Gold Foil Experiment

| Multimedia | Practice Homework | Homework |

Context

Scientists are constantly trying to model things that they can't see. The first real model of the atom was the raisin pudding (or plumb pudding) model. In 1907 Ernest Rutherford made new observations that were not consistent with the raisin pudding model.

What is the Gold Foil Experiment?

In 1909 Ernest Rutherford directed Hans Geiger and Ernest Marsden in an experiment which would shoot alpha-particles into gold foil and monitor where the alpha-particle ended up on a detector. This has become know as the Gold Foil Experiment (Google Search).

Explanation

Around the same time that the electron was discovered (around 1900) the Curies were doing their work with radioactivity (Google Search). 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 developed the "gun" to shoot the alpha-particles toward the gold foil.

In this experiment Rutherford as assuming that the atoms were as described in the raisin pudding model. With that assumption he predicted that the alpha-particles, which are much larger than the electrons, would go straight though the atoms and hit the detector directly across from the "gun" or source of alpha-particles. The electrons wouldn't provide a surface for the alpha-particles to bounce off of and there is not any other object in the atom, so the alpha-particles should go straight through and hit the detector on the other side.

This section will help you:

• Understand the background of the gold foil experiment.
• Know what the gold foil experiment is.
• Understand how scientific theories are developed.

Model

The only problem with Rutherford's prediction is that, while most of the alpha-particles went straight through, some of the alpha-particles were significantly deflected and some of them bounced back toward the source! The raisin pudding model has no way of explaining this. The raisin pudding model no longer agreed with observations and so it needed to be modified or discarded. There is no way to modify the raisin pudding model to account for these observations and so it had to be discarded.

The observations led them to believe that there must be a hard, dense part of the atom that the alpha-particles could bounce off of. They knew that the electrons were much too small and so they decided that the positive part of the atom must be hard and dense. They envisioned a large positive "nucleus" with the electrons outside of this nucleus. A physical model that they were familiar with that has a large object at the center and smaller objects going around it is the solar system. This is where the solar system model of the atom comes from. They thought that the nucleus was like the sun and that the electrons would orbit around the nucleus like planets around the sun. It was about 1911 when this theory was proposed. I have collected a few pages from http://particleadventure.org that describes this and has some pictures to help see what happened. You may want to view those pages.

A few years later the neutron was discovered and we include them in the nucleus with the protons. You are probably familiar with this solar system model of the atom, but it only lasted about 12 years. New experiments that couldn't be explained with the solar model quickly came along and this model also had to be discarded.

To summarize what we have learned so far and add some details that you have heard before:

Elements are the basic building blocks of all matter. Everything on earth is made from these elements. Atoms are the smallest piece of an element that retains the characteristics of that element. Atoms are composed of protons, neutrons and electrons. The protons and neutrons are almost 2000 times larger than the electrons and reside in the massive nucleus. The electrons are outside of the nucleus, but do NOT orbit around the nucleus like planets around the sun. If the nucleus of a hydrogen atom was the size of the earth, the electron could likely be half way to the sun. There is a lot of space in the atom.

Each element has a distinctive kind of atom. The number of protons in the nucleus determines which element it is. The periodic table allows one to easily determine how many protons are in the atoms of each element by looking at the number that runs consecutively across the table. That number not only numbers the elements, but also designates how many protons there are in the atom of that element. Neutral atoms have the same number of electrons as protons, since the electron has a charge of -1 and the proton has a charge of +1. Note that the number of electrons in an atom can change and that the number of electrons does not identify the element (unless the atom is neutral).

The atomic theory described above has been developed over many years. At one point in history the notion of matter being composed of discrete particles was a radical idea. But more and more evidence came forward to substantiate this view. One theory that was popular for a while was the raisin pudding model. According to this model the atom was composed of a spread-out positively charged cloud with electrons scattered throughout, like raisins (electrons) in pudding (positive cloud). This theory had to be discarded when Rutherford found that alpha particles were scattered by gold atoms. The original Bohr model, which is a planetary model, was popular for many years, but it too was shown to be wrong through experimentation.

Thinking Questions

1. Why was the solar system model of the atom developed?
2. What kind of experimental challenges did Geiger and Marsden have to overcome?
3. How do scientists come up with models for things that they can't see?