Shells split into sub-shells
At GCSE, shells filled 2, 8, 8 — a simplification that works up to calcium. The fuller picture: each shell is made of sub-shells, labelled s, p and d, and each sub-shell is made of orbitals that hold up to two electrons each (drawn as a box with up and down arrows — the two electrons have opposite spins).
This builds straight on the GCSE 2, 8, 8 shells — recap C1 — Electronic structure.
| Sub-shell | Orbitals | Holds |
|---|---|---|
| s | 1 | 2 electrons |
| p | 3 | 6 electrons |
| d | 5 | 10 electrons |
So shell 1 is just 1s (2 electrons); shell 2 is 2s + 2p (8); and shell 3 is 3s + 3p + 3d — 18, not 8. Electrons fill from the lowest energy upwards, one per orbital before any pair up — and the famous twist: 4s is slightly lower in energy than 3d, so 4s fills first.
Two columns on the left (the s block), six on the right (the p block), ten in the middle (the d block) — the table's shape IS the sub-shell structure.
Build the configurations
Tap a box to add an electron, tap again to pair it — then Check. Wrong arrangements aren't blocked; the checker names the rule you broke, exactly as a teacher would.
Interactive — four targets, unlocked in order
Tap a box to add an electron — tap again to pair it, again to clear. Nothing stops a wrong arrangement, so press Check answer when you think it’s right.
First ionisation energy
The evidence for all of this structure comes from measuring how much energy it takes to pull electrons off atoms.
First ionisation energy — the energy required to remove one mole of electrons from one mole of gaseous atoms, forming one mole of gaseous 1+ ions.
That wording — one mole, gaseous — is the A-Level standard, and it's checked word by word. The patterns in ionisation energies are where the sub-shell story earns its keep:
Why is helium's first ionisation energy higher than hydrogen's? Tick (✓) one box.
Sodium's first ionisation energy is much lower than neon's. Why? Tick (✓) one box.
Boron's first ionisation energy dips below beryllium's. At A-Level you'll learn this is evidence for… Tick (✓) one box.
How did you do?
Work through the builder and questions above and your score appears here.
That's the course. Retake the diagnostic in the last week of the holidays — fluent by lesson one.
That's fixable before September — I take on a small number of A-Level students each year →