How The Atom Was Proven To Exist

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Students will be able to explain how Einstein used Brownian Motion to finally prove scientifically that atoms exist.

Big Idea

In 1905, a young Albert Einstein published four groundbreaking theories. One of those theories used Brownian Motion to prove the existence of atoms. This lesson is part one of three describing how we discovered the atomic model used today.

NGSS Background

This lesson is based on California's Middle School Integrated Model of NGSS.

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

PS1.A: Structure and Properties of Matter - Gases and liquids are made of molecules or inert atoms that are moving relative to each other.

S & E Practices 2: Developing and Using Models

CCC: Cause and Effect

This lesson is a precursor to understanding the role atoms play in our world. Models are used (video of pollen grains) to assess the students' understanding of Brownian Motion (MS-PS1-1, SP2). In this lesson pollen grains observed under a microscope are used to confirm the existence of atoms, specifically water molecules colliding with the pollen grains. This demonstrates that in a liquid (water) molecules are moving about (PS1.A). The effect of the pollen grains needs a cause to explain Brownian Motion, which Einstein provided with his explanation that the pollen was being influenced by invisible water molecules (CCC: Cause and Effect).

This is part one of three lessons that review the historical experiments that lead to our current atomic model. The other two lessons are 1) How The Electron Was Discovered, and 2) How The Nucleus Was Discovered.

Background Science

Every year I'm asked the same question, "How can we know atoms exist is they are too small to see?" As a young teacher I typically gave the poor answer "because they do." Does this sound familiar?

I became tired of my answer and felt that I owed it to my students to explain how we have definitive proof that atoms are real and the answer is surprisingly simple. 

 In 1827 Robert Brown was observing pollen under a microscope. These pollen grains of Clarkia pulchella would always move about no matter how long the pollen was under the microscope. This was described as Brownian Motion and remained a mystery for many years. Social Studies Connection: This wildflower was first described by Lewis and Clark on their expedition of North America in 1806.




In 1905 Albert Einstein explained that the seemingly random motion of the pollen grains can be explained by water molecules colliding with the pollen grains. By using the known value of the pollen grain's mass and trajectory, Einstein was able to accurately determine the size of atoms, thereby proving their existence.



Pollen gain (blue) being bombarded by invisible water molecules (red). 


Teacher Lesson

20 minutes

I created a Powerpoint called How the atom was proven to exist - Teacher Lesson that will assist you in teaching this lesson.

I've also included a video that shows how to demonstrate Brownian Motion using an online simulator from - Brownian Motion

Student Activity

20 minutes

After the Teacher Lesson, students diagram four pictures to show how the atom was discovered. A single page in their science notebook was divided into four quadrants. Each quadrant has to contain a title, a colored diagram (3 colors minimum), labels describing the picture, and a multi-sentence explanation of the diagram. The boxes are titled as follows: 1) Robert Brown, 2) Brownian Motion, 3) Albert Einstein, and 4) How Brownian Motion proves atoms existed.

Student Examples