Title: Atomic Strcuture - Student pre-conceptions and Historic Experiments

  • Note: There are typos and mispelled words you should find and fix.
  • - fogleman fogleman

Grade level: 11th grade

Purpose: The purpose of this lesson is to introduce the basic concepts around atomic structure. We will explore what they have learned previously, both inside class and outside of class. We will discuss several of the key historic experiments that gave us information about the component parts of the atom, in terms of size, mass and location. We will attempt to place these experiments in historic perceptive and how one what go about exploring the "unseen". We will then explore the most significant of the various models of an atom, explaining the flaws in the models in particular and models in general. We would then end our discussion explaining the level of complexities we need in a model as chemists and what that means in practice.

Learning outcomes: In the first day of the lesson we hope to gather knowledge of the student's beliefs of atomic structure and discuss how scientists from the 18-20 centuries attempted to address this question with a series of historic experiments.

State Standards:
Michigan Chemistry Standards
C4.8 Atomic Structure
Electrons, protons, and neutrons are parts of the atom and have measurable properties, including mass and, in the
case of protons and electrons, charge. The nuclei of atoms are composed of protons and neutrons. A kind of force
that is only evident at nuclear distances holds the particles of the nucleus together against the electrical repulsion
between the protons.

C4.8A Identify the location, relative mass, and charge for electrons, protons, and neutrons.
C4.8B Describe the atom as mostly empty space with an extremely small, dense nucleus consisting of the
protons and neutrons and an electron cloud surrounding the nucleus.
C4.8C Recognize that protons repel each other and that a strong force needs to be present to keep the
nucleus intact.
C4.8D Give the number of electrons and protons present if the fluoride ion has a -1 charge.

An atom is the smallest particle of an element that retains the properties of that element. Atoms are thought of as "building blocks;" however, unlike blocks, they are actually mostly just empty space and do not have walls that bind them to a specific shape. The shape and structure of an atom are very important to chemists as it dictates the ways in which a particular atom interacts or perhaps bonds with other neighboring atoms. An atom is made up of electrons, protons, and neutrons, all of which have measurable properties. Measurable properties are characteristics that describe these atom parts. Examples include mass and charge/lack of charge. Protons and neutrons have a mass of 1 atomic mass unit; electrons are often not considered to have mass because the mass of an electron is so much smaller than the mass of a proton or neutron. The tiny mass of an electron does not have any measurable effect on many calculations and experiments. Atoms contain nuclei, which are protons and neutrons; protons, by convention, have a +1 charge and neutrons have no charge. Because protons are positively charged, they repel each other. Even though they repel each other, they, along with neutrons are held together due to a strong force that is only apparent at the nuclear level. The nucleus is surrounded by an electron cloud, a mostly-empty space where electrons are most likely to be found. With respect to the overall size of the atom, the nucleus is extremely small and dense. An electron, by convention, has a charge of -1. Because electrons are negatively charged, they repel each other. Neutral atoms do not have a charge because there are equal numbers of protons and electrons; however, ions can be formed with the addition or loss of electrons. Ions are classified as either anions or cations. Anions have more electrons than protons in the atom, thus having a negative charge. Cations have more protons than electrons in the atom, thus having a positive charge.

Instructional Activities
1. Engage the studnets in a discussion about the structure of the atom, stressing what they beleive they know rather than trying to get them to recallwhat they have learn. In Chemistry at Saline High School, they had to have previously had Science 2 which goes over the structure of the atom. While they should have been introduce to orbital theory, many students retain the Bohr model of the atom while others remember the plum pudding model. It will be important to recruit this information without judging it before we launch into modern theories.

2. The second part is begin a discussion about the history of research on atomic particles begining with J. J. Thomson and the cathode-ray tube and Millikan's oil drop experiment.

The two plates about midway in the CRT were connected to a powerful electric battery thereby creating a strong electrical field through which the cathode rays passed. Thomson also could use magnets, which were placed on either side of the straight portion of the tube just to the right of the electrical plates. This allowed him to use either electrical or magnetic or a combination of both to cause the cathode ray to bend.
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The amount the cathode ray bent from the straight line using either the electric field or the magnetic field allowed Thomson to calculate the e/m ratio.

What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this unit charge.

Here's how it worked. Have a look at the apparatus he used:

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An atomizer sprayed a fine mist of oil droplets into the chamber. Some of these tiny droplets fell through a hole in the upper floor. Millikan first let them fall until they reached terminal velocity. Using the microscope, he measured their terminal velocity, and by use of a formula, calculated the mass of each oil drop.

Next, Millikan applied a charge to the falling drops by illuminating the bottom chamber with x-rays. This caused the air to become ionized, and electrons to attach themselves to the oil drops.
By attaching a battery to the plates above and below this bottom chamber, he was able to apply an electric voltage. The electric field produced in the bottom chamber by this voltage would act on the charged oil drops; if the voltage was just right, the electromagnetic force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air.


The discovery of the proton was more gradual. Using hydrogen in a modified cathode ray tube , rays were f\discovered to be traveling in the direction opposite to that of electrons, Thompson later showed that they consisted of particles. Somewhere between Thomson and Chadwick, physicists realized that there are positively charged constituents of the nucleus, which we call 'protons'. The way this happened was a gradual process, and that is why it is hard to say exactly who discovered the proton, although if you had put a name against it, it would be Ruthford, sort of.

After the discovery of the electron, it was realized that there must be positive charge centers within the atom to balance the negative electrons and create electrically neutral atoms. Rutherford's discovery of the nucleus demonstrated that these positive charges were concentrated in a very small fraction of the atoms' volume. It was apparent that the hydrogen nucleus played a fundamental role in atomic structure, and by comparing nuclear masses to charges, it was realized that the positive charge of any nucleus could be accounted for by an integer number of hydrogen nuclei. By the late 1920's physicists were regularly referring to hydrogen nuclei as 'protons'. The term proton itself seems to have been coined by Rutherford, and first appears in print in 1920.

Ernest Rutherford** publishes his atomic theory describing the atom as having a central positive nucleus surrounded by negative orbiting electrons. This model suggested that most of the mass of the atom was contained in the small nucleus, and that the rest of the atom was mostly empty space. Rutherford came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide (a scintillator). Rutherford found that although the vast majority of particles passed straight through the foil approximately 1 in 8000 were deflected leading him to his theory that most of the atom was made up of 'empty space'.

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In 1920, Lord Ernest Rutherford postulated the existence of a neutral particle, with the approximate mass of a proton, that could result from the capture of an electron by a proton. This postulation stimulated a search for the particle. However, its electrical neutrality complicated the search because almost all experimental techniques of this period measured charged particles.
In 1928, a German physicist, Walter Bothe, and his student, Herbert Becker, took the initial step in the search. They bombarded beryllium with alpha particles emitted from polonium and found that it gave off a penetrating, electrically neutral radiation, which they interpreted to be high-energy gamma photons.

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The Joliot-Curies in their laboratory (Courtesy of the American Institute of Physics)
In 1932, Irene Joliot-Curie, one of Madame Curie’s daughters, and her husband, Frederic Joliot-Curie, decided to use their strong polonium alpha source to further investigate Bothe’s penetrating radiation. They found that this radiation ejected protons from a paraffin target. This discovery was amazing because photons have no mass. However, the Joliot-Curies interpreted the results as the action of photons on the hydrogen atoms in paraffin. They used the analogy of the Compton Effect, in which photons impinging on a metal surface eject electrons. The trouble was that the electron was 1,836 times lighter than the proton and, therefore, recoiled much more easily than the heavier proton after a collision with a gamma photon. We now know that gamma photons do not have enough energy to eject protons from paraffin.

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The Compton Effect
When James Chadwick reported to Lord Rutherford on the Joliot-Curies’ results, Lord Rutherford exclaimed, "I do not believe it!" Chadwick immediately repeated the experiments at the Cavendish Laboratory in Cambridge, England. He not only bombarded the hydrogen atoms in paraffin with the beryllium emissions, but also used helium, nitrogen, and other elements as targets. By comparing the energies of recoiling charged particles from different targets, he proved that the beryllium emissions contained a neutral component with a mass approximately equal to that of the proton. He called it the neutron in a paper published in the February 17, 1932, issue of //Nature//. In 1935, Sir James Chadwick received the Nobel Prize in physics for this work.
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Chadwick's Apparatus

  • How will you determine what your students know about electrons? How will you raise their curiosity so that you can allow them to learn this story? If you just tell them, it could be extremely boring. You have to include them in the process somehow. What about modeling the process of answering questions about an experiment and then having them work in groups on their own presentations? You could use a jigsaw approach, redistributing the groups and having each person present their group's work to another small group.
  • - fogleman fogleman **