Subatomic Particles and Relative Masses

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SWBAT name and describe relative masses of the three subatomic particles that make up an atom.

Big Idea

Proton mass = neutron mass >> electron mass; Electrons have about 1/1800th the mass of a proton or neutron.

Why this lesson?

This lesson addresses Disciplinary Core Idea Physical Science 1 (DCI PS1), which focuses on Matter and its interactions.  Specifically, two of the three subsections of DCI PS1 require student understanding of atomic and nuclear structure (structure of matter and nuclear processes).  Basic understanding of these concepts require students to understand that atoms are made from protons, neutrons, and electrons.  Further knowledge that protons and neutrons have almost equal mass and are relatively very massive compared to electrons helps students understand the scale of subatomic particles, addressing Cross-Cutting Concept 3: Scale, Proportion, and Quantity.  Getting a clearer idea of relative masses will help students understand in following lessons why it seems that an atom's mass is only determined by numbers of protons plus neutrons.  If students already know that the mass of an electron is negligible (as opposed to non-existent) compared to the masses of protons and neutrons, they are less likely to make the mistake of assuming electrons have no mass.

Through this activity, students choose something to represent an electron and then model what an equivalent relative mass for protons and neutrons would be, which engages them in Scientific Practice 2: Developing and Using Models.  Students who construct these models will be far more likely to understand the scale differences between protons/neutrons and electrons.  In order to successfully construct the models, students also will need to engage in Scientific Practice 5: Using Mathematics and Computational Thinking.  They will calculate, based on actual masses of subatomic masses, first what the relative mass relationships are between the particles.  Second, they will choose something to represent the least massive of the particles (which is the electron).  Once they measure the mass of their chosen electron, and calculate the required relative mass to model an equivalent proton and/or neutron, students might need to troubleshoot their electron choice because the equivalent proton/neutron is too massive to model.



5 minutes

While I take attendance, students do a warm-up activity in their composition Warm-Up/Reflection books.  I use warm-ups to either probe for students' prior knowledge about the day's upcoming lesson or to have them bring to mind and review what they should have learned during the Flipped Lesson recording the night before.  (To read more about Warm-Up/Reflection Books, please see the attached resource.)

Today's Warm-Up: "What is an atom made of?  How massive are these things?"

In this case, the warm-up is asking students to call upon any prior knowledge they may have about atomic structure and it is preparing them for today's activity during which they are investigating the relative masses of subatomic particles.  Potential misconceptions are that all subatomic particles have the same mass or that electrons have no mass at all.  I do not address any incorrect answers in today's warm-up because our lesson will address those issues directly.

If time permits, I walk around with a self-inking stamp to stamp the completed warm-ups indicating participation, but not necessarily accuracy.  On days when there is too much business keeping, I do not stamp.  For today, I will make sure to stamp student work because it is our first time using the Warm-Up/Reflection books.

I tell students that warm-ups are occasionally immediately checked and other times not.  At the end of each unit, I collect and spot-check Warm-Up/Reflection Books.  

As I am walking around stamping student responses, I might ask follow-up questions based on their answers (like "Why do you think that?").  After stamping all of the warm-ups, I ask students if anyone wants to share their answer.  My policy is to wait until there are five hands up or 30 seconds have elapsed, and then to call on someone.  I introduce this policy to my students during this warm-up because it is the first one we are doing in the school year, and it is most likely the first time I am asking them to share answers with the entire class.  I explain to my students that if five people volunteer within the 30 seconds, I will call on one of those five, usually.  I emphasize usually because I am concerned that if I only call on one of the five volunteers every time, it could become the same five students every time, which I do not want.  If it takes more than 30 seconds to get the minimum amount of volunteers, I have free choice, and will NOT choose a hand that is up.  This provides incentive to students to volunteer quickly, and ensures the wait time will not draw out beyond 30 seconds.  I reserve the right to call on any student at any time, but I typically do not want to use this technique as a way to embarrass students who are not prepared or paying attention.  (I will use proximity techniques to pick out students who are not doing what is expected and get them back on track, including walking near their seating area or putting my hand on his or her desk.)

As students share their answers, I do not confirm any answers as correct or incorrect.  This warm-up is strictly for examining prior knowledge and I want students to have thought about the question, but they do not need corrections at this point.


Introduction to the Activity

5 minutes

First, I pass out the Subatomic Particles Relative Masses Comparison student handout (a Subatomic Particles Relative Masses Key is available here).  Then, I tell students that they have 3 minutes to fill in the four blanks (masses of a proton, neutron, and electron, and which one has the least mass).  I tell them that they have to find the answers themselves, but they can use the textbooks in the room or their phones to do internet research.  I project a count down clock or verbally give time warnings at each minute mark and the 30 second mark.  I verbally count down the remaining 15 seconds even if a timer is projected so that I provide a verbal signal to settle back down and be ready for work.

It is important that students have correct values for subatomic particle masses.  I tell students to verify their answers with at least two other people.  Any one that finds any discrepancies needs to justify their answer and determine which person has the correct answer.  I remind students to pay attention to units: kilograms are not the same as grams!

I tell students that they need to decide how to show what the relative masses are for each particle.  I deliberately use the word "relative" in this context, then elaborate with a description of what that would look like.  I tell students that we need to see how much "stuff" a proton is made of, compared to how much "stuff" a neutron is made of, compared to how much "stuff" an electron is made of.  I do not tell students that an electron is the least massive and I am also careful not to use size descriptive words because we are looking at mass and not comparing densities or sizes of particles.  I want my students to discover the large mass discrepancy between the particles on their own.

I explain to the students that they can choose a lab station with supplies, but that they must spread out evenly and only stick to one lab station so that each of their lab station models use only one type of material for protons, neutrons, and electrons.  Some materials I make available for students:

  • granulated sugar (or salt)
  • rice grains
  • small beans
  • sand
  • metal shot
  • ground coffee
  • clay or playdough
  • strips of scratch paper

Any material that is available in quantities of about 1:1800:1800 will work for this activity.

Then, I tell my students that they can "go play" which is my cue for them to move from desk seating to our lab stations.  At this point, because it is still our first week together, I have not assigned lab groups and I allow students to choose their own groups/lab stations.  However, after the first week or two, I do create assigned lab groups and stations.

Group Work

35 minutes

As students work to complete the modeling activity, I walk around to each group asking and answering questions.  When students are calculating relative masses, I am checking to see that they are arriving at reasonable answers and troubleshooting their incorrect calculations.  Students should have the following information correct (although significant digits and/or units may vary slightly):

  • Proton mass = 1.673 x 10^-27 kg
  • Neutron mass = 1.675 x 10^-27 kg
  • Electron mass = 9.109 x 10^-31 kg

Students should recognize that electrons have the least mass and use that value to determine relative masses by dividing each mass by 9.109 x 10^-31 (or whatever value they have for an electron's mass).

Things to look for while troubleshooting:

  • Do students understand scientific notation?  I may have to remind students that a negative exponent indicates places that the decimal point moves to the left, making the coefficient number smaller.  Students may not understand 10^-31 is smaller than 10^-27 and I take the time to prove that it is by writing out similar powers of ten and explaining the difference between negative and positive powers of 10.
  • Are students using their calculators correctly?  Some students may not be familiar with using scientific notation on their calculators.  I can show students in a lab group how to enter the scientific notation using the specialized keys on their calculators.  In lab groups where one or more students do know how to use their calculators for scientific notation, I ask that those students show the rest of their group if it seems they are capable of doing so.
  • Are students getting off values due to calculator error (such as neglecting to use parentheses) or due to incorrect calculation?  I check to see that students are arriving at relative masses that approximate 1:1800 (for electrons:protons/neutrons) and if they are not, and they have correct values for particle masses, I need to determine where they are having the discrepancy.


Another issue students might have trouble with is determining an appropriate representation for the electron.  If students use something too massive, it will be difficult for them to get 1800 times that amount for their proton and their neutron portion of the model.  I do not warn students of this potential issue because I prefer that they discover the problem themselves.  This discovery leads to student understanding of the large difference in scale between proton/neutron mass and electron mass.

Photos of working groups and a sample of final models: Subatomic Particles Model 1Subatomic Particles Model 2Subatomic particles Model 3Subatomic Particles Model 4, and Subatomic Particles Model 5.

Once students have completed their models, I expect them to label them clearly and take a picture.  I use a free online grading program, engrade, that allows students to attach files as a "turn-in" function for assignments.  Students need to upload their photos to their engrade account as part of this in class assignment.

Sample student work for the calculations/data collection handout is here: Subatomic Particles handout Student Work.


Student Reflection

5 minutes

In student's Warm-Up/Reflection Books, students spend about 3-5 minutes writing a response to the day's reflection prompt. I design prompts to either help students focus on key learning goals from the day's lesson or to prompt deeper thinking.  I can also see if there are any students who are missing the mark in terms of understanding.  (A full discussion about how I use Warm-Up/Reflection books is included in the "Warm-Up" section of this lesson.)

Today's Reflection Prompt:  "In comparing the masses of protons, neutrons, and electrons, what did you discover?  Which particles affect an atom's mass the most?"

Desired student responses should indicate that:

  • Protons and neutrons have relatively the same mass.
  • Electrons have much less mass than protons or neutrons (somewhere in the order of 1/1800th).
  • An atom's mass is determined primarily by the mass from protons and neutrons.