• 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 fluorine is written ¹⁹₉F (mass number 19, atomic number 9). How many neutrons does it have? → 10Prompt: Atomic number = number of protons (= electrons in a neutral atom).
2. Oxygen has atomic number 8. How many neutrons are in the isotope oxygen-17? → 9Prompt: The number after the name is the mass number.
3. A sodium atom loses one electron to become a sodium ion. What is its charge? (0 / +11 / −1 / +1) → +1Prompt: It now has one more proton than electrons.

• 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 gallium contains 60 % of mass 69 and 40 % of mass 71. Calculate its relative atomic mass to 1 decimal place. → 69.8Prompt: Multiply each mass by its percentage abundance.
2. An element has two isotopes: mass 53 (90 %) and mass 55 (10 %). What is its relative atomic mass? Give 1 decimal place. → 53.2Prompt: 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 8 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 54 (60 %) and mass 56 (40 %). What is its relative atomic mass? Give 1 decimal place. → 54.8Prompt: 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.