Basic Atomic Theory Page

by Bud Schultz

This lesson was created as a part of the SMART website and is hosted by the Illinois Institute of Technology


Objective: To learn some basic facts about atomic structure and its history.

The Atom

Where did the idea come from?

The Greeks Democritus & Leucippus around 400 BC proponed that everything was made of very very small particles they called Atoms from the Greek word Atomos. Now remember that these guys had more written however the library at Alexandria burned and many older manuscripts were lost.

Peruse through this article: The Atomistic Philosophy of Leucippus and Democritus.

So now that you have an idea where they came up with the idea of Atoms, you have to wonder what?

a.) Where dinner is coming from? (smile)

b.) Where did these atoms come from?

c.) How do you combine them to make more marshmallow fluff. (Smile again, but remember some man    actually did this for a living.)

d.) How do you find out about something that is so small that it is impossible to see? (pick this one)

Leucippus and Democritus were great at guessing and observing the universe around them. They formulated these ideas with little experimental data to go on, just their everyday senses. Now the world was wrapped up into a nice neat package...or so we thought until some meddling physicists came along.

The meddling physicists enter the picture...

J. J.Thompson was playing with a cathode ray tube and wondered what the particles that were being emitted were. He decided they must be some part of the atom and he made a new model of the atom but with some flaws. J. J.Thompson looked at the atom as a small version of plum pudding, as in the components making up an atom are similar to a plum pudding, that is to say bits are encased and statically held in a medium. There are no clumps and lumps just a uniform goo with occasional bits of other stuff. The model seemed to be acceptable for the moment until Ernest Rutherford Came along. Ernest Rutherford found out that the atom was not the be all end all of matter or of physics. He came to realized that the atom was divisible and that it was therefore made up of component parts.

The question now was, what are these parts, how do they all fit together and how do you “see” them? Just how does one determine what an atom is made of when its way too small to see?

Of Bullets and watches...

A teacher of mine once had a great analogy.

There was once a man who wanted to know how a fine Swiss watch worked. He had a machine gun (a high energy particle, Alpha, beta, etc...) a camera (a camera or cloud chamber etc...) and an endless supply if Swiss watches (any material, atoms, a.k.a. targets). He placed the watches on a clothesline type conveyor belt. He lined the gun up so the watches are in the sights and adjusted the camera to photo any bullets hitting the watches. When the bullets fly the pictures snap and are developed. He looks at the photos and discovers how to build a complicated Swiss watch. We cannot see an atom. We cant even snap a photo if it, they are too small. We can infer what's happening by reactions that particles have with other objects though. Such as the trails left in Charles Wilson's experimental cloud chamber.  Wilson was studying cloud formation at the time.  Wilson was just doing what most guys at the time did, using observations made in one discipline to determine what is going on in another. In other words, they just came upon it and realized what they had.

Now at the time they realized they had two charges a positive and negative but no idea of the magnitude of the charge.  Along comes the hero of the day, Robert A. Millikan.

Millikan raided his moms perfume container and performed a wonderful experiment, read about his Oil Drop Experiment..

Now there are always ancidotes about these kind of guys, Millikan was no different.  He apparently upset a few grad students with his taking credit for their work.  On a billboard on the outskirts of town where he worked there was a billboard reading " God created the world in six days..."  The disgruntled student wrote in red paint, "but Millikan took the credit".

Now we had an atom with a massive center and a set charge, but no set form.  Along comes Neils Bohr and Max Plank who change the model of the atom.

Now the atom was explained.  The atom looked like a small solar system according to Bohr and was quantized according to Plank. ..or so we thought.

Take the short quiz for this section:

Smile and have fun with these.

Pick the analogy which best describes what Rutherford found.

a.) A thousand bullets fired from a gun hits a piece of paper and one bullet bounces straight back off the paper.
b.) A thousand bullets fired from a gun hits a piece of paper and fly straight through the paper.
c.) A thousand bullets fired from a gun hits a piece of paper and gives all of their energy to the paper.
d.) A bullet fired from a gun hits a piece of paper and bounces straight back off the paper leaving no hole in the paper.
e.) A bullet fired from a gun hits a piece of paper and bounces straight back off the paper leaving a hole in the paper.

Who was the first scientist to make a nuclear reaction?

a.) J.J. Thompson

b.) Ernest Rutherford

c.) Albert Einstein

d.) Democritus

e.) Niels Bohr


What did J.J. Thompson discover?

a.) The atom

b.) The proton

c.) The neutron

d.) The electron


Robert Millikan came along and discovered

a.) The charge on an atom.

b.) The charge on an oil drop.

c.) The exact value of pi.

d.) The fundamental electrical charge.


Who came up with the idea of quantization of an electrons orbit?

        a.) J.J. Thompson

        b.) Ernest Rutherford

        c.) Albert Einstein

        d.) Robert Millikan

        e.) Neils Bohr


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