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Bean BAG Isotopes Lab (5opts)

Introduction: John Dalton’s atomic theory that stated all atoms of the same element are identical and equal in mass was simple yet revolutionary. Unfortunately, it was not quite right. More research started to show that atoms of the same element could have different masses. These atoms were call isotopes. Isotopes have the same # of protons and electrons but different # of neutrons. Isotopes are chemically indistinguishable from one another; the only different is their mass number. Isotopes do not show up equally in nature. An element’s atomic mass represents the weighted average (based on percent abundance) of the masses of the isotopes. In today’s lab, we are going to learn how to calculate the atomic mass of a “bean bag” element!

Purpose: The purpose of this experiment is to investigate the mass properties and relative abundance of isotopes for the “bean bag” element and to calculate the atomic mass of this element.

A new element has been discovered and it is your responsibility to learn the following about the element:

• the number of naturally occurring isotopes present in the element

• the percentage of each isotope present in your sample of the element

• the atomic mass of the element

Materials: balance, 3 weighing boats, bag of beans

Procedure:

1. Answer the pre-lab questions.

2. Obtain a sample of the element from your teacher and record its ID number

3. Count the total number of beans (atoms) in your sample and record in the data section.

4. Assign each different bean (isotope) a code or letter (ie, W for white).

5. Sort the atoms from the “bean bag” element into three isotope groups according to the type of bean into a separate weighing boat. Count the number of atoms of each isotope and record in your data table under “Number of atoms in isotope group.”

6. Mass each isotope group of atoms and record in the third column under "Mass of Isotope Group”. (the weighing boat does not add any measurable weight)

Pre-Lab Questions (10 pts)

1. Neutrons were discovered in 1932, more than 10 years after the existence of isotopes was confirmed. What property of electrons and protons lead to their discovery? Suggest a possible reason why neutrons were the last of the three classic subatomic particles to be discovered?

2. In 1935 a scientist received the Nobel Prize in Physics for his discovery of the neutron. What is the name of the scientist that discovered the existence of neutrons and what scientist was he working with at that time?

3. Silicon occurs in nature in the form of three isotopes, Si-28, Si-29, and Si-30. Determine the number of protons, neutrons, and electrons in each isotope of silicon.

| |# of protons |# of neutrons |# of electrons |

|Silicon -28 | | | |

|Silicon -29 | | | |

|Silicon -30 | | | |

4. “The atomic mass of chlorine represents the mass of the most common naturally occurring isotope of chlorine.” Decide whether this statement is true or false and why.

5. What is a quick estimate for the average atomic mass of your bean bag element? (total weight of beans divided by the number of beans)

Data Table (10pts): Use the data table provided to collect the following information:

Total number of beans in sample: __________

|Code for Bean Bag Isotope |Number of atoms in group |Total Mass of isotope group (g) |

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Analysis (10pts):

• Calculate the average mass of one atom of that isotope (Mass of Isotope Group ÷ # of atoms of that isotope). Record your answer in the analysis table below.

• Calculate the percentage abundance of each isotope present in your element (# of atoms of isotope ÷ total # of atoms in sample). Record your answer in the analysis table below.

| |Average Mass of one atom |Percent abundance |

|Code for Bean Bag Isotope |Total mass of isotope group ÷ # of atoms of that |Number of atoms of isotope ÷ total number of atoms|

| |isotope |x 100 |

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Calculations (5pts):

• Determine the weighted atomic mass of your element to three significant figures using the below equation. Be sure to show your work and use correct significant digits!

Weighted atomic mass = (avg. mass of isotope 1)(% abundance) + (avg. mass of isotope 2)(%abundance) + (avg. mass of isotope 3)(%abundance)

(For percent abundance, use the decimal form rather than percentage form in your calculations.)

Space for you to calculate your Weighted Atomic Mass:

Conclusion (15 pts): Please answer in complete sentences!

1. The atomic mass of the “bean bag” element represents a weighted average of the mass of each isotope and its relative abundance. What did you calculate as the weighted atomic mass for your element? (Be sure to put your answer in the correct number of significant digits)

2. How close is your quick average atomic mass from pre-lab question 4 to the weighted atomic mass from post-lab question 1? Why was it not the same?

3. Did the most common individual beans or atoms have the same weight as your weighted atomic mass? If yes, which one? If not, did any come close? Why not? How does this relate to pre-lab question number 4?

4. The isotopes of magnesium (and their percent abundances) are Mg-24 (79%), Mg-25 (10.0%), and Mg-26 (11.0%). Calculate the atomic mass of magnesium. Use the mass number for the mass of each isotope of magnesium.

5. Give your element a name and symbol and record the information here. Remember that this is a new element so you, as a scientist, get to name this element whatever you want!

Bean Bag ID #: ______________

Element Name:______________ Symbol_____ Weighted Atomic Mass_________

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