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5.2: Lesson 5.1 Inside the Atom

Difficulty Level: At Grade Created by: CK-12
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Key Concept

The nucleus of an atom contains positive protons and neutral neutrons, which are held together by the strong force. Protons and neutrons consist of even smaller particles called quarks, which are held together by gluons. Negative electrons constantly move around the nucleus because of the attraction of opposite electric charges. The number of protons in an atom is the atomic number, which is unique for each element. The number of protons plus neutrons is the mass number, which equals the atom’s mass in atomic mass units.


  • SCI.CA.8.PS.3.a; SCI.CA.8.PS.7.b; SCI.AC.8.IE.9.f
  • MCR.6-8.SCI.8.1; MCR.6-8.SCI.9.1
  • AAAS.6-8.4.D.1, 2; AAAS.6-8.4.G.5; AAAS.6-8.11.B.1, 3; AAAS.6-8.11.D.3

Lesson Objectives

  • Compare and contrast protons, neutrons, and electrons.
  • Describe the forces that hold the particles of atoms together.
  • Define atomic number and mass number.
  • Describe ions and isotopes
  • Identify the particles called quarks.

Lesson Vocabulary

  • atomic mass unit (amu): SI unit for the mass of an atom, where 1 amu equals the mass of a proton (about 1.7 x 10-24 grams)
  • atomic number: number of protons in an atom
  • electron: negatively charged atomic particle that moves around the nucleus of the atom
  • ion: positively or negatively charged particle that forms when an atom gains or loses electrons
  • isotope: atom that differs in the number of its neutrons from other atoms of the same element
  • mass number: number of protons plus neutrons in an atom; equal to the atom’s mass in atomic mass units (amu)
  • neutron: electrically neutral atomic particle inside the nucleus of an atom
  • nucleus: tiny region at the center of an atom that contains protons and neutrons and makes up almost all of the atom’s mass
  • proton: positively charged atomic particle inside the nucleus of an atom
  • quark: type of fundamental particle of matter that makes up protons and neutrons

Teaching Strategies

Introducing the Lesson

Play a word association game with the class to introduce the lesson. Tell students to say whatever word first comes to mind when they hear the word “atom.” (Sample responses might include “matter,” “tiny,” “nucleus,” “molecule,” “electron,” and “particle.”) The activity will help students recall what they already know about atoms.


Help students learn the parts of the atom with the Atoms Family worksheet and song at the first URL below. Divide the class into small groups and have each group complete the worksheet. Encourage students to sing the Atoms Family song. Finally, have groups complete the Atoms Family math challenge at the second URL.

Building Science Skills

Stress the importance of using models to study atoms because they are so small. Then have students create models of the atom using one or more ideas suggested at the following URL.


Have students examine the periodic table .You can use the interactive periodic table at the following URL with a projector or TV screen. Focus on one element in the table, such as carbon. Point out its atomic number and atomic mass (atomic weight). Based on these two numbers, have students calculate the number of neutrons in the atom.

Differentiated Instruction

Pair English language learners and native English speakers, and have partners make a cluster diagram centered on the atom. They should add surrounding circles for parts of the atom, atomic forces, atomic number and mass number, ions and isotopes, and quarks. Tell them to add important details to each circle.


Ask students to make a word-search or criss-cross puzzle using lesson vocabulary terms. They can make the puzzle by hand or use the free puzzle maker at the URL below. Distribute copies of their puzzles for other students to solve.

Science Inquiry

Divide the class into pairs and have partners explore the interactive “Build an Atom” simulation at the URL below. Students will build an atom with subatomic particles and observe how the element, charge, and mass of the atom change as they add the different particles.

Common Misconceptions

The URL below lists several common misconceptions that middle schoolers may have about atoms. Use some or all of the misconceptions to create a true-or-false quiz. For any misconceptions that students think are true, explain why they are false.

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 5.1 Quiz in CK-12 Physical Science for Middle School Quizzes and Tests.

Points to Consider

In this lesson, you saw several simple models of atoms. Models are useful for representing things that are very small. Scientists have used models to represent atoms for more than 200 years. In the next lesson, you’ll read about some of the earlier models.

  • How might scientists have modeled atoms before the particles inside atoms were discovered?
  • How do you think earlier models might have differed from the models in this lesson?

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Date Created:
Nov 11, 2013
Last Modified:
Jul 31, 2016
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