Day 17

Context

After looking at matter, elements and physical and chemical changes we now look at models of an atom.

Explanation

Because matter is composed of atoms, it is important to know how atoms are constructed and how they behave.

After this class you should be able to:

• Describe the parts of an atom.
• Describe the first model for the atom.
• Describe the set-up for the Gold Foil experiment.

Our understanding of what is in an atom and how they are oriented.

• What is in an atom?
• How would you draw a picture of the atom?

The first model of the atom - the raisin pudding model

Just before 1900 the existance of the electron was becoming known and during the early 1900's the electron was characterized by a very talented scientist by the name of Robert Millikan who performed the famous Millikan oil-drop experiment which allowed him to determine the charge on an electron. He and others found out that the electron was extremely small and that it has a negative charge.

With the discovery of the electron a model of the atom could be constructed. Since they knew that the electrons were negatively charged and that most materials are neutral, they had to postulate something with a positive charge that would balance the charge of the electron. What they proposed was a fluid-like substance with a positive charge that the electrons could be scattered around in. The electons would be like the raisins in a raisin pudding. The pudding would be the positve part of the atom and the electrons would be suspended around the atom in this positive "pudding" material. This atomic theory first seems to be proposed by J. J. Thompson around 1898 and is called the raisin pudding model. Notice that there isn't any nucleus or other "large" object in this model of the atom, the only particles are the very small electrons.

The gold foil experiment

Around the same time that the electron was discovered (around 1900) the Curies were doing their work with radioactivity. They found that one of the particles that can be given off during radioactive decay is the alpha-particle. The important characteristic of alpha-particles for our purposes is that it is much larger (about 8000 times larger) than an electron.

The Geiger counter, used to monitor radioactivity, is named after Hans Geiger who participated in the gold foil experiment. His ideas would develop the detector for the alpha-particles.

It is thought that Ernest Marsden, who participated in the gold foil experiment, developed a "gun" to shoot the alpha-particles toward the gold foil.

The experiment was then set up so that Marsden's gun would shoot the alpha particles toward the detector developed by Geiger. Between the gun and the detector they placed some gold foil so that the alpha-particles would have to go through the gold foil in order to get to the detector.

What do you expect to happen when they shoot the alpha-particles at the gold foil?

Homework

There is no homework associated with Day 17.