100% sure how this experiment worked -- I would watch this super helpful video!

Big take away: Rutherford did the Golf Foil experiment (shot alpha particles at gold foil and observed the pattern of where those alpha particles ended up) He discovered that atoms were made of mostly empty space! (Most particles went straight through) He determined that there must be a positively charged nucleus in the center of the atom, with some electrons floating around on the outside.

This is called the "peach model" with a positively charged "pit" a the center

Bohr then went on to say the the electrons aren't just randomly floating around the positive center, they must be in orbits, like so:

And then quantum physicists took it even FURTHER and said we can't ever really know where the electrons are with certainty, we just know where they are most LIKELY to be, but it's kind of a mess, not necessarily perfect rings like Bohr said. That's why we have the electron cloud model now. (Watch first videos on this page to learn more about Bohr's model and the electron cloud model)

Practice

1. Without looking at this page, try to map out a timeline of atomic theory -- with the name of the model and who came up with it.

2. Who discovered the electron? What did he use to discover it? How did he know they were negatively charged?

3. Explain how the Gold Foil experiment led to the Plum Model. Draw pictures to aid in your explanation.

4. Who discovered that atoms are mostly empty space?

5. Define Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple proportions. Read about them in your textbook (pgs. 67-69) for help!

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