Relative Atomic Mass of Candium: Isotopes and Atomic Mass
OBJECTIVES
 Determine the average atomic mass of each isotope of the fictitious element candium.
 Determine the relative abundance of isotopes of candium.
 Calculate from experimental data, the relative atomic mass (Ar) candium.
INTRODUCTION
Isotopes are atoms of the same atomic number having different mass numbers (A) due to different numbers of neutrons. The
average or relative atomic mass (Ar) of an element is the weighted average of the masses of the isotopes of that element. The weighted
average takes into account both the masses and relative abundance of each isotope as it occurs in nature. The relative abundances and
masses of small atomic particles are measured in the laboratory by an instrument called a mass spectrometer. The mass spectrometer
separates particles by mass and measures the mass and relative abundance of each. From these data, a weighted average is calculated
to determine the atomic mass of the element.
In this lab, you will carry out experiments and perform the necessary calculations to determine the relative atomic mass of the
fictitious element candium. The three different isotopes of candium are M&Mium, skittleium, and Reesespiecesium. As in real
elements, these isotopes are collections of particles having different masses. Your job will be to obtain a sample of candium and
determine the relative abundance of each isotope and the average mass of each type of particle. From this data you will calculate the
weighted average mass of candium. Unlike real isotopes, the individual isotopic particles of candium may vary slightly in mass; this is
why you will use an average mass for each type of isotopic particle. Then you can calculate the weighted or relative atomic mass (Ar) of
candium.
MATERIALS
 A sample of candium.
 Electronic balance.
 Plastic weigh boats.
PROCEDURE
Record your data on the table provided (if necessary, reproduce the data table in your lab book) as you carry out
the following steps. Be sure to add across the rows to get all the totals.
1. Find the mass of all the M&M’s. Repeat for the Skittles and Reese’s pieces.
2. Count up how many M&M’s, Skittles, and Reese’s pieces you have.
3. Divide the mass of each isotope (M&M’s, Skittles, and Reese’s pieces) by the number of each isotope to
get the average mass of each isotope.
4. Divide the number of each isotope by the total number of all the candies together to get the relative
abundance of each isotope. (Note: multiplying these results by 100 would give you the percent
abundance of each isotope, however, there is no need to do this at this point).
5. Multiply the relative abundance from step 4 by the average mass of each isotope to get the relative mass
of each isotope.
6. Add the relative masses across the row to get the average atomic mass (Ar) of all the particles of candium.
7. Select 3 M&M’s from your sample—the largest, the smallest, and one that appears “average” in size.
Determine the mass of each of the three and record them. Compute the average mass of the largest and
smallest only. Record your result.
8. Clean-up: Eat the candium or toss in the garbage. Discard the weigh boat. Turn off the balance. Do not
leave candy on the table, floor or sink please.
DATA
Step
M&M’s
Skittles
Reese’s pieces
Total
1. Mass of each
isotope.
2. Number of
each isotope.
3. Average
Mass of each
isotope.
4. Relative
Abundance
5. Relative
Mass
6. Ar
Calculations: be sure to calculate and record your data correctly using appropriate sig figs that reflect the
sensitivity of your measurement equipment.
ANALYSIS
1. Which of the data in the table above is measured? Counted? Calculated?
2. In all the rows in the data table, you added across to get the Total. The total for row 3 does not take into
account the relative abundance of the candium isotopes. Recalculate this total by dividing the total mass by the
total number of each isotope. How does you answer compare to the total for row 3 in the data table?
3. Find the percent of each isotope in your sample. What is the difference between percent and relative
abundance? Hint: What is the total for each?
4. Compare the value for steps 3 and 6 totals. Why are they different?
5. a) Compare your relative atomic mass (Ar) with at least one other group (their names are ________________
and _______________) Explain any differences between your results.
b) How would using larger sample sizes affect this difference?
6. a) Compare the average from step 7 above the mass of the M&M that appeared “average” and to the average
from the total for row 2 in the data table.
b) Which of the three averages do you think is most reliable? WHY?