Small, Smaller, Smallest

Version 2


    Unit Summary

    Students learn how our current understanding of the structure of atoms has developed over centuries through the study of indirect evidence. They continue their study of atomic structure as they explore concepts, such as subatomic particles and their charges, probable location of electrons, nuclear and electric forces, and isotopes of different elements. In small groups, they create timelines to trace the history, and explain the scientific processes and knowledge used by each landmark scientist.

    This unit is designed for a flipped classroom, where students acquire content information at home or outside the classroom. In a face-to-face setting, they engage in student-centered activities with the teacher and peers. Throughout this unit, various resources are suggested for online viewing and interaction. The activities and structure of the unit can be modified for a more traditional classroom environment.


    At a Glance

    • Grade Level: 9-12
    • Subject: Science
    • Topic: Chemistry
    • Higher-Order Thinking Skills: Analysis, Creativity, Communication
    • Key Learnings: Atomic Structure, Scientific Inquiry
    • Time Needed: 3 weeks, 1 hour classes, daily


    Things You Need




    Download this Unit (zip)


    Curriculum-Framing Questions

    • Essential Question
      How do we learn about the world around us?
    • Unit Questions
      How do we use indirect evidence to learn about the world?
      How do "wrong" ideas help the progression of scientific knowledge?
      What are some advantages and disadvantages of using models to describe the world?
    • Content Questions
      What are the components of an atom?
      How are electrons arranged in atoms?
      How are different isotopes of carbon used in the real world?


    Assessment Processes

    View how a variety of student-centered assessments are used in the Small, Smaller, Smallest Unit Plan. These assessments help students and teachers set goals; monitor student progress; provide feedback; assess thinking, processes, performances, and products; and reflect on learning throughout the learning cycle.


    Instructional Procedures

    Because this unit is designed for a flipped classroom, students will be expected to do some kinds of activities, such as watching online lectures or videos, at home or outside of regular classroom time. Face-to-face classroom time will be used for activities that benefit from peer collaboration and individual or small-group interactions with the teacher.

    Unit Introduction

    Introduce the unit with a whole-class or small-group discussion based on the Essential Question, How do we learn about the world around us? Encourage students to consider the following questions and take notes on their thoughts:

    • What about the world can our senses tell us?
    • Do our senses always give us reliable information? When do they? When don't they?
    • How does technology help us use our senses to learn about the world around us?
    • What do scientists need to know that their senses cannot tell them? How can they learn these things?


    To help students understand the concept of using indirect evidence to understand objects that cannot be directly observed, pose the following scenario to students in small groups:

    Seeing the Invisible

    Your island has been at war with a nearby island. So far, your thorough surveillance of the surrounding ocean has made enemy approaches impossible. You have just learned, however, that they have invented a new kind of ship with an invisibility screen that they are going to use to sneak onto your island and steal all of your weapons. You don't know what the ship will be like, but you must prevent its attack by thinking of strategies for learning about an enemy that you cannot see.

    • How can you learn anything about the ship as it approaches?
    • What kinds of information can you learn?
    • What processes would make the information scientifically accurate?
    • What can you know for certain and what can you determine is merely probable or improbable?


    Some possible responses could be:

    • Look at the wake that the ship would leave, and estimate the ship's size, shape direction, and speed.
    • Listen for sounds of an engine or other method of propulsion.
    • Shoot something at different heights around the ship's wake to determine how tall or long it is.


    After students have completed the activity, ask them to participate in an online discussion to the following prompts, using the online discussion rubric for self-assessment:

    • What strategies did your group come up with to describe the invisible ship?
    • What is "indirect evidence," and how do we use it in everyday life and more formal science inquiry?

    Basic Atom Structure Review

    As a prior knowledge activity, have students review what they learned about the structure of atoms in earlier grades by taking an online quiz, such as Atoms Basics Quiz*. Ask students to reflect in their journals about what they feel they understand well and what they still need to review.


    If you feel students need more review about basic concepts of the atom, assign home readings or videos, such as the Khan Academy video Introduction to the Atom*. Use face-to-face class time for construction of models of atoms, such as the Light-Brites Activity*. While students are working on the activity, take anecdotal observational notes to determine students' understanding. You may want to provide an online tool for students to record concept descriptions and vocabulary for future reference.

    Atomic Structure and Theory


    In this section of the unit, students develop a more sophisticated understanding about the structure of the atom. During class time, have students work with partners on the PhET science simulation Build an Atom*. Select appropriate materials, such as Build a Molecule* or Build an Atom: Introduction*, from the Teaching Resources for materials to support students while they work with the simulation and to deepen their understanding. Observe students while they work on the activity. Refer to Misconceptions about the Structure of Atoms for ideas about what to look for as students work through the various activities in this unit to identify topics that need further instruction.


    Assign viewing of the video Bohr's Model of the Atom* outside of class. Ask students to take notes, noting questions they have, and preparing for a face-to-face discussion the following day by considering the following questions:

    • How is Bohr's view of the atom the same as earlier views? How is it different?
    • How did Bohr use indirect evidence to explain the structure of the atom? (Remind them of the ship activity they completed earlier.)


    Another possible activity is to ask students to make a representation of a Bohr atom. Students can use found materials to construct a physical model or use technology to make a digital representation.


    In class, pair students and ask them to work through the Models of the Hydrogen Atom*, a PhET simulation. Adapt and distribute the Bohr Atom Simulation* worksheet from the Teaching Resources section of that simulation for students to complete together. Explore other resources and simulations, such as Rutherford Scattering*, that you might want to use to build students' understanding of the important concepts.

    For homework, ask students to review the concepts of atomic number, atomic mass, forces in an atom, and electron orbits from their textbook or the videos assigned earlier. Ask students to explore the concepts of nuclear forces further-strong force, electromagnetic force, fusion, and fission-through the textbook or online videos, such as Binding Energy, Fission and the Strong Nuclear Force*. In class, have students complete all or parts of the Composition of an Atom* activities to reinforce the concepts.


    At the end of this section, ask students to do a quickwrite*, where they describe what they learned and what concepts they still find confusing or are unsure about. Address any learning challenges through additional activities, resources, or conferences.


    Introduce the topic of isotopes and radioactive decay with a reading, an online video*, or a presentation you create for students to watch at home. Assign students a set of short questions to answer while they are watching the video or presentation that will prepare them for the classroom activity. In class, ask students to complete one or more of the following activities, depending on their understanding:


    Isotopes and Atomic Mass*

    A PhET simulation that explores isotopes and how abundance relates to the average atomic mass of an element.



    An activity using plastic eggs to simulate different isotopes of an element.


    Isotopic Pennies*
    An activity that demonstrates how objects with the same appearance can have different weights


    For a quick assessment of students' understanding of isotopes, ask them to take an online quiz*, a clicker quiz, or post your own question online for students to answer. Follow up the assessment with review or additional activities, if necessary.

    To study a practical use of isotopes, ask students to investigate how carbon isotopes are used to identify the age of material with bits of organic matter. For an extension, interested students can work through Carbon 14 Dating* exercises.


    To conclude this section, place students in small groups to review the activities they completed or read about in light of the following questions:

    • Which experiments relied on direct evidence?
    • Which experiments used indirect evidence?
    • Which experiments were neither, but merely illustrated in a concrete way processes or conditions that cannot be easily observed or measured?


    Take anecdotal notes during this discussion to determine students' level of understanding of the concepts of indirect and direct evidence in the process of scientific inquiry. If necessary, provide additional instruction to fill in gaps or correct misconceptions.


    Have students reflect on their learning in online journals, to describe new ideas and lingering questions.

    Small, Smaller, Smallest Timeline Project

    Place students in small groups to complete the Small, Smaller, Smallest Timeline project. This project is designed to help students think about the methods that scientists use to develop theories about the world, and how these theories connect, disprove, confirm, and build on each other. You might consider having students complete this project before the preceding activities as a more inquiry-based activity. Encourage students to use their creativity in the project, to try out new kinds of technology, and to take advantage of the special skills and abilities of their group members.


    Recommend resources about the history of the study of the atom, such as textbook readings, online videos (like The Structure of the Atom* series), Web sites, or teacher-made presentations. Use Discussion Questions: The History of Atomic Theories to promote critical thinking and build content knowledge as they work on the project. Emphasize the following parts of the project:

    • Critical components of the various theories and models
    • Connections among theories and scientists
    • Scientific processes used to develop the various theories


    Distribute the project description and rubric to help students plan and self-assess their timelines. Consider the following options when assigning the project:

    • Assign each group a different time period, so the class, as a whole, will complete a detailed timeline.
    • Select scientists and thinkers you want to emphasize, and divide the names among the groups.
    • Allow students to choose the names and events they want to research, based on criteria that you give them.


    Give students several days to work on the project, in class and out of class. Use this time to interact with groups and individual students to prompt critical thinking and draw conclusions about how student learning is progressing. You may want to intersperse the activity with journal entries, short demonstrations or presentations, news articles, and other activities that can be completed both at home and in the classroom. When the projects are finished, give students time to view each other's timelines, using the timeline peer review, and prepare some study questions to help them review any information or concepts you want to make sure they retain from the project. Follow up the project with a reflection and a final exam over the content.


    Complete the unit with a face-to-face or online discussion about the Essential Question, How do we learn about the world around us? and Unit Questions, How do we use indirect evidence to learn about the world? and, What are some advantages and disadvantages of using models to describe the world?

    Beyond the Atom

    To prepare students for moving on to the study of chemical reactions and how atoms in elements combine with other elements to form compounds, have students explore the following Online Labs* simulations:


    Chemical Reactions

    Single Displacement Reactions

    Combination Reaction


    Prerequisite Skills

    • Basic understanding of how scientists develop theories about the world
    • Ability to work collaboratively with other students
    • Ability to use technology to access information, communicate with others, consume content, and create digital products


    Differentiated Instruction

    Resource Student

    • Identify a reduced number of concepts for students to master
    • Encourage metacognition through journaling and self-assessment
    • Use concrete activities and visuals whenever possible to explain concepts
    • Encourage students to use their interests and special abilities, such as art or technology, in completing the unit project
    • Enlist students as tutors for younger children

    Gifted Student

    • Encourage students to research more advanced or theoretical topics
    • Encourage experimentation with technology to create sophisticated digital products
    • Assign mentors to encourage interested students to study advanced concepts

    English Language Learner

    • Place students in groups so ELL students can participate in a smaller group
    • Allow students to locate Web resources in their native languages
    • Use visuals and hands-on activities whenever possible
    • Encourage students to use their own interests and culture when completing open-ended tasks.

    Students develop a creative digital product that shows how scientists’ understanding of the building blocks of matter has evolved over time.