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5.4: Lesson 5.3 Modern Atomic Theory

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Key Concept

Bohr introduced the idea of energy levels, which confine electrons to certain fixed distances from the nucleus. Today, electrons are represented by an electron cloud model. Denser regions of the cloud, called orbitals, show where electrons are most likely to be.


  • MCR.6-8.SCI.11.3, 4
  • NSES.5-8.A.2.5; NSES.5-8.G.2.1
  • AAAS.6-8.1.A.3; AAAS.6-8.1.B.2; AAAS.6-8.11.B.1, 3, 5; AAAS.6-8.12.D.9

Lesson Objectives

  • Define energy levels.
  • Describe the electron cloud and orbitals.

Lesson Vocabulary

  • electron cloud: area surrounding the nucleus of an atom where electrons are likely to be
  • energy level: area located at a fixed distance from the nucleus of an atom where electrons can orbit the nucleus
  • orbital: dense region in the electron cloud around the nucleus of an atom where electrons are most likely to be

Teaching Strategies

Introducing the Lesson

Start a simple sketch of a hydrogen atom on the board by drawing a small circle labeled “nucleus.” Add a smaller circle labeled “proton” inside the nucleus. Call on a student to come to the board and add the electron and its orbit to the sketch. (The student is likely to draw a circle around the nucleus and add a symbol such as a dot for the electron.) Point out that this is how electrons are usually represented but it’s misleading. Explain that if the nucleus were the size of the nucleus in the sketch, then the electron orbit would actually be about half a mile away with nothing but empty space in between. Tell students they will learn more about electrons and where they are located in the atom when they read this lesson.

Using Visuals

Students may confuse energy levels and orbitals. Relate the energy levels in Figure 5.15 to the number of orbitals per energy level in Table 5.1.

Question: If an atom has electrons only in the first energy level, how many orbitals and electrons can it have?
Answer: It can have a maximum of one orbital and two electrons.
Question: If an atom has electrons in just the first two energy levels, how many orbitals and electrons can it have?
Answer: It can have a maximum of six orbitals (one in energy level 1 and four in energy level 2) and a maximum of 12 electrons (2 per orbital).


Differentiated Instruction

Have students make a KWL chart for the position of electrons in atoms. Tell them to divide a sheet of paper into three columns and label them from left to right “Know,” “Want to Know,” and “Learned.” Before they start reading, they should fill in the first column with what they already know (e.g., electrons are located outside the nucleus). They should also fill in the second column with questions they still have (e.g., How far from the nucleus are the electrons?). Tell students to try to find answers to their questions when they read the lesson. Respond to any questions that remain unanswered after they finish reading.


Interested students can dig deeper into orbitals and learn about s, f, p, and other specific orbitals. The URLs below are good sources of information and images. You might want to have them teach the topic to the class. If so, have them use visuals in their presentation.

Science Inquiry

Challenge the class to brainstorm how they might create a two- or three-dimensional representation of the electron cloud model. How could they represent the nucleus? The electron cloud? For ideas, students can see animated two-dimensional representations of the electron cloud model at these URLs:

Real-World Connection

Relate energy levels to the real world. Virtually all students will be familiar with fluorescent lights. Tell them that the light is energy emitted by electrons jumping from higher to lower energy levels. They can learn more at this URL: http://home.howstuffworks.com/fluorescent-lamp.htm.

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

Points to Consider

In this chapter, you learned that atoms of each element have a unique number of protons. This is one way that each element differs from all other elements.

  • How could the number of protons be used to organize elements?
  • If one element has more protons than another element, how do their numbers of electrons compare?

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Date Created:

Nov 11, 2013

Last Modified:

Aug 21, 2014
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