Mini-Lab 1-01: Foiled Again
Purpose:
In this activity, you will discover that a few simple measurements can lead you to
information about individual atoms!
Think about this:
Avogadro’s number: 6.022 x 1023
Density = mass/volume
Aluminum weighs 26.98 g/mol
The standard deviation of a group of 3 or more measurements of the same quantity will
give you an estimate of the uncertainty in the average.
Hint: cut a square of aluminum foil
Safety
1)
2)
3)
4)
Don’t eat or drink anything in the lab.
Always wear eye protection.
Wear protective clothing (lab coats, etc.).
Don’t play around – treat the lab with respect.
Questions
1) What’s the density of Aluminum?
2) How thick is a sheet of aluminum foil…
a. …in centimeters?
b. …in inches?
3) Estimate the volume of a single atom of aluminum.
4) How thick is a sheet of aluminum foil in numbers of atoms?
5) What assumptions do you have to make in order to come up with an answer to the
above questions?
6) OPTIONAL QUESTION: How well do you really know these numbers? Pick
one of your answers for questions 1), 2), 3) or 4) and calculate the uncertainty in it
based on your measurements.
WAIT! Do not write down an answer to the Final question until your Instructor
tells you to.
7) FINAL: What’s the distance between the centers of two neighboring aluminum
atoms?
Instructor’s Page
1-01: Foiled Again
Source: SCALE-UP
Concepts: measurement, dimensional analysis, atomic-level dimensions
Materials: Aluminum slugs, graduated cylinders, rulers, balances, foil, scissors (have
them cut a small square of foil with the scissors)
Hints: Once they determine the density of aluminum and the mass of the piece of foil,
they can calculate numbers of moles. After calculating the volume (with density and
mass), they can get the thickness and also calculate atoms/cm3, by assuming that the
atoms are packed in some fashion. The easiest assumption is that they are cubes
packed side-by-side in three dimensions. According to Webelements
(http://www.webelements.com/webelements/elements/text/Al/radii.html) the
internuclear distance in Aluminum is 2.86 Angstroms. Another necessary assumption
is that the foil is of equal thickness, but that’s probably a pretty good assumption to
make. This could be confirmed by use of a micrometer, if desired.
Calculating the uncertainty in their answer should be kept at a level appropriate for
the student. At the simplest level, the uncertainty in the density will be approximately
equal to the uncertainty in their volume measurement (as a percentage), since most
lab balances will give much less uncertainty than a graduated cylinder.
Discussion Ideas: Packing of atoms, actual size of an aluminum atom – how well did
they do in their estimation? What changes in their procedure, reasoning or
assumptions could have led them to a better estimate? What evidence is there that
there really are “atoms” in the first place (i.e. – Rutherford’s gold foil experiment)?
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