Reflection: Grappling with Complexity Finding Atomic Mass Lab  Section 4: Publishing (PostLab)
This lab requires students to calculate the internal mass of washers in a film canister then divide by the mass of a single washer (canister A  representing Hydrogen) to determine the number of washers in a film canister. This replicates the process chemists employed to find the atomic mass of the elements. Many students have trouble understanding that the published atomic masses are averages of various isotopes (one type of atom with different weights) of a single element and are listed as decimal values. A misconception can occur when students think that the atomic mass may incorrectly represent a mass of partial neutrons (neutrons cut in pieces). I have students that have conceptional difficulty relating their calculated atomic mass (whole number) to an averaged published atomic mass (decimal number).
Finding Atomic Mass Lab
Lesson 3 of 6
Objective: Students will simulate the efforts of deterring the atomic mass of four elements by weighing film canisters.
This lesson is based on California's Middle School Integrated Model of NGSS.
MSPS11 Develop models to describe the atomic composition of simple molecules and extended structures.
DCI: PS1.A: Structure and Properties of Matter  Substances are made from different types of atoms, which combine with one another in various ways.
Science and Engineering Practice: (4) Analyze and Interpret Data
CCC: Energy and Matter
This lesson recreates the efforts chemists experienced when they were determining atomic mass and using hydrogen (H) as their basis of comparison. Students will weigh multiple film canisters (representing atoms) filled with washers (representing the nucleus) in an attempt to determine the amount of washers in the film canisters, akin to chemists trying to determine the atomic mass (total number of protons and neutrons in the nucleus) of elements (MSPS1, PS1.A).
Once atomic mass has been found for these mystery atoms, the students will compare their values to the periodic table in an attempt to determine the names of each atom (SEP 4). This lesson fits in an overarching theme of matter and how it is classified within the realm of science (CCC Energy & Matter). As such students develop scientific discourse within their groups in an attempt to determine which atomic mass their measurements reference on the periodic table.
Common Core Writing Standards (need help)  students summarize atomic mass writing sentences of what they know about atomic mass with formal writing structure.
Common Core Math Standards (need help)  students use the physical mass of a film canister filled with washers as a model for atomic mass, using simple division to determine atomic mass.
Due to measurement and rounding errors, along with atomic masses being reported as averages, there is a wider degree of acceptable answers. This leads to discussions among the students as to which atomic mass correctly matches the data. Proper analysis of the available data is key to understanding the major points of this lesson.
Another firm aspect of this lesson is the use of models (film canisters) representing atomic structure. Students must be able to recognize the limitations and sacrifices necessary whenever a model is used to describe a scientific principle. Students may point out that the film canister may represent the atom, but the outside edge of the canister (electron cloud) is far too close to the washers (nucleus) for this to be a truly accurate representation of the atom.
This lesson was originally inspired by 'Science  Focus on Physical Sciences 8th Grade' CA Edition. 2007. McDougal Littell.
Setup (Prelab)
Needed Supplies
 35 mm film canisters
 washers
 triplebeam balance scales
 calculators
To perform this activity you will need to acquire a lot of old 35mm film canisters (5/group). Film canisters used to be easy to obtain with a letter home to parents for donations, but with digital cameras film canisters have become scarce. The best prices I have found online are at B&H. Some online research may turn up better results, as they're not hard to find. Make sure to get the black film canisters as opposed to the clear.
TIP: You can use any metallic pieces instead of the washers, such as hex nuts or pennies. I choose washers because they are the cheapest to purchase.
If your students need experience/practice with triplebeam balance scales see: TripleBeam Balance Activity or Equipment Measurement Lab.
Once you've acquired the canisters you will want five per group. Leave one canister blank (identified as the EMPTY canister). Label the remaining canisters as A, B, C, and D. If you are using clear canisters you might want to wrap them with electrical tape or paint the canisters. fill each canister with the following washers:
 EMPTY canister (leave empty)
 A canister  1 washer (Hydrogen)
 B canister  4 washers (Helium)
 C canister  12 washers (Carbon)
 D canister  20 washers (Neon)
The actual number of washers you place in each film canister is up to you, as long as canister A stays at 1 washer. I routinely have different groups working with different washer counts. Make sure the washer count corresponds with a rounded atomic mass on the Periodic Table.
The film canisters are standin for atoms, specifically the electron shell. The washers represent the nucleus. The eventual student goal is to figure out, through deductive reasoning, how many 'washers' are in each canister. Each individual washer will represent a particle in the nucleus (proton/neutron). The EMPTY canister will be used to subtract the mass of the canister so the students can determine only how much the contents (particles/washers) weigh and thereby determining how many particles/washers are in the nucleus. Your students may be tempted to simple open the canisters and count the washers. I tell my students that splitting an atom (opening the canister) will cause a nuclear explosion, i.e. every member of the group receives a ZERO for this activity for releasing deadly radiation into the classroom. You could also wrap each canister with glue or tape. In my experience the treat of a group zero keeps the canisters closed.
Once the groups have determined how many particles are in the nucleus (atomic mass) they will use a periodic table to figure out what element is represented. Make sure your students are using 'Atomic Mass' to figure out the elements. My students will use 'Atomic Number' if I haven't made a fuss about the students needing to use 'Atomic Mass'. The biggest 'fail' for this activity happens when the students use 'Atomic Number' instead of 'Atomic Mass'.
I tell my students from the beginning that canister A is Hydrogen and only contains one washer/particle (Hydrogen's Atomic Mass = 1.01). The students will use this value to figure out the rest of the canister, similar to how chemists used the mass of hydrogen to figure out the mass of other elements.
They will weight the EMPTY canister and subtract that value from the weight of canisters A, B, C, and D, which will provide the weight of the washers only (think  atomic mass). They will then divide that subtracted weight by the wight of canister A. Since canister A only has 1 washer (1 proton) the resulting answer should tell your students the number of washers in the canisters (atomic mass). Round each value to the nearest whole number and compare to the atomic masses on the periodic table. Due to weight variations and rounding errors the computed atomic mass may not exactly match the values published on the periodic table. Make sure your students understand that these computed atomic masses are approximations and judgement should be used to compare the published atomic masses on the periodic table.
Student Directions
 Find the mass of the empty film can. Record the data in box (1). (Record the data 4 times – once in each column.)
 Measure the mass of each weighted film can. Record the data in box (2)
 Subtract the mass of the empty can from each weighted can. Record the data in box (3). Think of this as the mass of the nuclei.
 Divide the ‘Mass of Washers’ of Can ‘B’, ‘C’, and ‘D’ by the ‘Mass of Washers’ Can ‘A’. Record in box (4).
 Round each number to the nearest whole number. Record in box (5).
 Estimated the number of washers in each can. Record in box (6). HINT: use Can ’A’ to help guide your investigation. DO NOT OPEN THE CANS. SPLITTING ATOM MAY HAVE EXPLOSIVE CONSEQUENCES.
 Using box (6) as an estimated ATOMIC MASS. Make a determination as to what element you have been investigating. Record in box (7). HINT: Use a modern Periodic Table.
TIP: I purposely place Box 2 'Mass of Film Can' over Box 1 'Mass of Empty Film Can' so as to make the subtraction of the two values more intuitive to the students.
Investigation (Lab)
Pass out Atomic Mass Lab  Directions and Atomic Mass Lab  Data Table. I copy these to pages backtoback so as to only hand one paper to each student.
One of the main goals of this lab is to allow students the opportunity to differentiate between atomic number (protons) and atomic mass (protons+neutron).
Resources (2)
Publishing (PostLab)
Student Data Table
At the conclusion of the lab your students should have been able to identify three elements (B, C, & D), comparing their calculated atomic mass to the published atomic masses on the periodic table.
TIP: Watch for students comparing their calculated atomic mass (whole number) to atomic number (whole number) instead of the published average atomic mass (decimal number). I have some students who lack the abstract ability to compare a whole number to a decimal number.
Some groups arrive at calculated atomic masses that when compared to the atomic masses on the periodic table point to the same element (example  canister B has a calculated atomic mass of 3 and canister C has a calculated atomic mass of 5, both canisters point to Helium which has an atomic mass of 4). When this happens (measurement and/or rounding error) I tell the kids that the same element may have different mass and still be the same atom (definition of an isotope). I tell my students to trust their data and stand by their conclusion that they can reasonably defend.
Atomic Mass Student Explanation  Notebook
As a summarizing activity I have my students draw a picture of what atomic mass represents. They must include at least three colors and labels. Many kids understand that the decimal number on the periodic table lists how much one single atoms weighs, but do not understand contextually what atomic mass represents. When asked what they should draw I ask them what should they count to arrive at atomic mass. Through gentle guiding I can get them to understand that the particles in the nucleus (protons + neutrons) make up the atomic mass. If they believe that electrons should also count I ask them, "If a feather (electron) landed on an elephant (nucleus) would the element weigh more?" Most kids can grasp this analogy and understand that technically the elephant would increase in mass, but the increased mass would not be detectible.
On the bottom half of the page the students are required to write five sentences summarizing what atomic mass is to them. The sentences do not have to be related to one another. They start the next sentence right after the period of the last sentence. At the end of each sentence they must number that sentence (makes it easier to spot grade). I can quickly scan and assign relative points for understanding. The whole process allows for quick and easy assessment. I have the students place the numbers after the sentence because if sentences are first numbered and then written, many students would turn this into a list.
I prefer this format because it allows me the opportunity to find out what my students know, as opposed to a multiple choice quiz that tests what I think they should know.




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