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# 5.3: Lesson 5.2 History of the Atom

Difficulty Level: At Grade Created by: CK-12

## Key Concept

The ancient Greek philosopher Democritus first introduced the idea of the atom. The idea was re-introduced by John Dalton in 1800. He provided evidence for atoms and developed atomic theory. J.J. Thomson discovered electrons and proposed the plum pudding model of the atom. Ernest Rutherford discovered the nucleus and protons and proposed the planetary model of the atom.

## Standards

• MCR.6-8.SCI.11.1, 3, 4; MCR.6-8.SCI.13.5
• NSES.5-8.A.2.5, 7; NSES.5-8.G.2.1; NSES.5-8.G.3.3
• AAAS.6-8.1.A.3, 4; AAAS.6-8.1.B.2; AAAS.6-8.4.D.1, 2; AAAS.6-8.10.F.1, 2; AAAS.6-8.11.B.1, 3, 5; AAAS.6-8.12.D.9

## Lesson Objectives

• State Democritus’s ideas about the atom.
• Outline Dalton’s atomic theory.
• Explain how Thomson discovered electrons.
• Describe how Rutherford found the nucleus.

## Lesson Vocabulary

No new vocabulary terms are introduced in this lesson.

## Teaching Strategies

### Introducing the Lesson

Before you teach this lesson, demonstrate to students the thinking behind Democritus’s idea of the atom. As students watch, cut an apple in half. Then cut one of the halves in half. Continue cutting the pieces in half until they are too small to cut into smaller pieces. Relate the demonstration to Democritus’s idea of “uncuttable” pieces, or atomos. He proposed that matter is made up of tiny particles that cannot be reduced to smaller pieces. Tell students they will learn in this lesson more about Democritus’s idea as well as other early ideas about the atom.

### Demonstrate

You can demonstrate Dalton’s, Thomson’s, and Rutherford’s atomic models using bowls of gelatin with and without fruit such as berries. Dalton’s model can be represented by a bowl of plain gelatin, Thomson’s by a bowl of gelatin with berries scattered throughout it, and Rutherford’s by a bowl of gelatin with a single berry in the middle. Have students match the bowls of gelatin with the scientists’ models of the atom.

### Activity

Have students do the “Rutherford Roller” activity at the following URL. The activity simulates Rutherford’s famous gold-foil experiments in which he deduced the existence of the atomic nucleus from indirect evidence. At the end of the activity, relate it to Rutherford’s experiments.

### Discussion

Discuss Dalton’s atomic theory and how well it has withstood the test of time. List the parts of the theory on the board, and ask students which part or parts they think may no longer be accepted (atoms are the smallest particles of matter, which cannot be divided into small particles). Remind students that scientists now know there are particles smaller than the atom, including extremely small particles called quarks.

### Differentiated Instruction

Assign the two questions below for students to think about. Then pair any English language learners or less proficient readers with other students, and have partners discuss the questions. Finally, have the students work together to write answers to the questions.

1. Atoms are too small to be seen without special microscopes that were invented in the 1980s, but scientists had already learned a lot about atoms before then. How could scientists learn about atoms without being able to see them?
2. What did early scientists think atoms were like? How did early ideas differ from today’s ideas about atoms?

### Enrichment

Students with an interest in history might enjoy researching and creating a timeline of the history of the atom. Tell them to include modern discoveries and ideas as well as the events described in this lesson. They can start with the URLs below. Display their timeline in the classroom and refer to it as you teach this lesson and the next.

### Science Inquiry

Have students do one or more of the hands-on simulation activities described at the URL below. The simulations will allow them to model how subatomic particles were discovered on the basis of indirect evidence.

### History Connection

Place the achievements of Ernest Rutherford in historical context. Have students read about the life and discoveries of Ernest Rutherford at one or more of the URLs below. Based on their reading, ask them to identify what they think is Rutherford’s most important scientific contribution. Also ask whether they agree or disagree with a Rutherford biographer’s claim that “Rutherford was one of the most illustrious scientists of all time.”

## Reinforce and Review

### Lesson Worksheets

Copy and distribute the lesson worksheets in the CK-12 Physical Science for Middle School Workbook. Ask students to complete the worksheets alone or in pairs to reinforce lesson content.

### Lesson Review Questions

Have students answer the Review Questions listed at the end of the lesson in the FlexBook® student edition.

### Lesson Quiz

Check students’ mastery of the lesson with Lesson 5.2 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.

## Points to Consider

In this lesson, you read how models of the atom changed as scientists discovered the particles that make up atoms. In the next lesson, you will read how Rutherford’s model was revised as scientists learned even more about electrons. For example, they discovered that electrons do not travel around the nucleus in random orbits as Rutherford thought.

• Can you predict how electrons might move around the nucleus?
• How might Rutherford’s model be changed to show this?

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