Isotopes are atoms of the same atomic number, but have dissimilar masses due to the different
numbers of neutrons they contain. The atomic mass of an element is the weighted average of all
its isotope’s masses. The weighted average takes into account both the mass and relative
abundances of each isotope and is measured in the laboratory by an instrument called a mass
spectrometer. The mass spectrometer enables scientists to calculate the atomic mass of an
element by separating its isotopes by mass and measuring their relative abundances.
The purpose of this lab is to carry out an experiment and perform the necessary calculations to
determine the atomic mass of the fictitious element Vegium (vegetable). The three different
isotopes of vegium are beanium, peaium and cornium (beans, peas, and corn). As in real
elements, isotopes are collections of particles having different masses. The objective is to obtain
a sample of vegium and determine the relative abundance and mass of each isotope. Unlike real
isotopes, the individual isotopic particles of vegium differ slightly in mass, so you must first
determine the “average” mass of each isotope before calculating the weighted average mass
(atomic mass) of vegium.
1. Obtain a cup of the vegium.
2. Separate all the beans, all the peas and all the corn seeds.
3. Weigh all the beans, all the peas, and all the corn seeds on an electric balance.
4. Record the weights onto the table.
5. Count all the beans, all the peas, and all the corn seeds.
6. Record the number of beans peas and corn seeds in the table.
7. Place the beans, peas, and corn seeds back into the cup.
1. Calculate the average mass and record the answer onto the table
2. Calculate the percent of each isotope and record it.
3. Calculate the relative abundance
4. Calculate the relative weight
Average mass = total mass of isotope / number of isotope
Percent of each = (number of isotope / total number of isotopes) x 100
Relative abundance = number of isotope / total number of isotopes
Relative weight = relative abundance x average mass
Results (use applicable “Formulas” above)
Beans Peas Corn Total
Total mass of each isotope
Number of each isotope
Average mass of each
Percent of each
Relative weight (Atomic Mass)
Analysis and Conclusion
1. What is the difference between percent and relative abundance?
2. What is the result when you total the percent and the relative abundances?
3. How is the atomic mass and the total average mass of isotopes different?
4. Compare your data with your classmate and explain the difference between the atomic mass
of your vegium sample and that of your classmate. Explain why the difference would be
smaller if larger samples were used.
5. Calculate the percent error of your vegium’s atomic mass using your classmate’s data as the
6. Complete the following table of the fictitious element vegium.
Atomic number Mass number Number of protons Number of Number of