Names:______

Date: ______Period: ____

Lab: Beanium (Bn)

(Isotopes and Average Atomic Mass)

Introduction:

The average atomic mass of an element is the weighted average of the masses of the isotopes of that element. The weighted average takes into account both the mass and relative abundance of each isotope as it occurs in nature.

Purpose:

In this lab you will determine theaverage atomic mass of the fictitious element beaniuim, Bn,from a mixture of three naturally occurring isotopeswith different masses.

You willcalculate the weighted average atomic mass, which is “weighted” by the different natural abundances (%’s) of each isotope.

Equipment and Materials Needed:

triple-beam balance

a sample of beanium in a plastic cup

two additional empty plastic cups

one or two stiff cards (for handling beanium)

Procedure:

1)Sort the isotopes of beanium into separate piles and count the “particles” of each isotope in your sample. Record the number of particles of each type in the Data Table.

2)Weigh an empty cup on the balance and record its mass.

Place all of one isotope in the cup and reweigh it. Record that mass also.

Subtract to find the mass of just the isotope.

3)Using the other two cups, repeat the process for the other two isotopes and record your results. Be careful: all similar-looking cups do NOT have the same mass.

4)When you are sure you have gathered all the required data, mix the three isotopes together again and return them. Return the empty cups to the prep area for reuse.

Check to make sure that no “isotopes” ended up in the sink or on the floor.

5)Complete the Calculations table to determine the Average Atomic Mass of beanium.

DATA TABLE

Color of isotope: / ______/ ______/ ______/ Total:
Mass (g):
# of particles:

Calculations: (Use the Data Table on page 1 to complete the calculations for the table below)

CALCULATIONS TABLE

Color of isotope: / ______/ ______/ ______/ Overall:
  1. Average
Mass (g):
  1. %
Abundance: / = 100 %

Calculations: (Follow these instructions CAREFULLY to complete the calculations for the table above)

  1. From the Data Table on the previous page, divide the massby the # of particlesto find theaverage mass of the eachisotope.

To get the Overall average,Do NOT add the averages,divide the Total massby the Total # of particles.

  1. From the Data Table on the previous page, divide the # of particlesby the Total# of particlesand multiply by 100 to get the %relative abundance of each isotope.
  1. Calculate the weighted Average Atomic Mass of beanium from its 3 isotopes using the following formula:

Avg. At. Mass = (%)(Mass1) + (%)(Mass2) + (%)(Mass3)

(Note: the %’s must be in decimal form, that is, 78.5% must be 0.785 or 2.2% must be 0.022)

SHOW YOUR WORK:

Record your answer:

Questions:

1)What is an isotope?

______

2)If you compared your answers with those of another lab group and found a difference, how would you explain that, assuming neither group made a “mistake?”

______

______

3)Why isn’t the atomic mass of each element a whole number?

(like Neon: 10 protons + 10 neutrons = 20 atomic mass of neon)

______

______

4)The “mass number” of “an atom” is not on the periodic table because the periodic table lists the ______of each element. Explain.

______

______

Conclusion: Write a paragraph discussing the lab’s purpose (on the first page)

and what you observed and learned about the average atomic mass of elements.

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