During the lesson, emphasize how mass spectrometry helps scientists understand the type of atomic particles given off and the mass of these particles.

• Ask students questions and listen carefully to their answers during the activity involving the physical model of the mass spectrometer to determine if they understand how the instrument works and why it is important.
• Informally assess through your anecdotal observations and their answers to the two questions if students are able to summarize what they have learned.
• Use the two questions that students turned in to see if they understand how the mass spectrometer works and how it can be used to find average atomic masses.
• Use the following checklist to evaluate students’ understanding:

o   The student demonstrates a general understanding of the different parts of the mass spectrometer.

o   The student accurately describes how the magnetic field in a mass spectrometer changes the path of charged particles of different mass.

o   The student is able to explain how measurements made with the mass spectrometer can be used to find the average atomic mass.

If additional study is needed, have students read “The Mass Spectrometer” by Jim Clark, (S-C-2-2_The Mass Spectrometer.doc) at http://www.chemguide.co.uk/analysis/masspec/howitworks.html.

Students may also click on links to other topics on the mass spectrometer and learn about how it is used to identify organic compounds. This work may be done individually or with a partner.

Tell students, “We are going to take another look at the Westinghouse Atom Smasher built in 1937. It was the world’s first industrial atom smasher and it was built in Forest Hills, Pennsylvania! The atom smasher was designed to break apart the atom, the foundation stone for matter. The atom smasher was a large, pear-shaped steel tank which stretched about as high as a six-story building. A device inside the unit generated an electric current that accelerated particles of matter through a 40-foot vacuum tube. These particles, moving at 30 to 100 million miles per hour struck target atoms, causing them to disintegrate into smaller particles. Today we are going to learn how the scientists identified the particles that they produced. They used an instrument called a mass spectrometer. This instrument separates ionized particles of isotopes and measures their mass-to-charge ratios by using magnetic fields of different strengths. We are now going to set up a model to show how a mass spectrometer works.”

Place a wooden ramp on a lab table and put three steel balls of different sizes next to the ramp. Then tape two neodymium magnets to the table, one a few inches to the right and slightly behind the ramp, and the other a few inches and about 20 degrees to the right of the ramp. When the steel balls roll down the ramp they should be deflected at different angles. The arrangement should be similar to that shown in the diagram below.

Say to the class, “We are now going to let three steel balls each with a different mass roll down the ramp. May I have two of you to catch the balls as they roll off the table and one to release the steel balls for me?” After the three students are situated at strategic positions around the table, have one of the students place the large steel ball at the top of the ramp and release it. Ask students to observe the angle of deflection. Then have the student release the medium steel ball from the same height on the ramp and have students observe the angle again. Finally have the student release the small steel ball from the same height as the other two steel balls and observe the angle once more. Then ask the class, “Why do you think each steel ball had a different path after it rolled down the ramp?” Write the answers students suggest on the whiteboard. Then say, “The mass of the steel ball is one factor. Also the strength of the magnetic field pulling on the ball is another factor. Now let’s see how all this applies to a real mass spectrometer.” Pass out a color copy of the diagram shown below or draw the diagram on a whiteboard before the class period begins.

Say to the class, “The mass spectrometer makes it possible to sort out atoms differing in mass, even though they may be chemically identical. For example, the mass spectrometer can separate a sample of uranium atoms having an average atomic mass of 238.029 amu into uranium-235 (mass = 235.044 amu) and uranium-238 (mass = 238.051 amu). It separates the atoms of different elements, and also protons from atom fragments.” Refer to the diagram for the following points. Say, “Let’s see how the mass spectrometer works:

1.      Particles formed by bombarding the target atoms enter the mass spectrometer at the top.

2.      Electrons strike the particles ionizing them, giving most of the particles a +1 charge.

3.      The ionized particles, or ions, are accelerated by an electric field.

4.      The ions pass through a magnetic field produced by an electromagnet. The strength of the magnetic field can be controlled by manipulating the current in the electromagnet.

5.      The magnetic field deflects the ions by different amounts.

6.      The ions then hit a detector, where their signal is amplified and recorded.

By manipulating the magnetic field to change the deflection of the various ions, scientists can calculate the mass and number of each particle produced by the atom smasher.”

Extension

• For students requiring extra practice with the standards, have them create a poster of mass spectrometer with a detailed diagram and description of its history, including:

o   a diagram with labels

o   a timeline of invention

o   a statement of the uses and impact on science

• For students performing above and beyond the standards, break students up into groups of two or three. Have them do an Internet activity at computer stations set up around the room or on laptop computers placed on lab tables. Have students go to the Web site: http://www.magnet.fsu.edu/education/tutorials/java/singlesector2/index.html

There they will read about the single-sector mass spectrometer and play with the applet on the page. Tell students, “When you see the applet on the Web page, play with the magnetic field strength and observe what happens to the atoms. Then answer the following questions.” Write the following questions (without the answers) on the whiteboard while students are playing with the applet.

1.      What is the relationship between the magnetic field strength and the mass of the particles? (Answer: The higher the magnetic field strength, the higher the mass of the particles passing through the detector slit.)

2.      How does the ability of the mass spectrometer to count particles of different mass allow scientists to find the atomic mass of an element? (Answer: The mass spectrometer can count atoms of different isotopes of an element and find the relative abundance of each isotope. Given the mass of each isotope and the relative abundance, scientists can then find the atomic mass.)

Have students turn in their answers to the questions just before the end of the class period, evaluate their answers, make comments, and return their papers the following day.