Lesson Plan

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Lesson Plan
Grade: Date: 17/01/2026
Subject: Physics
Lesson Topic: Know the relationship between the nucleon number and the relative mass of a nucleus
Learning Objective/s:
  • Describe the concept of nucleon (mass) number and its role in determining a nucleus’s relative mass.
  • Explain why electron mass is negligible and how binding energy creates a small mass defect.
  • Apply the approximation m ≈ A u to calculate the relative mass of a given nucleus.
  • Analyse common misconceptions about nuclear mass and correct them.
Materials Needed:
  • Projector or interactive whiteboard
  • Printed worksheets with nucleon‑number tables
  • Calculator
  • Periodic‑table handout
  • Sample isotope cards (e.g., N‑14, O‑16, Ca‑40)
  • Whiteboard and markers
 Introduction:
Begin with a quick visual of a simple atomic model, asking students to estimate the mass of a nucleus based on the number of protons and neutrons they see. Recall that protons, neutrons and electrons have different masses and that the nucleon number (A) counts only protons and neutrons. Today we will determine how A relates directly to the nucleus’s relative mass (≈ A u) and identify the limits of this approximation.
Lesson Structure:
  1. Do‑now (5'): Students answer quick‑check questions on nucleon number and relative mass on a sticky note.
  2. Mini‑lecture (10'): Review nucleons, mass number, and the approximation m ≈ A u, using the particle‑mass table.
  3. Guided example (10'): Work through the nitrogen‑14 example together, highlighting the mass‑defect concept.
  4. Collaborative activity (15'): In pairs, calculate relative masses for isotopes (O‑16, Ca‑40) and compare with textbook values, noting discrepancies.
  5. Concept check (5'): Whole‑class quiz (clickers or show of hands) to address misconceptions.
Conclusion:
Summarise that the nucleon number provides a quick estimate of a nucleus’s relative mass, with only a tiny correction from binding energy and electron mass. Ask students to write one exit‑ticket sentence explaining why carbon‑12 is defined as exactly 12 u while other nuclei show a slight mass defect. For homework, assign a worksheet requiring calculation of relative masses for three additional isotopes and a short paragraph on the role of binding energy.