Lesson Plan

• Describe the concept of half‑life and its mathematical link to the decay constant.
• Explain how exponential decay governs the reduction of undecayed nuclei over time.
• Calculate a half‑life from a given decay constant using the formula \(t_{1/2}= \ln 2 / \lambda\).
• Interpret typical half‑life values for different radionuclides.
• Analyse why half‑life is independent of sample size and external conditions.

• Projector or interactive whiteboard
• PowerPoint/slide deck with equations and examples
• Student handout containing sample calculations and a half‑life table
• Graph paper or digital plotting tool
• Scientific calculators (or calculator apps)
• Optional: radioactive decay simulation software

Begin with a quick “mystery” image of a decaying banana to spark curiosity about invisible changes. Ask students what they already know about radioactive decay and how scientists measure its speed. State that by the end of the lesson they will be able to define half‑life, link it to the decay constant, and solve simple calculations.

1. Do‑Now (5′): Students answer a short prompt – “If you start with 1000 atoms, how many remain after one half‑life?” Collect responses on sticky notes.
2. Mini‑lecture (10′): Present the exponential decay law, derive \(t_{1/2}= \ln 2 / \lambda\) on the board, and highlight key concepts.
3. Guided practice (12′): Work through the example calculation (λ = 2.5 × 10⁻³ s⁻¹) together, projecting each step.
4. Interactive activity (10′): In pairs, students use calculators or simulation software to compute half‑lives for the radionuclides in the provided table and plot one decay curve.
5. Check for understanding (5′): Quick quiz via Kahoot or hand‑raise – ask for the relationship between half‑life and decay constant and why external conditions don’t affect it.
6. Summative task (8′): Students complete a short worksheet where they must explain half‑life in their own words and solve a new problem.

Recap the definition of half‑life, its formula, and its independence from external factors. Have students write one‑sentence exit tickets answering “What is the most important thing you learned about half‑life today?” Assign homework: a set of three half‑life calculations using different decay constants.