# History of the Atom

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## Objective

Students will be able to model the atom and explain how theories regarding the atom have changed throughout history as demonstrated by taking notes, watching videos, drawing models, and analyzing models.

#### Big Idea

Scientists have investigated atoms throughout history which has led to many changes regarding our understanding of the structure of the atom.

## Introduction

In this lesson students are led through the history of the atom.  I emphasize how our understanding of the model of the atom has improved as scientists have tested hypotheses, and improved on previous theories.

In this lesson I lead students through the history by using a PowerPoint but also showing videos of the experiments used and drawing models on the board.  At the end of the lesson they do a group activity where they compare models of the atom and write down evidence related to the models.

This lesson aligns with the NGSS Performance expectation: 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.

It also aligns with the NGSS Science and Engineering Practice 2: Developing and Using Models.  Modeling is used in several ways in the lesson.  First, students are encouraged to begin by making their own model of what they think and atom looks like.  Second, as I go through the history of the atom I show how the model of an atom changed.

## Engage

5 minutes

To help students begin to think about models, and how we use models to think about the atom, I begin the lesson my having student draw their own model of an atom.

I present the idea on the first slide of the PowerPoint.

I have them do this on an atoms model paper which has a box for their initial drawing of an atom and a box for their final drawing of the atom.  I emphasize that a model is a way to show a conceptual idea and that they should try to show as much detail as possible on their models.  I also make sure to tell them that there is no right or wrong answers and that I am only interested in what they already know about atoms.

See the Modeling Reflection for more details about how I do this in my classroom.

My goals of having students model an atom are twofold:

1. For students to actually put down what they already know so they can build on their knowledge in the the lesson.
2. So that I can start to gauge misconceptions that students hold such as the size of atoms, the size of the subatomic particles, how electrons move, and where the subatomic particles are located.

This is an example of a student's modeling paper.  Notice that his initial idea of an atom is more like a molecule with more than one atom together.

## Explain

40 minutes

Once students have recorded their initial idea of the atom I begin to explain the history of the atom.

I go through the history of the atom starting with Democritus and ending with the Quantum Mechanical model.  I do this by presenting a PowerPoint while students fill in information on their notes graphic organizer

As I present the information I use several videos:

1. cathoderaytube video which explains how Thompson was able to determine that electrons have a negative charge.

2.Millikan's Oil Drop Experiment video which explains how Millikan was able to determine the charge of an electron.

3. Rutherford's Goil Foil Experiment video which explains how Rutherford was able to determine that atoms have a very dense positively charged nucleus.

I also draw models on the White Board to show the models of the atom have changed through history.  This document shows the six different models I draw beginning with Dalton's idea of a solid neutral sphere and ending with the Quantum theory with protons and neutrons in the nucleus.  I emphasize to students how there is not one correct model for the atom and how scientists are still doing research to better understand atoms and atomic structure.

These two pages (page1 and page2) show one student's filled in notes regarding the history of the atom.

## Elaborate/Evaluate

30 minutes

To elaborate on the idea of models and atoms I have students work with partners on a worksheet where they read through five different models of atoms (A through E) and then compare the models to each other.  This part of the lesson was adapted from Living By Chemistry's lessons on the atom. Living By Chemistry is a great book that often leads students into understanding concepts using inquiry and real-world examples.  To learn more about this great resource visit their website.

I explain to students what they will be doing on the last slide of the PowerPoint.

This worksheet is unique because it has students use evidence from the models to back up their answers to various ideas.  Many students struggle within thinking through the models so I have them work with partners to have someone to talk through the problem with.  I make partners based on their table groups and have them work with the person next to them.  If there is an odd number of students at a table I will have students move to make partners and occasionally will have student work in groups of three.

While students are working I walk around to check their answers and make sure that they are citing evidence from the models.

Because of copyright issues I cannot post the actual worksheet that I use from Living By Chemistry.

If you are interested in doing a similar activity and do not have access to Living by Chemistry I found a similar lesson from the American Association for the Advancement of Science (AAAS), The History of the Atom 5: The Modern Theory.  This is a great lesson which could be used to have students do reading related to the modern theory.