Make your own free website on

Atomic Theory

Isotopes and Atomic Mass Lab, or Beanium Lab

In this lab you will carry out experiments and perform the necessary calculations to determine the atomic mass of the fictitious element beanium. The three different elements are blackium, brownium, and whitium. As in real elements, these isotopes are collections of particles having different masses. Your job will be to obtain a sample of beanium and determine the relative abundance of each isotope and the mass of each type of particle. From this data you will calculate the massed average mass, or atomic mass, of beanium. Unlike real isotopes, the individual isotopic particles of beanium differ slightly in mass, so you will determine the average mass of each type of isotopic particle. Then you can calculate the massed average mass, or "atomic mass" of beanium.

Before the lab:
1) Write an introduction/background section in your lab book. Be sure to include what isotopes are(with an example), why an atomic mass is a decimal, define relative abundance, and how the atomic mass is found if the abundances are known of each isotope.
2) Prepare a data table which will have these headings across the top: Blackium, brownium, whitium, and total. The rows will be labeled along the side as follows: Mass of each isotope, number of each isotope, average mass of each, and percent abundance. Then make a separate data table for step #3 of the procedure.


  1. Count the number of each color of bean. Make sure your number matches that on the board.
  2. Weigh all the black beans. Record the mass. Do the same with the brown and white beans.
  3. Select three black beans from your sample--the largest, the smallest, and one that appears to be "average" in size. Determine the mass of each of the three.
  4. Place all the beans back in the container.

Data Analysis:
Calculations: Show one sample of each calculation. Remember significant digits for all calculations.

  1. Average Mass: Divide the total mass of the black beans by the number of black beans. Do the same for the brown and white beans.
  2. Percent abundance: Divide the number of each isotope by the total number of particles (beans). Multiply this by 100 to get percent. (The sum of the three percent abundances should equal 100. Why?)
  3. Atomic Mass: Use the percent abundances and the average masses in the atomic mass equation.
    (Atomic mass = (% of isotope #1/100)(mass of isotope #1) + (% of isotope #2/100)(mass of isotope #2) + . . .

Questions: (Answer these in your data analysis section.)

  1. Which pieces of your data must be measured and which can be calculated?
  2. Explain any differences between the atomic mass of your beanium sample and that of a different lab group. Explain why the difference would be smaller if larger samples were used.
  3. Compute the average mass of the largest and smallest black beans from step 7. Compare this average to the mass of the "average" black bean and to the average mass of all the black beans. Which average mass is most reliable? Why?

Evaluation: As usual.
Conclusion: As usual.


Return to Chemistry 20