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A new atomic theory, in which all atoms of the same element are identical to one another and equal in mass, was proposed by the scientist Dalton. Although the theory had its flaws and was simple, it was revolutionary. Scientists became able to study the actual structure and mass of atoms after the discovery of radioactivity. Soon, isotopes were discovered, as atoms of the same element which have been built up to have different masses.
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. Do not taste or ingest any materials in the chemistry laboratory.
Wash hands thoroughly with soap and water before leaving the laboratory Prelab Questions
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 larger for scientists to discover them.
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. “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
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. Count and record the number of Bg atoms in each isotope group. 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
Data Table
“Bean Bag” Isotope (Bg)
Number of Atoms
Total Mass of Atoms
1
170 (white)
49.2 g
2
56 (brown)
27.83 g
3
354 (green)
27.12 g
Results Table
“Bean Bag” Isotope (Bg)
Average Mass
Percent Abundance
1
0.2894 g
29.3%
2
0.4970 g
9.7%
3
0.0766 g
61.0%
Questions
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)
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. 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. Note: Divide the percent abundance of each isotope by 100 to obtain its relative abundance. Relative abundance = Percent abundance/100 293 x 0.2894) + (0.097 x 0.4970) + (0.610 x 0.0766) = atomic mass (Bg) 0.0847942 + 0.048209 + 0.046726 = atomic mass (Bg)
0.1797292 amu = atomic mass (Bg)
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.
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. Note: To one decimal place, the mass of each isotope is equal to the mass number. This, the mass of an atom of Mg-24 is 24.0 amu. (0.790 x 24.0) + (0.100 x 25.0) + (0.110 x 26.0) = atomic mass (Mg) (18.96) + (2.50) + (2.86) = atomic mass (Mg)
24.32 amu = atomic mass (Mg)
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. 63.5 amu = (0.75 x 63.0) + (0.25 x 65.0)
Cu-63 percent abundance: 75%
Cu-65 percent abundance: 25%
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, but is midway between the mass numbers of copper-63 and copper-65 because it takes into account the abundance of each isotope of the element. The abundances are of course not the same, so the atomic mass will not be 64. Radioactive isotopes (radioisotopes) are widely used in medicine. Because isotopes have identical chemical properties, the reaction and distribution of radioisotopes in the body is similar to that of their natural isotopes. Iodine-131, for example, is an artificial radioisotope that is used to diagnose thyroid disorders. When administered to a patient, the radioisotope is taken up by the thyroid gland, where it is incorporated into the thyroid hormones, just as iodine in the diet would be. Based on where the following elements are likely to be found in the body, match each radioisotope with its medical use. Sodium-24: C-Tracing blood circulation
Phosphorus-32: D-Genetics (DNA) research
Calcium 47: A-Studies of born formation
Iron-55: B-Red blood cell studies
Conclusion
The atoms of an element can have several isotopes, which have the same number of protons and electrons but differs in the number of neutrons. Reslting 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 they are used to study the element and for medical purposes.
