1. Atomic Structure

2. Dalton's Atomic Theory
Held up well to a lot of the different chemical experiments that scientists performed to test it. In fact, for almost 100 years, it seemed as if Dalton's Atomic Theory was the whole truth.
As it turns out, Dalton had a lot right.
He was right in saying matter is made up of atoms
He was right in saying there are different kinds of atoms with different mass and other properties
He was "almost" right in saying atoms of a given element are identical
He was right in saying during a chemical reaction, atoms are merely rearranged
He was right in saying a given compound always has atoms present in the same relative numbers.
But he was wrong in saying atoms were indivisible or indestructible.

3. J.J. Thompson
In 1897, a scientist named J. J. Thompson conducted some research which suggested that Atomic Theory wasn’t the entire story.
As it turns out, atoms are divisible. In fact, atoms are composed of smaller subatomic particles.

4. Discovering Electrons
In the mid-1800s, scientists were beginning to realize that the study of chemistry and the study of electricity were actually related.
First, a man named Michael Faraday showed how passing electricity through mixtures of different chemicals could cause chemical reactions.
Shortly after that, scientists found that by forcing electricity through a tube filled with gas, the electricity made the gas glow!
Scientists didn't, however, understand the relationship between chemicals and electricity until a British physicist named J. J. Thomson began experimenting with what is known as a cathode ray tube.
Cathode Ray Tube Experiment

5. Cathode Ray Tube Experiment

6. Cathode Ray Tube Experiment

7. J. J. Thomson made a radical proposal:
Maybe atoms are divisible!
J. J. Thomson suggested that the small, negatively charged particles making up the cathode ray were actually pieces of atoms.
He called these pieces "corpuscles", although today we know them as "electrons".
Thanks to his clever experiments and careful reasoning J. J. Thomson is credited with the discovery of the electron.

8. Thomson's Model of the Atom
Thomson's "plum-pudding" model for the atom. Notice how the "plums" are the negatively charged electrons, while the positive charge is spread throughout the entire pudding batter.

9. Thomson's Model of the Atom
Negative electrons = Pieces of Fruit-Plums (chocolate chips)
Positive material = Pudding (cookie dough batter)
How this fits Thompson’s Observations:
Isolated electrons using a cathode ray tube
Never managed to isolate positive particles
Result: Thomson theorized that the positive material in the atom must form something like the "batter" in a plum pudding, while the negative electrons must be scattered through this "batter".

10. Disproving the "plum pudding" model
A man named Ernest Rutherford used alpha particles (Helium atoms that lost their electrons and had a positive charge)as tiny bullets to fire at all kinds of different materials.
One experiment in particular, however, surprised Rutherford, and everyone else.
Fired alpha particles at a very thin piece of gold foil
Almost all of the alpha particles went straight through the foil as if they'd hit nothing at all
Every so often, though, one of the alpha particles would be deflected slightly as if it had bounced off of something hard
Structure of the Atom: The Rutherford Model

11. Discovering the Nucleus
Rutherford Observation
Most alpha particles passed through the gold foil without hitting anything at all
Once in a while, though, the alpha particles would actually collide with a gold nucleus, causing the alpha particles to be deflected, or even to bounce right back in the direction they came from.
Rutherford Explanation
The positive matter forming the gold atoms was not distributed like the batter in plum pudding
The positive matter was concentrated in one spot
forming a small positively charged particle somewhere in the center of the gold atom.
We now call this clump of positively charged mass the nucleus.

12. What about our “Batter” of electrons?
Rutherford Suggestion
Electrons might be circling or "orbiting" the positively charged nucleus as some type of negatively charged cloud instead
Not much evidence at this time to suggest exactly how the electrons were held in the atom
Modern Reconstruction of Rutherford's Experiment