Lab—Find the Average Atomic Mass of the
Element Runtium
All of the elements on the Periodic Chart exist in at least two isotopic forms. Isotopes are
atoms whose atomic numbers are the same but whose mass numbers are different due to
differing numbers of neutrons present in each isotope. The atomic mass which is shown on
the Periodic Chart for that element is actually an average of all the isotopes of the element,
weighted by the percentage of the abundance in which they occur. IT SHOULD BE NOTED
THAT DIFFERENT ISOTOPES OF AN ELEMENT ARE NOT FOUND IN DIFFERENT
PLACES--EVERY SAMPLE OF THAT ELEMENT CONTAINS ALL OF THE ISOTOPES IN
FIXED PERCENTAGES, NO MATTER WHERE THE ELEMENT IS FOUND.
NEWS FLASH!!! A NEW ELEMENT HAS JUST BEEN DISCOVERED. Borger, USA.
Nuclear chemists, performing basic research on food products have discovered what is
believed to be a new element. The researchers have tentatively named this element
Runtium derived from irradiated Wal-mart packages. Further research of the new element
will be conducted in more suitable surroundings, namely laboratories in a nearby high school.
STUDENT EXCITEMENT REGARDING THIS DISCOVERY IS RUNNING AT FEVER
PITCH! Many chemistry students have generously volunteered their time and expertise to
help with the follow-up experiments involving the new element.
A reliable source was overheard to say that the first follow-up experiments conducted in the
high schools will determine how many isotopes of this element exist, the mass of each of
these isotopes, the percentage of abundance of each isotope and the average atomic mass
of the new element. One unique property of Runtium should make these experiments
particularly easy--unlike normal atoms, Runtium atoms are very large. They can be easily
seen, and the different isotopes can be recognized and sorted by hand. Scientists are
expecting a complete, comprehensive summary of this new element within two days,
including pictures and collected data tables. THIS IS THE MOST EXCITING DISCOVERY
IN CHEMISTRY OF THE CENTURY!!!
PURPOSE: To identify the number of isotopes of Runtium, determine the mass of each of
the Runtium isotopes, find the percent of abundance of each isotope and calculate the
average atomic mass of the new element Runtium.
EQUIPMENT:
balance
sample of Runtium
plastic sorting cups
plastic counting plate
calculator
PROCEDURE:
1. Use the small plastic cups at each lab station to sort the sample of into its
Runtium various isotopes.
Pick one of the isotopes to be #1.
Place an empty plastic cup on the balance and TARE it (reset it to zero).
Pour all of the atoms of that isotope into the cup on the balance and
record the total mass of all of the atoms of that particular isotope.
Now pour this sample of isotope #1 out on the plastic plate and count the
number of ATOMS of isotope #1 and record the number on the
Observation Sheet.
Have your lab partner verify this number by counting again. If you do
not agree on the number, count them again together.
To find the mass of ONE ATOM of isotope #1, divide the total mass of isotope #1
by the number of atoms of isotope #1. THIS IS THE MASS OF ONE ATOM OF
ISOTOPE #`1.
When you are through with that isotope, put it back into the plastic Zip-Lock bag.
BE CAREFUL NOT TO SPILL ANY OF THE ATOMS ON THE FLOOR!!
2. Repeat the same procedure for isotope #2, #3, #4 etc., recording the total
mass of the isotope and the exact number of atoms of that isotope.
3. Now you must determine the percentage of abundance of each of the different
isotopes.
First, find the total number of atoms present in your sample by adding the
numbers of atoms of each of the isotopes together.
Divide the number of atoms of ONE of the isotopes by the TOTAL
number of all atoms and multiply by 100 to convert it to a percentage.
4. Repeat the same process for each of the other isotopes.
5. Calculate the average atomic mass which should be placed on the Periodic
Table for the new element Runtium.
5. Wash your hands before you leave the lab.