| Lesson Plan |
| Grade: |
Date: 25/02/2026 |
| Subject: Chemistry |
| Lesson Topic: Describe relative atomic mass, $A_r$ as the average mass of the isotopes of an element compared to $1 / 12$ th of the mass of an atom of ${ }^{12} \mathrm{C}$ |
Learning Objective/s:
- Describe the concept of relative atomic mass ($A_r$) and how it is calculated from isotopic masses and abundances.
- Explain the relationship between $A_r$, the carbon‑12 scale, and molar mass.
- Apply the weighted‑average formula to determine $A_r$ for elements such as chlorine and bromine.
- Convert between relative atomic mass, molar mass, and number of atoms using the mole concept.
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Materials Needed:
- Projector or interactive whiteboard
- Chemistry textbook or teacher guide
- Worksheet with isotope tables and practice problems
- Scientific calculator
- Periodic table handouts
- Printed isotopic data charts (Cl, Br)
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Introduction:
Begin with a quick poll asking students what they know about isotopes and why atoms have different masses. Connect this to prior learning on the mole and Avogadro’s number, highlighting the need for a common mass scale. State that by the end of the lesson they will be able to calculate and use relative atomic mass in stoichiometric problems.
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Lesson Structure:
- Do‑now (5') – short quiz on isotopes and Avogadro’s number to activate prior knowledge.
- Mini‑lecture (10') – introduce the definition of $A_r$, the carbon‑12 reference, and the weighted‑average formula.
- Guided example (12') – work through the chlorine calculation on the board while students follow step‑by‑step.
- Collaborative activity (15') – groups calculate $A_r$ for bromine using the provided table and discuss the hydrogen $A_r$ value.
- Check for understanding (8') – exit ticket: one‑sentence explanation of how $A_r$ links to molar mass.
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Conclusion:
Recap the definition of relative atomic mass and its direct equivalence to molar mass in g mol⁻¹. Collect exit tickets to gauge understanding and clarify any lingering misconceptions. Assign homework: complete the worksheet calculating $A_r$ for two additional elements and convert the values to molar masses.
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