The Atomic Theory and Rutherford’s Experiment

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Objective

SWBAT explain our current understanding of the atom citing evidence from experiments.

Big Idea

Learn how scientists determined the size and charges of subatomic particles by modeling Rutherford's Gold Foil Experiment.

Introduction

This lesson addresses the NGSS HS-PS1-1, "Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms". 

The goal of the lesson is to build on students' knowledge about the basic structure of the atom, its subatomic parts, their charges and the relative size of the atom from second and third lessons in this unit, and show them scientist and experimentation was part of the discover. This is aligned with the NGSS Disciplinary Core Idea (DCI) PS1.A (Structure and Properties of Matter): “Each atom has a charged substructure consisting of a nucleus, which is made of protons and neutrons, surrounded by electrons.”

Students explore the atom using the NGSS Practice of Developing and Using Models. Students will practice modeling using a simulation that illustrates how Ernest Rutherford determined the atom contains a dense, positively charged nucleus.

The Cross Cutting Concept (CCC) that is illustrated throughout the lesson is Systems and System Models. Students explore different models of the atom, recognizing that within a system (the atom) phenomenon can be explained. This is an underlying theme; students will not be summatively evaluated on this part of the lesson.

Warm-up/Engage

10 minutes

As students enter into the room I have a warm-up statement on the board: Draw and label your interpretation of the atom in your science journals. I have them get started on this right away, so we can get to the explore section of the lesson quickly.

The objective of the warm-up is not to have students draw the correct model, it’s to have them think about why there are many different models of the atom, how scientists made discoveries about something so small and what an accurate representation of an atom looks like.  

As students draw their models, there are a variety of student atom diagrams, ranging from the Bohr’s model to something from the cartoon Jimmy Neutron. The warm-up serves two purposes: (1) a review of knowledge about the subatomic parts from lessons 1-3 which will be seen in their drawings and (2) an introduction to how scientists determined the structure of the atom which occurs during the class discussion.

To stimulate discussion I have 5 or 6 student volunteers draw their picture on the board. I follow this up by asking, "Why are there so many models of the atom"?

Common student responses include, "Many scientists discovered the atom" and "Knowledge of the atom has changed over time." When students get to this point, I ask students for possible explanations about how the atom was discovered. Students' responses include from "experimentation" and "technology". At this point I move to the next part of the lesson, where I introduce the students to the Rutherford investigation.

Explain/Explore

30 minutes

My goal is to get students to understand how Rutherford was able to determine the charge and shape of the nucleus of an atom. I show the first four slides of the Atomic Theory ppt so that students get a basic knowledge of each of the scientist involved in the development of early atomic theory, so they can appreciate the scientific process that went into the discovery of the atom.

I do not have students take notes on the PowerPoint, as it’s only meant to be an introduction to the Rutherford investigation. This takes only 5 minutes.

I then introduce students to the Rutherford activity. Usually they are curious to hear about it because they have seen that materials are set up at stations around the room. After, I give a brief explanation of the investigation and student expectations as follows:

  • Around the room are a set of 8 white boards that have a different shape block attached to each, with towels over them (Rutherford ppt), so that the shape cannot be seen. 
  • The goal is to roll the marble under the board, at the block, and determine the shape of the block (which I demonstrate). Remember that the marble represents an alpha particle (+ charge particle) and simulates Rutherford’s experiment. A dry erase marker has been provided to for student attempts to sketch the shape on the board as the marble is rolled from various angles.
  • 3-4 minutes are provided for each station. At the end of the time, sketch the shape of the block in the hypothesis section of the worksheet.
  • Another goal is to think like a scientist and evaluate how this experiment is like Rutherford’s Gold Foil Experiment.
  • While the investigation is going on you [students] should also be thinking about how this investigation is different from Rutherford’s experiment, in addition to what possible other things could be determined from the investigation that were not mentioned during the ppt.

During the investigation:

  • The beginning of the investigation takes a little time, but by the time students finish 2 or 3, it takes them less time to complete each station.
  • I let students know that looking under the boards will result in a point deduction. This requires monitoring.
  • I notice students discussing hypotheses with one another.
  • I remind students that I do not deduct point for wrong guesses. The process is supposed to be inquiry, based on the process of scientific thinking, not the student’s ability to guess right.  

While students are working on the investigation I walk around and ask them, “What are we doing this for?” or “What have you concluded so far about Rutherford’s experiment?"  I am trying to elicit from students that there is a dense nucleus that has a positive charge with a lot of space around it that contains negatively charged electrons.   

After the activity has been completed, I provide students with the actual shapes of the blocks from the Rutherford experiment ppt (Rutherford Activity Pics) and have them write the shapes in the boxes on the worksheets.  

This satisfies their curiosity and provides room for conversation about the representation of the marble as an alpha particle and the charge of the blocks. See my Atomic Theory Refection for more thoughts on student outcomes 

Elaboration/Evaluation

10 minutes

The elaboration portion of this lesson is answering the post investigation questions (Rutherford activity key (1) (2) that are on the worksheet. These questions make them think beyond rolling a marble by getting them to think about the shape and the structure of the atom (DCI:  PS1.A).  This occurs because students often think that it is an easy task to guess the shape, but it turns out it is not as easy as they think.  This coupled with questioning during the explore portion of the lesson encourages thought about the structure of the atom.