Electrons fill shells around the nucleus in order: 2 in the first, 8 in the second, 8 in the third (for the first 20 elements). The electrons in the outer shell decide how an element behaves.
Neon glows in signs but never reacts with anything. Sodium, one proton heavier, catches fire in water. The difference is one electron.
In Year 9 you met the nuclear atom. This lesson arranges its electrons.
Make your prediction first. Help opens once you've locked it in.
Add protons, neutrons and electrons one at a time. Watch the electrons: when does a new shell start?
Build sodium-23 and watch where each electron goes.
Use the board. This ticks when you get there.
In your book: draw Bohr diagrams for lithium (3), oxygen (8) and potassium (19). Circle the outer shell of each and write how many electrons it has.
Magnesium has 12 electrons. How many are in its outer shell?
Calcium has 20 electrons. How many shells do they fill (or start)?
Finish argon-40: it needs one more electron.
Use the board. This ticks when you get there.
Write one sentence: "At first I thought… Now I think… because…"
Revision summary
Students arrange the electrons of elements 1 to 20 in shells (2, 8, 8, 2) and identify the outer-shell electrons, the variable the periodic table depends on (AC9S10U06, node 1, a hinge node). About 25 minutes.
Open the teacher guide: answers, solutions, slides and a worksheet.
A concept tutorial for introducing or revising the idea; it builds on Year 9's nuclear atom (What's inside an atom?). Set the width with ?width=minimum, typical (default) or wider.
The hinge is fluency: students who can't place outer electrons quickly can't use the periodic table later. The Explain checks and the book task are practice, not breadth.
Board notes: the Bohr model, not to scale. Shells hold 2, 8, 8, then 2 (the simple rule for elements 1 to 20; the third shell holds more in heavier atoms, which Year 10 doesn't need).
Filling shells in order, 2 then 8 then 8. When a student writes 2, 8, 9, ask how many the third shell holds for the first 20 elements.
Stretch: Why does the periodic table have 2 elements in its first row and 8 in the next two?
Support: Give a shell template (three circles with 2, 8, 8 boxes) to fill in for each element.
Next: Why do elements in a group behave alike? (node 2), then periodic trends (node 3); predicting properties from position is a class task (node 4).
In class: flame tests (sodium, potassium, lithium, copper chlorides) with a hand spectroscope if available.