• Quick retrieval from earlier learning (Atomic number, mass number and isotopes). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. An atom of chlorine is written ³⁵₁₇Cl (mass number 35, atomic number 17). How many neutrons does it have? → 18Prompt: Atomic number = number of protons (= electrons in a neutral atom).
2. Chlorine has atomic number 17. How many neutrons are in the isotope chlorine-35? → 18Prompt: The number after the name is the mass number.
3. What are isotopes? (atoms of different elements with the same mass / atoms of the same element with different numbers of neutrons / atoms with different numbers of protons / charged atoms) → atoms of the same element with different numbers of neutronsPrompt: The number of protons decides which element it is.

• Show the periodic table and ask what patterns students can see before anything is explained.
• Share the objective: Relative atomic mass from isotope abundances. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Natural boron contains 20 % of mass 10 and 80 % of mass 11. Calculate its relative atomic mass to 1 decimal place.
• Step 1: 10 × 20 + 11 × 80 = 1080
• Step 2: 1080 ÷ 100 = 10.8
• Step 3: So the answer is: 10.8
• Answer: 10.8
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Natural boron contains 20 % of mass 10 and 80 % of mass 11. Calculate its relative atomic mass to 1 decimal place. → 10.8Prompt: Multiply each mass by its percentage abundance.
2. An element has two isotopes: mass 47 (25 %) and mass 49 (75 %). What is its relative atomic mass? Give 1 decimal place. → 48.5Prompt: The answer must lie between the two isotope masses.

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 6 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. An element has two isotopes: mass 79 (20 %) and mass 81 (80 %). What is its relative atomic mass? Give 1 decimal place. → 80.6Prompt: The answer must lie between the two isotope masses.
2. Natural magnesium contains 79 % of mass 24 and 10 % of mass 25 and 11 % of mass 26. Calculate its relative atomic mass to 1 decimal place. → 24.3Prompt: Multiply each mass by its percentage abundance.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-10.