Objectives (5 - 7 minutes)

1. Understanding the concept of Atomic Mass Unit (AMU): Students should understand that AMU is a standardized unit that represents the relative mass of an atom. They should learn that AMU is based on the isotope of carbon-12 and that the mass of any other atom is expressed in terms of how many times it is larger or smaller than the mass of carbon-12.

2. Calculation of Average Atomic Mass: Students should be able to calculate the average atomic mass of an element, taking into consideration the natural abundance of the isotopes of that element. This involves using percentages of isotopes and their atomic masses to calculate the average atomic mass.

3. Identification of Isotopes and their relevance: Students should learn to identify isotopes of an element and understand why the atomic mass of an element is not a whole number. They should understand that the atomic mass of an element in the periodic table is the weighted average of the masses of all the isotopes of that element, taking into consideration their natural abundance.

Secondary Objectives:

• Application of acquired knowledge: Students should be able to apply the concept of AMU and calculation of average atomic mass in practical problems, such as determining the mass of a sample of an element based on its isotopic composition.

• Development of critical thinking skills: By understanding the concept of AMU and the calculation of average atomic mass, students will develop critical thinking skills, such as the ability to analyze data and make inferences.

• Stimulate scientific curiosity: Through examples and practical applications, students should be encouraged to ask questions and seek answers about the world around them, thus fostering scientific curiosity.

Introduction (10 - 15 minutes)

1. Review of previous concepts: The teacher should start the lesson by reviewing the concepts of atom, isotopes, and the current atomic model. These concepts are fundamental to understanding the lesson topic. One possible way to do this is through a review game, where students are challenged to remember and explain these concepts.

2. Problem situation: The teacher can present two problem situations that pique students' interest and encourage them to think about the lesson topic. For example:

   • "Imagine you are a scientist and have two samples of carbon. One has a mass of 12 atomic mass units and the other has a mass of 14 atomic mass units. How can you determine the percentage of each isotope in the sample?"

   • "Imagine you are an archaeologist and found a sample of an unknown metal. How can you determine its atomic mass?"

3. Contextualization: The teacher should then explain the importance of the lesson topic, showing how understanding the Atomic Mass Unit and the calculation of average atomic mass is crucial in various areas of science and technology. For example, it can be mentioned that the Atomic Mass Unit is used to determine the mass of a variety of subatomic particles, including protons, neutrons, and electrons, and that the calculation of average atomic mass is fundamental to chemistry, as most elements in nature consist of a mixture of isotopes.

4. Capturing students' attention: To captivate students' attention and spark their interest in the topic, the teacher can share some related curiosities. For example:

   • "Did you know that the Atomic Mass Unit is so small that a single drop of water contains over 3 trillion hydrogen atoms?"

   • "Did you know that the calculation of average atomic mass is so precise that scientists can determine the isotopic composition of rock samples from the moon brought back to Earth by the Apollo missions, allowing us to learn more about the formation of the moon and our solar system?"

The purpose of these curiosities is to show students that Chemistry is not just a theoretical discipline, but has fascinating practical applications and relevance in the real world.

Development (20 - 25 minutes)

1. Activity "Building Atoms" (10 - 12 minutes):

   • Materials needed: Small magnets of different sizes and colors, colored markers, small pieces of paper, tape, and cardboard.
   • Objective: This activity aims to allow students to visualize the structure of an atom and understand the idea of isotopes.
   • Procedure: Students, divided into groups, will receive a set of small magnets of different sizes and colors, representing protons, neutrons, and electrons. Each group will also receive small pieces of paper and colored markers to write down the number and type of particles in each atom they build.
   • Step by step:
     1. Students should choose an element from the periodic table to represent.
     2. Using the magnets, they should build several atoms of that element, varying the number of protons, neutrons, and electrons.
     3. For each atom built, students should write down the number and type of particles on their pieces of paper.
     4. At the end of the activity, students should have a collection of different atoms of the same element, representing different isotopes of that element.
     5. They should compare the masses of these isotopes and understand that atomic mass is not a whole number due to the existence of isotopes.

2. Activity "Discovering Average Atomic Mass" (10 - 12 minutes):

   • Materials needed: Cards with symbols of elements, cards with numbers representing the natural abundance of the isotopes of each element, cards with the atomic masses of the isotopes of each element.
   • Objective: This activity aims to allow students to practice calculating the average atomic mass of an element.
   • Procedure: Students, still divided into groups, will receive a set of cards. Each card will represent a chemical element, the natural abundance of the isotopes of that element, and the atomic masses of the isotopes.
   • Step by step:
     1. Students should organize the cards for each element according to the natural abundance of the isotopes.
     2. Then, they should calculate the average atomic mass of that element by multiplying the natural abundance of each isotope by its atomic mass and summing the results.
     3. Students should repeat this process for several elements.
     4. At the end of the activity, students should have a better understanding of how the average atomic mass of an element is calculated and why it is not a whole number.

The teacher should circulate around the room during the activities, guiding the groups, answering questions, and ensuring that all students are engaged and understanding the concepts.

Return (8 - 10 minutes)

1. Group Discussion (3 - 4 minutes): The teacher should gather all students and promote a group discussion about the solutions or conclusions of each group. This is an opportunity for students to share their findings, explain their problem-solving strategies, and clarify any doubts they may have. During the discussion, the teacher should ask guiding questions to ensure that students are understanding the key concepts and to encourage critical thinking. Examples of questions may include:

   • "How did you determine the average atomic mass of each element?"
   • "Why is the atomic mass of an element in the periodic table not a whole number?"
   • "How can understanding the Atomic Mass Unit and the calculation of average atomic mass be useful in everyday situations or in other scientific disciplines?"

2. Connection to Theory (2 - 3 minutes): After the discussion, the teacher should provide a quick review of the theoretical concepts covered in the lesson. The goal is to ensure that students understand the connection between theory and the practical activities carried out. The teacher can do this through a brief presentation, using concrete and everyday examples to illustrate the concepts. For example, the teacher can explain how the calculation of average atomic mass is used in laboratories to determine the isotopic composition of samples, or how the Atomic Mass Unit is used in particle physics to describe the mass of protons, neutrons, and electrons.

3. Individual Reflection (3 - 4 minutes): To conclude the lesson, the teacher should propose that students reflect individually on what they have learned. The teacher can do this through guiding questions, such as:

   1. "What was the most important concept you learned today?"
   2. "What questions have not been answered for you yet?"
   3. "How can you apply what you learned today in other situations or disciplines?"

   Students should have a minute to think about these questions. They are then encouraged to share their answers with the class, if they wish. The teacher should listen carefully to the students' responses, as they can provide valuable feedback on the effectiveness of the lesson and on which concepts may need further reinforcement in future classes.

4. Closure: The teacher should end the lesson by reinforcing the key concepts learned and the importance of the Atomic Mass Unit and the calculation of average atomic mass. The teacher should also refer to any reading or homework assignment that students may have for the next class and answer any final questions from students.

Conclusion (5 - 7 minutes)

1. Summary of Key Points (2 - 3 minutes): The teacher should start the Conclusion by summarizing the fundamental concepts that were covered during the lesson. This includes the concept of Atomic Mass Unit (AMU), the definition of isotopes, and how to calculate the average atomic mass of an element. The teacher should emphasize the importance of these concepts and how they are used in Chemistry and other areas of science.

2. Connection between Theory and Practice (1 - 2 minutes): Next, the teacher should explain how the lesson connected theory, practice, and applications. They should highlight how the practical activities of building atoms and calculating average atomic mass helped solidify the theoretical concepts presented. The teacher should also reinforce how understanding the AMU and the calculation of average atomic mass has practical applications in laboratories and in everyday situations.

3. Additional Materials (1 - 2 minutes): The teacher should then suggest some additional study materials for students who wish to deepen their understanding of the topic. This may include textbook chapters, online educational videos, interactive chemistry websites, and practice problems. The teacher should encourage students to explore these resources and seek additional help if needed.

4. Importance of the Topic (1 minute): Finally, the teacher should summarize the importance of the lesson topic. They should explain how understanding the AMU and the calculation of average atomic mass is fundamental to many other concepts and applications in Chemistry. The teacher should also emphasize how the ability to apply these concepts in real-world situations can be valuable not only in Chemistry but in many other scientific disciplines and careers.

5. Closure: The teacher should end the lesson by thanking the students for their participation and encouraging them to continue exploring and questioning the world around them. They should also remind students of any homework or reading assignment for the next class and answer any final questions.