How The Electron Was Discovered

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Students will be able to explain how the electron was discovered by J.J. Thomson and the contribution it made to our current atomic model.

Big Idea

Students always ask me how can we know what at atom looks like if we can't see them. This lesson is part two of three explaining how we developed our modern atomic model using the discovery of the electron.

NGSS Background

NGSS Standard: MS-PS1-1: Develop models to describe the atomic composition of simple molecules and extended structures.

DCI: PS1.A: Structure and Properties of Matter - Substances are made of different types of atoms, which combine with one another in various ways.

Science & Engineering Practices 2:  Developing and Using Models

CCC: Energy and Matter

The intent of this lesson is to provide the students with an understanding of how we developed the atomic model that is in current use. By explaining how we discovered the atom, students gain ownership in the atomic model. This lesson can be employed as an answer to the student question, "How do we know what the atom looks like if we've never seen it?" I've been able to anticipate this question and I typically provide this explanation immediately after a student has voiced their doubt about the accuracy of the atomic model.

I believe that students should ask this question and I have heavily emphasized our inability to ever see atoms, since they are smaller than light particles and can never been seen by optical methods. I want my students to have the courage to ask questions and cast doubt on what they do not understand.

Teacher Demonstration

25 minutes

Every year I am asked by my students, "Since we can't see atoms how do we know they exist?"

I make a point to explain to my students early on that we will never be able to use optical microscopes to see inside an atom, no matter how advanced our technology becomes. In essence atoms are smaller then light particles, called photons, and since atoms are smaller then photons there isn't anything to bounce off of and create an image.

This causes many students to question how we can know so much about the atomic structure without ever being able to see it. For many years I tried to avoid the question and ask the students to trust that the text book was right. I finally got tired of not having an answer and decided to find an explanation an eighth grader would understand. This lesson is based largely on the book 'The Story of Science - Einstein Adds a New Dimension' (chapter 5) by Joy Hakim.

I previously referenced this book in another lesson called History of Science. This lesson explains which scientists made major advances within the field. The explanations I give to my students have been simplified for an eighth grade audience and some liberties are used to present the formulas. I've included a video that does a good job of showing the complexities of J.J. Thomson's work.

Background science video - formulas used to measure the size of an electron.


J.J. Thomson

 J.J. Thomson was credited for discovering the electron, by using cathode rays, and was awarded the 1906 Nobel Prize in Physics. He has largely been forgotten by textbooks because he proposed an incorrect atomic model made up of positive and negative regions, nicknamed the Plum Pudding Atomic Model. I tell my students to not look harshly on this discovery, since it was one of the first atomic models and science has to start somewhere even if it is wrong. 

To recreate J.J. Thomson's work I purchased a Cathode Ray Tube.

I start out the demonstration by explaining to students that the green beam was called a cathode ray by scientists like J.J. Thomson, but today we know it is a stream of electrons. It was believed at the time that atoms had a magnetic charge and scientists were searching for the source of this electrical charge, something that wasn't explained by John Dalton's original model of 'impenetrable spheres'.

As seen in the above video I place a magnet near the electron stream and watch the stream bend. The kids will 'ooh' and 'aah' over this. I explain that J.J. Thompson performed a similar type of experiment trying to figure out what this green beam was and concluded that the stream must be negatively charged since it responds to the magnet.

While J.J. Thomson was working with this device and magnets, he stumbled upon a comparison to a cannonball shot out of a cannon. He compared the deflection the cannonball experiences as it is shot out of a canon due to the force of gravity (a cannon ball shot level will gradually fall toward the ground because the force of gravity, pulling downward,  independent of the forward velocity) to the deflection of the electron beam. Now at the time he didn't know it was an electron beam.

This sets up three known variables, of 'mass of cannon ball', 'deflection of the cannon ball due to gravity', and 'deflection of the electron steam due to magnetism' and one unknown variable 'mass of electron.

Setting up a simple ratio looks like this:

I simplified the math somewhat for an eighth grader to understand, but basically if you multiply the 'mass of the cannon ball' by the 'deflection of the electron stream due to gravity' and divide that value by the 'deflection of the cannon ball due to gravity' you arrive at the value for the 'mass of the electron'.

J.J. Thomson discovered that the electron was negatively charged (response to a magnet) and the mass of the electron was significantly smaller than the mass of an atom (see: How The Atom Was Proven To Exist). This small mass of the electron helped provide evidence for small subatomic particles (see: How The Nucleus Was Discovered). He hypothesized that the election must be imbedded with the atom, floating freely about like plums mixed in with pudding (a popular dish at the time) and thus providing an explanation of the source of the electrical charge within the atom. J.J. Thomson called his new atomic model 'The Plum Pudding Model'. 

Student Activity

20 minutes

At the conclusion of the lesson I have my students make four diagrams (divide paper into fourths) retelling the story of how the electron was proven to exist. Each box is titled 1) Dalton's Atomic Model, 2) How the electron was discovered, 3) How the mass of the electron was found, and 4) J.J. Thomson's Atomic Model. Each diagram has to have a minimum of three colors, labels, and a minimum of two sentences explaining the diagram.

TIP: Do not overemphasize Dalton's Atomic Model, and J.J. Thomson's Model as they are incorrect models and are only referenced to show how we arrived at our current atomic model. 

Student Work