# Name: & Period: Bean Bag Isotopes – Relative Abundance & Atomic Mass Purpose

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Name: & Period:
Bean Bag Isotopes – Relative Abundance & Atomic Mass
Purpose: To gain a better understanding of the concepts of Isotope, Mass Number, Percent Abundance and Atomic Mass by studying the mass properties and relative abundance of isotopes for the ‘bean bag” element (symbol, Bg) and to calculate the atomic mass of this element.
Background: When John Dalton proposed his Atomic Theory in 1808, he stated that all atoms of the same element were identical to one another and equal in mass. With the discovery of radioactivity in the early 1900s, this theory was found not to be completely true. In 1913, Frederick Soddy (Scotland) coined the term isotope to define atoms of the same element that have the same chemical properties but different atomic masses. It comes from the Greek words meaning “same place” to denote the fact that isotopes occupy the same place on the periodic table (they are the same element) even though the atomic masses were different. Soddy received a Nobel Prize in 1921 for his study into the nature and origin of isotopes. Conclusive scientific evidence came from Francis Aston (England) who modified JJ Thompson’s ‘positive ray” tube experiment that studied ions. He worked with Neon-as he described them, as major and minor isotopes having mass numbers of 20 and 22. He received the Nobel Prize in Chemistry in 1922 for his actual discovery of isotopes.
Sample Calculation: Cl-35 and Cl-37 have a characteristic percent abundance in nature. If analyzed from common sources i.e. sea water or salt deposits the percentage is always the same; 75.8% Cl-35 and 24.2% Cl-37. The weighted average is what we find on the Periodic Table.

Atomic mass (chlorine) = (0.758)(35.0amu) + (0.242)(37.0amu) = 35.5 amu
Pre-Lab Questions:

1. In 1932, Chadwick discovered the neutron, more than 10 years after the existence of isotopes were confirmed.

1. What property of electrons and protons led to their discovery first?

2. What is a plausible reason for neutrons being discovered last?

1. Silicon occurs in nature in the form of 3 isotopes, Si-28, Si-29 and Si-30. What is the number of p+, e- and no for each?

Si-28 p+

Si-29 p+

Si-30 p+

e-

e-

e-

no

no

no

Materials:

• Balance, centigram (0.01 g precision)

• Weighing dishes or small cups, 4

• Labeling pen or marker

• “Bean bag” element, symbol Bg, approximately 50 g

Safety:

Although the materials used in this lab are considered nonhazardous, you will observe all normal laboratory safety guidelines. The food stuffs that have been brought into the lab are considered laboratory chemicals and are for lab use only. Do not taste or ingest any materials. Wash hands thoroughly before leaving the lab.

Procedure:

1. Sort the atoms in the “bean bag” element sample (Bg) into three isotope groups (1, 2, & 3) according to the type of bean. (Assume that each type of bean represents a different isotope and that each bean represents a separate atom).

2. Place each isotope group into a separate weighing dish or small cup.

3. Count and record the number of Bg atoms in each isotope group.

4. Measure the total mass of Bg atoms belonging to each isotope group.

5. Record each mass to the nearest 0.01 g in the data table below. *Note: Zero (tare) the balance with an empty weighing dish and record the mass.

Data Table:
 “Bean Bag” Isotope (Bg) Number of Atoms Total Mass of Atoms 1 2 3

Name: & Period:
Bean Bag Isotopes – Relative Abundance & Atomic Mass

Results Table:
 “Bean Bag” Isotope (Bg) Average Mass Percent Abundance 1 2 3

Post Lab Questions & Calculations:

1. Calculate the average mass of each Bg isotope to three sig. figures (Show your work). Enter the answers in the Results Table.

Bg #1

Bg #2
Bg #3

1. What is the total number of Bg atoms in the original sample?

3. Calculate the Percent Abundance of each isotope by dividing the number of atoms of each isotope by the total number of atoms and multiply the result by 100. Enter the answers to one decimal place in the Results table.

Bg #1
Bg #2
Bg #3

1. The atomic mass of the Bg element represents a weighted average of the mass of each isotope and its relative abundance. Use the following equation to calculate the atomic mass of Bg. *Note: Divide the percent abundance of each isotope by 100 to obtain its relative abundance.

Percent abundance iso = isotope

Relative abundance = RA = relative abundance

100

Atomic Mass = (RAiso1 x massiso1) + (RAiso2 x massiso2) + (RAiso3 x massiso3)

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