How big is an Atom?


Physicists have developed powerful Scanning Tunneling Microscopes that can now photograph individual atoms. From the accompanying photograph, students will measure the size of a typical atom. This photograph was made by Peter Zeppenfeld and Donal Eigler at IBM and is called "Molecule Man" because it was constructed by moving 28 individual molecules of carbon monoxide on a platinum surface. Molecule man stands 50 Angstroms tall, and 25 Angstroms wide!

Purpose:

De-mystify atoms and make students 'see' them as real objects in the world that can be studied and measured.

Procedure:

  1. With a millimeter ruler, measure the size of the vertical edge of the accompanying 'scanning tunneling microscope' image.
  2. Divide the number of millimeters you measured by the actual size of the field of the microscope based on the dimensions of the Molecule Man given in the text above in Angstroms.
  3. Measure 5 or 10 of the round molecule images at a point just where they begin to blend together, but measure them the same way each time.
  4. Average the 5 - 10 measurements together to get the average diameter of the carbon monoxide molecule image in the photograph.
  5. Multiply the molecular diameters in millimeters by the scale factor you computed in Step 1. The answer will be the average diameter of the molecule in Angstrom units. In actuality, the carbon and oxygen atoms are actually stacked up vertically in the photograph so the diameters you have measured are pretty close to the diameter of an individual carbon or oxygen atom!

Assessment:

Students will see that atoms can be measured in a very simple way. Students individual answers may vary, so you might plot the student's answers on a graph with 'Number of Estimates' on the vertical axis, and 'Estimated Size' on the horizontal axis. Discuss the possible reasons for the differences in the answers in terms of how a student decided where the edge of an atom 'ended' in the photograph.


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