Bean Bag Isotope: Abundance and Atomic Mass Lab

Abstract

In this experiment, we investigate the mass properties and relative abundance of isotopes for the "bean bag" element (symbol, Bg), and calculate the atomic mass of this element.

Introduction

A new atomic theory, proposed by scientist Dalton, suggests that all atoms of the same element are identical and equal in mass. This theory was revolutionary, and with the discovery of radioactivity, scientists gained the ability to study the actual structure and mass of atoms. Isotopes, atoms of the same element with different masses, were soon discovered.

Purpose

The purpose of this lab is to investigate the mass properties and relative abundance of isotopes for the "bean bag" element (symbol, Bg), and to calculate the atomic mass of this element.

Equipment and Materials

• Balance centigram (0.01-g precision)
• "Bean Bag" element, Symbol Bg, approximately 50 g
• 4x Weighing dishes or small cups
• Marker or pen for labeling

Safety

Observe all normal laboratory safety guidelines. The food-grade items that have been brought into the lab are considered laboratory chemicals and are for lab use only.

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Do not taste or ingest any materials in the chemistry laboratory. Wash hands thoroughly with soap and water before leaving the laboratory.

Prelab Questions

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

The property that led to the discovery of electrons and protons was their charges.

Neutrons were the last of the three subatomic particles to be discovered because they have no charge and therefore it was harder and took longer for scientists to discover them.

Question 2: 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.

• • • Si-28 has 14 protons, 14 neutrons, and 14 electrons.
    • Si-29 has 14 protons, 15 neutrons, and 14 electrons.
    • Si-30 has 14 protons, 16 neutrons, and 14 electrons.

Question 3: "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 explain why.

This statement is false. The atomic mass of chlorine represents the average mass of all the isotopes, and it takes into account the relative abundance of chlorine isotopes.

Procedure

1. 1. Sort the atoms in the "bean bag" element sample (Bg) into three isotope groups (1, 2, and 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.) Place each isotope group into a separate weighing dish or small cup.
   2. Count and record the number of Bg atoms in each isotope group.
   3. Measure the total mass of Bg atoms belonging to each isotope group, and record each mass to the nearest 0.01 g in the data table.

*Note: Zero the balance with an empty weighing dish on the balance pan, THEN add all of the Bg atoms of one type to the weighing dish and record the mass. (Do this for each isotope group.)

Observations and Data

Results Table

Questions

Question 1: Determine the average mass of each Bg isotope to three significant figures. Enter the results in the Results Table.

(See Data and Observations: Data Table)

Question 2: What is the total number of "bean bag" (Bg) atoms in the original sample? Calculate the percent abundance of each isotope: Divide the number of atoms of each isotope by the total number of atoms and multiply the result by 100. Enter the results to one decimal place in the Results Table.

The total number of "bean bag" (Bg) atoms in the original sample is 580, including what’s in the table. Percent abundances:

• • • Isotope 1: 170/580 * 100 = 29.3%
    • Isotope 2: 56/580 * 100 = 9.7%
    • Isotope 3: 354/580 * 100 = 61.0%

Question 3: The atomic mass of the "bean bag" element (Bg) represents a weighted average of the mass of each isotope and its relative abundance. Use the equation on the lab sheet to calculate the atomic mass of Bg.

Atomic mass (Bg) = (Isotope 1 Mass x Isotope 1 Percent Abundance) + (Isotope 2 Mass x Isotope 2 Percent Abundance) + (Isotope 3 Mass x Isotope 3 Percent Abundance)

Atomic mass (Bg) = (0.2894 g x 0.293) + (0.4970 g x 0.097) + (0.0766 g x 0.610) = 0.0847942 + 0.048209 + 0.046726 = 0.1797292 amu

Question 4: How many Bg atoms in the original sample would be expected to have the same mass as the calculated atomic mass of the element? Explain.

None of the Bg atoms in the original sample would be expected to have the same mass as the calculated atomic mass of the element because the atomic mass is the average of the masses of each atom. Each atom has its own abundance percentage, and the atomic mass is an average of all the masses. As a result, the atoms would not have the same mass as the calculated atomic mass, but they would probably have very similar masses.

Question 5: The isotopes of magnesium (and their percent abundance) are Mg-24 (79.0%), Mg-25 (10.0%), and Mg-26 (11.0%). Calculate the atomic mass of magnesium.

Atomic mass (Mg) = (Isotope 1 Mass x Isotope 1 Percent Abundance) + (Isotope 2 Mass x Isotope 2 Percent Abundance) + (Isotope 3 Mass x Isotope 3 Percent Abundance)

Atomic mass (Mg) = (24.0 amu x 0.790) + (25.0 amu x 0.100) + (26.0 amu x 0.110) = 18.96 + 2.50 + 2.86 = 24.32 amu

Question 6: Copper (atomic mass 63.5) occurs in nature in the form of two isotopes, Cu-63 and Cu-65. Use this information to calculate the percent abundance of each copper isotope.

Atomic mass (Cu) = (Isotope 1 Mass x Isotope 1 Percent Abundance) + (Isotope 2 Mass x Isotope 2 Percent Abundance)

63.5 amu = (63.0 amu x 0.75) + (65.0 amu x 0.25)

Cu-63 percent abundance: 75%

Cu-65 percent abundance: 25%

Question 7: Explain why the atomic mass of copper is not exactly equal to 64, midway between the mass numbers of copper-63 and copper-65.

The atomic mass of copper is not exactly equal to 64 because it takes into account the abundance of each isotope of the element. The abundances are not the same, so the atomic mass will not be exactly midway between the mass numbers of copper-63 and copper-65.

Conclusion

The atoms of an element can have several isotopes, which have the same number of protons and electrons but differ in the number of neutrons, resulting in a mass difference. To get the atomic mass that is on the periodic table, you must use a sample of the element to calculate the relative abundance of the isotope, and then multiply it by the mass of the isotope, and add the result of each isotope to the others. Some isotopes are radioactive and are used to study the element and for medical purposes.