Lesson Plan

• Describe what isotopes are and how they differ in mass and natural abundance.
• Explain the concept of relative atomic mass as a weighted average of isotopic masses.
• Apply the calculation formula to determine the relative atomic mass of an element from given isotope data.
• Interpret calculation results and compare them with tabulated values.

• Projector or interactive whiteboard
• Chemistry textbook or teacher notes on isotopes
• Worksheet with isotope tables and practice problems
• Calculator or spreadsheet software
• Periodic‑table handout
• Coloured markers for a bar‑chart activity

Begin with a quick poll: “What determines the atomic mass shown on the periodic table?” Students recall that isotopes share the same number of protons but differ in neutrons. Explain that today they will learn how to calculate the relative atomic mass using isotope masses and natural abundances, and they will be able to verify tabulated values by the end of the lesson.

1. Do‑now (5'): Students answer the poll question and discuss their ideas.
2. Mini‑lecture (10'): Review isotopes and introduce the weighted‑average formula for relative atomic mass.
3. Guided example – Chlorine (10'): Walk through each calculation step while students follow on their worksheets.
4. Paired activity – Magnesium (12'): Learners calculate Ar for magnesium and compare with the accepted value.
5. Practice problems (15'): Work on bromine, element X, and element Y calculations; teacher provides support.
6. Check for understanding (5'): Quick quiz or exit ticket requiring one new calculation.

Recap that relative atomic mass is obtained by weighting each isotope’s mass by its fractional abundance and that careful conversion of percentages is essential. Students hand in an exit ticket showing a new calculation, and homework is to complete a worksheet with three additional elements. Emphasise the importance of using appropriate significant figures.