• 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 sodium is written ²³₁₁Na (mass number 23, atomic number 11). How many neutrons does it have? → 12Prompt: Atomic number = number of protons (= electrons in a neutral atom).
2. Uranium has atomic number 92. How many neutrons are in the isotope uranium-238? → 146Prompt: The number after the name is the mass number.
3. What are isotopes? (atoms with different numbers of protons / charged atoms / atoms of the same element with different numbers of neutrons / atoms of different elements with the same mass) → 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: Electron shells and the periodic table. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: How many electrons does an atom of neon (atomic number 10) have in its outer shell?
• Step 1: 2,8
• Step 2: So the answer is: 8
• Answer: 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. How many electrons does an atom of carbon (atomic number 6) have in its outer shell? → 4Prompt: Fill the shells 2, 8, 8 in order.
2. Why does potassium react more vigorously with water than sodium? (Its outer electron is further from the nucleus, so it is lost more easily / It has a full outer shell / It has fewer protons / It is a non-metal) → Its outer electron is further from the nucleus, so it is lost more easilyPrompt: Potassium has one more shell than sodium.

• 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. How many electrons does an atom of silicon (atomic number 14) have in its outer shell? → 4Prompt: Fill the shells 2, 8, 8 in order.
2. Why do elements in the same group react in similar ways? (They have the same mass / They have the same number of electrons in their outer shell / They have the same number of neutrons / They were discovered at the same time) → They have the same number of electrons in their outer shellPrompt: Reactions involve the outer electrons.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-9.