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ASIM Calculating Average Atomic Mass - Beadium student handout, revised 10/
Calculating Average Atomic Mass - Beadium
Introduction In a random sample of any element found in nature, it is common to find atoms with slightly differing masses. Since the atoms are the same element, they must have the same number of protons. Since electrons have relatively no mass when compared to protons and neutrons, the mass differences must reside in differing numbers of neutrons. These slightly different family members are called isotopes. Isotopes have different nuclear properties and masses, but have the same chemical properties. The sum of the protons and neutrons is called the mass number of the isotope, and the mass number is used to distinguish isotopes from each other. By expressing the mass number and atomic or proton number of an element, we can distinguish in written material between isotopes. This is done by writing the mass number as a superscript, and the atomic or proton number as a subscript, as in 146 C^ , or by writing the element’s name followed by a hyphen and its mass number, as in carbon-14. In the following lab activity models of atomic nuclei will be used to calculate the average atomic mass of isotopes. Then a graphical model will be developed and used to explain why the abundance-weighted average of isotopes of an element yields the published atomic mass.
Essential Question How are the atomic masses on the periodic table calculated?
Pre-Lab Exercise Prior to doing the Beadium activity, complete the Isotopes POGIL and the Average Atomic Mass POGIL activities provided by your teacher.
Lab Activity When scientists analyze isotopes, the sample of atoms is random or randomly selected meaning that although the number of each isotope is not known when the sample is obtained it should be representative of the relative abundances of each isotope as they occur in nature. In this lab, the average atomic mass of the rare element beadium will be determined. The symbol of this fictitious element is Bd. Beadium’s isotopes can be distinguished by color according the chart below.
Isotope name Isotope Color Atomic Mass (amu) Isotopic Symbol Beadium – 64 red 64.0 64 Bd Beadium – 65 green 65.0 65 Bd Beadium – 66 yellow 66.0 66 Bd Beadium – 67 blue 67.0 67 Bd Beadium – 68 white 68.0 68 Bd
Materials Randomly chosen sample of beadium isotopes, writing utensil, calculator
Groups of 2 – each person hands in a lab summary after the white board session including data from the lab sheet and graph.
ASIM Calculating Average Atomic Mass - Beadium student handout, revised 10/
Procedure
- Obtain a random sampling of beadium isotopes. Do not allow any atoms to fall on the floor!
- Remember in the POGIL prelab activity, the average atomic masses were calculated based on a 100 atom sample. You probably do not have 100 atoms in your random sampling of beadium. Group the isotopes in such a way as to allow you to calculate the percent of each isotope in your sample.
- Develop a model by creating a graph using your beads that displays the abundance of each isotope in your sample. (Include your graphical model in your white board session.)
- Sketch your graph in the box provided on your data sheet. Be sure to give it a title and label the axes.
- Read through the calculations below to complete the data table.
Calculations (place all results in data table)
- Using your graph, record the number of atoms of each isotope that are present in your sample.
- Determine the percent of each isotope in your sample. Report your answer to the hundredths place.
- Calculate the weighted fraction of each isotope in your sample. 4. Calculate the average atomic mass of your sample by using the equation that you developed in the pre-lab POGIL activity and report your answer to the correct number of significant figures.
ASIM Calculating Average Atomic Mass - Beadium student handout, revised 10/
White Board Session Develop your initial argument. Your argument must include a claim, which is your answer to the essential question. You must include evidence to support your claim. The evidence is the analysis of the data collected by your group and your interpretation of what the analysis means. Finally, you must include a justification for your argument. You will use a scientific principle to explain the evidence you decided to use. Make sure your evidence is relevant and important. You will create a poster that will include all the information shown below. You may include data tables and graphs on your board to substantiate your claims.
The Essential Question: Group Members Names:
Our Claim: ( What is your answer to the Essential Question?)
Our Evidence: ( Analyzed data and an
interpretation of the analysis. What does the analysis mean?)
Our Justification: (Why does the evidence matter?
Using scientific concepts, explain the importance and relevance of the evidence.)
Argumentation Session One member of your group will remain with your poster to defend your argument. The presenter is responsible for explaining how your group analyzed your data, and why you decided to do it that way.
All other group members will travel to other groups’ posters to listen to their argument and critique their work. As your critique others work, decide if their conclusions are valid and how well did they support their ideas? You need to determine if their argument is persuasive and convincing.
Ask the following questions.
- Did they use genuine evidence and did they use enough evidence to support their claim? o Is their data organized to show trends over time? A relationship between variables? o Did they use more than one piece of evidence? o Are all of their ideas supported by evidence?
- Is their evidence of high quality (valid)? o Did they use appropriate methods to gather the data? o Did they make an attempt to reduce errors in measurements?
- Does their explanation fit all of their evidence?
- Is their explanation sufficient and clear? o Does it explain everything? o Is it free from contradictions? o Did they think of any alternative explanations? Why did they accept or reject those ideas or conclusions? o How confident is the group in their conclusions?
ASIM Calculating Average Atomic Mass - Beadium student handout, revised 10/
Exit Slip Name _______________________________________
Answer the following questions and turn them in before you leave.
- The average atomic mass of nitrogen is 14.0067 amu. Predict which isotope listed below is most abundant in nature. Explain your thinking. a. Nitrogen- b. Nitrogen- c. Nitrogen- d. Cannot make prediction with the given information
- Based on your models of atomic nuclei explain why the abundance-weighted average of isotopes of an element yields the published atomic mass.
- Why is it not sufficient to calculate a simple average of the isotope data?
- Using the information in the table below calculate the average atomic mass and identify the element using the periodic table. Isotope Abundance Mass A 75.76% 34.9689 amu B 24.24% 36.9659 amu
